The Dynamics of Dating

The Dynamics of Dating

by Dr. Roger C. Wiens

Bewildered, Janet watches her son gaze in awe at the dinosaur exhibit. A sign tells her that the Tyrannosaurus Rex is millions of years old. But last Sunday, her Bible teacher stated emphatically that God made Earth only six thousand years ago. The confusion over dates makes her stomach churn. The age assigned to the fossils in front of her seems to contradict the creation account, and Janet’s heartbeats accelerate at the implication. Is Earth young or old? If old, did that mean the Bible is wrong? Or could science be wrong? And what is she going to tell her son?

Scientists agree that radiometric-dating techniques offer the most concrete evidence of any dating system for answering questions about the age of Earth. Yet, many people challenge the accuracy of radiometric dating, and misinformation describing the various radiometric techniques abounds. Debunking mysterious and complicated explanations of radiometric dating can be accomplished with a simple understanding of its general principles. Credible answers to common misconceptions about radiometric dating and a proper understanding of Scripture can help people like Janet reconcile creation accounts regarding the age of Earth.

General Principles of Radiometric Dating

Radiometric dating can be compared to an hourglass. When the timepiece is turned over, sand grains fall from the top of the hourglass to the bottom. No one can predict the moment when a particular grain will fall through the neck, but an estimate can be made for how long the whole pile of sand will take to fall.

A similar process takes place with the radioactive decay of atoms. (For a brief science review, see sidebar and figure 1.) The timepiece that allows dating is the “radioactive” decay of certain kinds of atoms from one form into another. Radioactive decay results from unstable combinations of protons and neutrons in the atom’s nucleus. Though most atoms contain stable nuclei and do not decay, some types do. When radioactive decay occurs, no one can predict which individual atoms will decay when. But, for a large number of atoms, the number that will decay within a given time can be predicted. The original (parent) atom changes into a daughter atom having different chemical properties.

However, one significant difference exists between radiometric dating and the hourglass design. Unlike the hourglass, the rate of radioactive decays in a rock depends on the number of parent (original) atoms at any given time (N0). As fewer parent atoms are left, fewer decays occur. If it takes a certain length of time for half of the parent atoms of a radioactive isotope to decay (half-life), it will take the same amount of time for half of the remaining parent atoms (a fourth of the original total) to decay. In the next interval, with only a fourth remaining, only an eighth of the original total will decay. This produces an exponentially decreasing curve as described by the equation and displayed as the decreasing curved line in figure 2.

All radiometric dating is based on this very simple equation and the exponentially decreasing curve. In other words, N is the present abundance of parent atoms, the original abundance of parent atoms equals N0, t is time, and k is a constant related to the half-life (the time it takes for half of the parent atoms of a radioactive isotope to decay). The simplicity of this equation combined with the fact that it works with many different dating methods produces great confidence in its reliability.

An hourglass measures the specific amount of time that has passed since being turned over. Radiometric dating also tells how much time has passed since a particular event took place. For igneous rocks (those formed from magma or lava), the method measures how much time has passed since molten material cooled and turned into rock. In other cases, the event may be the end of a period of metamorphic heating (e.g., heating to over a thousand degrees Fahrenheit underground) or, for radiocarbon dating, the length of time since a plant or animal died. The different dating techniques provide accurate timetables for determining the age of rocks or other artifacts.

The Accuracy of Radiometric Dating. Though work on radiometric dating first started around 1910, relatively slow progress was made before the late 1940s. Many dating methods have now been tested and retested for over fifty years. Radiation detectors measure the half-lives of radioactive isotopes either directly by counting the number of atoms decaying in a given amount of time from a known amount of the parent material, or by measuring the ratio of daughter-to-parent atoms in a sample that originally consisted of parent atoms only. While the number of atoms to decay in fifty years may be a small fraction of the total, extremely precise counting of the daughter atoms can be accomplished.

Table I gives the half-lives for a few of the most commonly used radiometric dating methods. The uncertainty levels of these half-lives are very small—only about plus or minus 2 percent for all except for rhenium (5%), lutetium (3%), and beryllium (3%).1 At this level of certainty, though an age may vary by a small percentage, no question remains as to whether Earth was created recently, or a long time ago. However, to measure ages of things accurately, one must apply the appropriate dating method.

Which Dating Method is Appropriate? A number of different devices measure time in everyday life. A stopwatch measures time in a one-hundred-meter race. An ordinary alarm clock measures how long a person sleeps. A calendar counts the number of days before Christmas. A calendar can’t measure time in the one-hundred-meter race, and a stopwatch can’t measure the days before Christmas.

As with other timepieces, radiometric-dating methods must be appropriate to the sample being dated. Though many people are familiar with carbon-14 dating, this technique dates organic material such as bones, wood, cloth, paper, and other dead tissue from either plants or animals and is not effective for determining the age of rocks. The best results are usually obtained if one uses a method whose half-life lies within a factor of ten of the sample’s estimated age. In the rare case that prior clues are absent, trying more than one method in order to obtain the correct age may be required. If the first attempt yields insufficient daughter atoms, a method with a shorter half-life needs to be tried, or samples with more parent atoms should be used in order for more daughter atoms to be present.

Most of the dating methods being discussed in the following paragraphs apply well when determining how long ago igneous rocks cooled and hardened from magma or lava. Atoms usually mix well in a liquid such as magma. When the molten material cools and hardens, the atoms no longer freely move about. Daughter atoms from radioactive decay occurring after the rocks cooled become trapped where they originated within the rocks. Like the sand grains accumulating in the bottom of the hourglass, the age of the rocks can be determined by measuring the number of daughter atoms and the number of remaining parent atoms, then using the half-life to calculate the time it took to make those daughter atoms.

However, a small complication remains. One cannot always assume that no daughter atoms existed to begin with, so the initial amount of the daughter product must be determined. Each dating method solves this problem in its own way. Particular types of dating work better in some rocks; others perform better in other rocks, depending on the rocks’ composition and age.

Examples of Individual Dating Methods

Over forty different radiometric dating methods have been successfully used. Of these forty, three brief examples show how some of these methods work.

Potassium-Argon. Potassium, an abundant element in Earth’s crust, has one radioactive isotope, of which 11.2 percent becomes the gas isotope, argon-40. Whenever rock melts and becomes magma or lava, the argon gas tends to escape. When the molten material hardens, argon (produced by later decays of potassium-40) is once again trapped. In this way, formation of an igneous rock resets the potassium-argon clock. The geologist simply measures the relative amounts of potassium-40 and argon-40 to date the rock.

However, there are often instances of small amounts of argon remaining in the rock when it hardens, due either to trapped atmospheric argon or from argon escaping from decays deep underground. Air argon can easily be corrected for. But the argon from underground can have a higher concentration of argon-40 escaping from the melting of older rocks. Called parentless argon-40, its parent potassium does not come from within the rock being dated, nor from the air. In these slightly unusual cases, the date given by the normal potassium-argon method is too old. However, scientists in the mid-1960s came up with a way around this problem—the argon-argon method.

Though understood for over a third of a century, groups critical of dating methods seldom discuss the argon-argon method. This method uses exactly the same parent and daughter isotopes as the potassium-argon method, in effect, presenting a different way of telling time from the same clock. More accurate than the potassium-argon method, this method is less susceptible to parentless argon. The argon-argon method can determine if a system has been disturbed. In such cases rather than giving a wrong date, the rock gives no date.2

Rubidium-Strontium. In nearly all dating methods (except potassium-argon and argon-argon), some amount of the daughter product already exists in rocks when they cool. Using these methods is like trying to tell time with an hourglass that was turned over before all of the sand had fallen to the bottom. Good techniques exist to determine precisely how much of the daughter product resided in the rock when it began to cool and harden.

In the rubidium-strontium method, rubidium-87 decays to strontium-87. Several other isotopes in strontium are stable and do not decay. The ratio of strontium-87 to one of the stable isotopes, for instance strontium-86, increases over time as more rubidium-87 turns to strontium-87. But when the rock first cools, all parts of the rock have the same ratio of strontium-87/strontium-86 because the isotopes were well mixed in the liquid magma. Some of the minerals in the rock start out with a higher ratio of rubidium to strontium than others. Rubidium has a larger atomic size than strontium, so rubidium does not fit into the crystal structure of some minerals as well as others. Figure 3 presents an important concept used in rubidium-strontium dating.

Several things can, on rare occasions, cause problems for the rubidium-strontium dating method. If a rock contains some minerals that are older than the main part of the rock, dating can be difficult. Sometimes magma inside the earth picks up unmelted minerals from the surrounding rock as it moves through a magma chamber. Usually a geologist can distinguish these “xenoliths” from the younger minerals around them. If he or she does happen to use them for dating the rock, the points represented by these minerals reveal unreliability when plotted on a graph. Other difficulties arise if a rock has undergone metamorphism, that is, if the rock became very hot, but not hot enough to completely melt (or remelt). In these cases, the dates also appear as unreliable. Some of the minerals may have completely melted, while others did not melt at all, so thus some minerals express the igneous age while others minerals express the metamorphic age. In these cases no date is determined, as the different ages within the same rock appear inconsistent.

In rare instances, the rubidium-strontium method has given straight lines that produce wrong ages. This can happen when the rock being dated was formed from magma that was not well mixed, and which contained two distinct batches of rubidium and strontium. One magma batch had rubidium and strontium compositions near the upper end of a line (such as in figure 3), and one batch had compositions near the lower end of the line. In this case, the minerals got a mixture of these two batches, and their resulting composition ended up near a line between the two batches. This is called a two-component mixing line. Only about thirty cases of this mixing line have been documented among the tens of thousands of rubidium-strontium measurements made.

If a two-component mixture is suspected, a second dating method must be used to confirm or disprove the rubidium-strontium date. The agreement of several dating methods is the most fail-safe way of dating rocks. Researchers have made comparisons of numerous dating methods on the same rocks and have shown them in close agreement, even on very old samples.3

Many dating methods work similarly to the rubidium-strontium method. Some of the more common ones include samarium-neodymium, rhenium-osmium, and lutetium-hafnium. These methods all use three-isotope diagrams similar to figure 3 to determine the age. They differ from each other primarily in the types of minerals these element pairs prefer, in the length of their half-lives, and the measuring techniques they employ.

Uranium-Lead and Related Methods. The uranium-lead method, first used in 1907, is the longest-used dating method. More complicated than other parent-daughter systems, the uranium-lead system actually puts several dating methods together. Natural uranium consists primarily of two isotopes, U-235 and U-238, and these isotopes decay with different half-lives to produce lead-207 and lead-206, respectively. In addition, lead-208 is produced by thorium-232. Three independent estimates of the age of a rock can be ascertained by measuring the lead isotopes and their parent isotopes, uranium-235, uranium-238, and thorium-232. These are often used in combination to check for concordance, or agreement, between more than one chronometer.

Extinct Radionuclides: Hourglasses That Ran Out

After the sand has run down in an hourglass, the hourglass itself offers no way to determine how long ago it finished running down. In a similar manner, finding that a once abundant radioactive parent no longer exists indicates that a longer interval of time has elapsed than the one that isotope can help to measure. In this case, the parent isotope is said to be “extinct.”

A number of extinct isotopes have been identified by the measured presence of excessive amounts of the daughter isotope. These measurements show once abundant parent isotopes shortly after the creation of the solar system. Among these parents are calcium-41 (t1/2 = 130,000 years), aluminum-26 (700,000 years), iron-60 (1.5 million years), manganese-53 (3.7 million years), iodine-129 (16 million years), and plutonium-244 (82 million years). Extinct radioisotopes provide conclusive evidence that the solar system was created longer ago than the span of these half-lives.  Earth was created so long ago that radioactive isotopes with half-lives shorter than half a billion years have decayed away, but not so long ago that radioactive isotopes with much longer half-lives are gone.4 This scenario is equivalent to finding the sand still falling in an hour-measuring hourglass, while the sand in an “egg-timer” hourglass has run out.

Addressing the Challenges

Radiometric dating has proven reliable from relatively short timescales of seconds, minutes, days, and years (calibrated with laboratory clocks), to a few thousand years (cross-calibrated with other reliable age indicators), to many millions of years (cross-comparison performed between dating methods). Some people question whether data from so far in the past can be credible. But trusting dating methods is similar to trusting other events of history. Why do people believe Abraham Lincoln lived? An extremely elaborate scheme would be required to fabricate his existence, including forgeries, fake photos, false quotations, and many other things. In short, to believe he existed seems far more reasonable than to believe his existence was feigned. The situation with radiometric dating is similar, only examination of rock data rather than of historical records reveals the story. Multiple corroborations of radiometric dating make a very strong case for its validity.

  1. Radiometric dates agree with astronomical timescales.In astronomy, decay rate constancy can be tested easily by studying stars at varying distances. Since these distances represent different light travel times (hence different astronomical eras), astronomers can observe whether or not decay rates were slower or faster at different eras. Their research reveals constancy, and constancy confirms established radiometric dates.
  2. Vast amounts of evidence for the reliability of dating have appeared in periodicals such as Science, Nature, and specific geology journals. In 1999 alone, more than a thousand papers published on radiometric dating essentially agreed on a very old age for Earth.
  3. Most rocks are, for practical purposes, closed systems. Some doubters have tried to dismiss geologic dating by saying that no rocks are completely closed systems (i.e., rocks are not isolated from their surroundings and as a result have lost or gained some isotopes used for dating). From an extremely technical perspective this point may be true—perhaps one atom out of a trillion has leaked out of nearly all rocks—but such a change makes an unmeasurably small change in the result. Many books written over the past forty years detail the precise conditions under which dating mechanisms work.
  4. The presence of only two quantities in the exponent of the equation, half-life and time, make equations for radiometric decay extremely simple. No evidence in the past century suggests that decay rates might slow down over time, leading to incorrect dates. The following argument makes such an idea meaningless in terms of “apparent” but false ages: Based on the equation, in order for ages to appear longer than actual, all half-lives would have to change in sync with each other. Since different dating methods all produce agreement, all of the half-lives must have slowed. Such an occurrence would be as if time itself slowed down.
  5. A misconception exists that radiometric dating is based on index fossils with dates assigned long before radioactivity was discovered. In truth, radiometric dating is based on the half-lives of radioactive isotopes measured over the last forty to eighty years. Fossils do not calibrate them. Radiometric dating is most often used on igneous rocks while fossils are found in sedimentary rocks.
  6. Decay rates have been directly measured over the last fifty to eighty years. In some cases, a batch of pure parent material is weighed, then set aside for a long time. The resulting daughter material can then be weighed. Often, radioactive decays can be detected more easily by the energy bursts each decay gives off. For this detection, a batch of the pure parent material is carefully weighed and then put in front of a Geiger counter (or gamma-ray detector), which counts the number of decays over a long time period.
  7. If decay rates were poorly known, dates could be inaccurate. However, most decay rates used for dating rocks are known to within about 2 percent accuracy. Uncertainties are only slightly higher on rhenium (5%), lutetium (3%), and beryllium (3%).6 Such small uncertainties provide no reason to dismiss radiometric dating. Whether a rock is 100 million years old or 102 million years old makes little difference.
  8. Since exponents are used in the dating equations, some people believe that a small error in the half-lives could lead to very large errors in the dates. In reality, a half-life off by 2 percent, leads only to a 2 percent error in the date.
  9. Some individuals have suggested that a small change in the nuclear forces might have accelerated nuclear clocks during a certain period just a few thousand years ago, causing spuriously old radiometric dates. Since methods date rocks from the time of their formation, such a change of nuclear forces would have to have occurred after Earth (and the rocks) were formed. To make a difference, the half-lives would require shortening from several billion years down to several thousand years—a factor of at least a million. Such a shortening would cause large physical effects. For example, Earth is heated substantially by radioactive decay. If that decay is sped up by a factor of a million or so, the tremendous heat pulse would easily melt the whole planet, including the rocks in question. 
  10. Some people suggest that the “full-life” (the time at which all of the parent is gone) should be measured rather than the half-life (the time when half of it is gone). Unlike sand in an hourglass, which drops at a constant rate independent of how much is remaining, the number of radioactive decays is proportional to the amount of parent remaining. Figure 2 shows how after two half-lives, ½ x ½ = ¼ is left, and so on. After 10 half-lives there is 2-10 = 0.098% remaining. Scientists sometimes instead use the term “mean life,” that is, the average life of a parent atom. The mean life is always 1/ln(2) = 1.44 times the half-life. Most people more easily understand half-life.
  11. Subjecting rocks used in dating methods to heat, cold, pressure, vacuum, acceleration, and strong chemical reactions that could be experienced on Earth or other planets yields no significant change in radioactive decay rates.
  12. Claims of unreliability have been made based on the inaccurate dating of a rock from the Mount Saint Helens eruption (1980). The dating lab reported it as several million years old. Does this mean radiometric dating can’t be trusted? Not when proper procedures are observed. Radiometric dating can be “tricked” if a single dating method is improperly used on a sample. Anyone can move the hands on a clock to indicate the time incorrectly. Likewise, people actively looking for incorrect radiometric dates can find them. However, multiple dating methods used together on igneous rocks are typically trustworthy.
  13. Some people propose that since radiogenic helium and argon continue to escape from Earth’s interior, Earth must be young. However, the radioactive parent isotopes, uranium and potassium, have very long half-lives, as shown in Table 1. These parents still exist and still produce helium and argon in abundance in Earth’s interior. Further, a time lag exists between the production of daughter products and their escape (or degassing). If Earth were geologically young, very little helium and argon would have been produced by now. What does the evidence show? Researchers have compared the amount of argon in the atmosphere to the amount expected from decay of potassium over 4.6 billion years, and they find consistency.

  14. Unsubstantiated speculation can produce the idea that only nontheists and others who dismiss the inerrancy of the Bible give credence to radiometric dating techniques. However, the roots of the scientific age can be traced to the idea that God’s creation is testable, trustable, and worthy of systematic study. The key concept of such study details God’s revelation of Himself, not only through the Bible (special revelation) but also through creation (general revelation). A great number of other Christians recognize with conviction that radiometric dating substantiates evidence that God created Earth billions, not thousands, of years ago. Many Christians work in the field of radiometric dating.

God’s Word Validates Scientific Conclusions

Accepting the reliability of radiometric dating cannot be considered equivalent to compromising the spiritual and historical inerrancy of God’s word. Many Christians view a proper reading of Genesis 1 to indicate that “day” refers literally to a long period of time.

The psalmist marveled at the scope of God’s creation. Today, the length and breadth of God’s creation, both in temporal and spatial dimensions, speak ever more clearly of the Creator’s awesome nature. The heavens do declare the Lord’s glory, and the Earth indeed shows God’s handiwork. Radiometric dating testifies to the magnificence of God’s power. Careful consideration of all the scientific facts and all the relevant Scripture passages can help people like Janet discern both the age of Earth and the validity of the biblical creation account. Together science and Scripture provide the answer Janet needs for herself—and for her son.

Roger C. Wiens wrote his Ph.D. dissertation on isotope ratios in meteorites. He worked for ten years in the geology departments at Caltech and the University of California, San Diego, characterizing oceanic rocks and isotope ratios in diamonds, and studying the feasibility of a space mission for NASA. He presently works in the Space and Atmospheric Sciences Department of the Los Alamos National Laboratory. He has published over 20 scientific research papers and has also published articles in Christian magazines. Dr. Wiens has been a member of Mennonite, Baptist, and Conservative Congregational churches.


  • Atom: The smallest unit that materials can be divided into. An atom is about ten billionths of an inch in diameter and consists of a nucleus of nucleons (protons and neutrons) surrounded by electrons.
  • Closed system: A system (rock, planet, etc.) that has no influence or exchange with the outside world. In reality there is always some exchange or influence, but if this amount is completely insignificant for the process under consideration (e.g., for dating, if the loss or gain of atoms is insignificant) for practical purposes the system can be considered closed.
  • Daughter: The element or isotope that is produced by radioactive decay.
  • Decay: The change from one element or isotope to another. Only certain isotopes decay. The rest are said to be stable.
  • Element: A substance that has a certain number of protons in the nucleus and unique properties. Elements may be further broken down into isotopes, which have nearly all of the same properties except for their mass and their radioactive decay characteristics.
  • Half-life: The amount of time it takes for half the atoms of a radioactive isotope to decay.
  • Igneous rock: Rock formed from molten lava. The other two types of rock are sedimentary (formed by the cementing together of soil or sand) and metamorphic (rocks re-formed by heat over long periods of time).
  • Isotope: Atoms of a given element that have the same atomic number. Most elements have more than one isotope. Most radioactive elements used for dating have one radioactive isotope and at least one stable isotope. For example carbon-14 (which weighs 14 atomic mass units) is radioactive, while the more common isotopes, carbon-12 and carbon-13 are not.
  • Magma: Hot molten material from which rocks are formed. When magma erupts on the surface of the earth it is called lava.
  • Metamorphism: The heating of rocks over long time periods at temperatures which are hot enough to change the crystal structure but not hot enough to completely melt the rock. Metamorphism tends to alter or reset the radiometric time clocks, though some radiometric techniques are more resistant to resetting than others.
  • Nucleons: Neutrons and protons, which make up the nucleus of an atom.
  • Parent: The element or isotope which decays. The element it produces is called the daughter.
  • Radioactive: Subject to change from one element to another. During the change, or decay, energy is released either in the form of light or energetic particles.
  • Radiocarbon: Carbon-14, which is used to date dead plant and animal matter. Radiocarbon is not used for dating rocks.
  • Radiometric dating: Determination of a time interval (e.g., the time since formation of a rock) by means of the radioactive decay of its material. Radiometric dating is one subset of the many dating methods used in geology.
  • Three-isotope plot:  In dating, this is a plot in which one axis represents the parent isotope and the other axis represents the daughter isotope. Both parent and daughter isotopes are ratioed to a daughter element isotope that is not produced by radioactive decay. This type of plot gives the age independent of the original amounts of the isotopes.
  • Two-component mixing: The mixing of two different source materials to produce a rock. On rare occasions this can result in an incorrect age for certain techniques that use three-isotope plots. Two-component mixing can be recognized if more than one dating technique is used, or if surrounding rocks are dated.
  • Xenolith: Literally, a foreign chunk of rock within a rock. Some rocks contain pieces of older rocks within them. These pieces were ripped off of the magma chamber in which the main rock formed and were incorporated into the rock without melting. Xenoliths do not occur in most rocks, and they are usually recognizable by eye where they do occur. If unrecognized, they can result in an incorrect date for a rock (the date may be of the older xenolith).


  1. Norman E. Holden, “Total Half-Lives for Selected Nuclides,” Pure Applied Chemistry 62 (1990), 941-58. See also geochronology textbooks such as Alan P. Dickin, Radiogenic Isotope Geology (New York: Cambridge Press, 1995); Gunter Faure, Principles of Isotope Geology, 2d ed. (New York: Wiley, 1986).
  2. 2. Roger C. Wiens, Radiometric Dating: A Christian Perspective, available from ASA Web site (1995); Internet; accessed 8/01/01. See also geochronology textbooks such as those by Dickin; Faure.
  3. Wiens; G. Brent Dalrymple, The Age of the Earth (Stanford, CA: Stanford University Press, 1991).
  4. Some isotopes with half-lives shorter than several hundred million years exist, but only because they are constantly being replenished by either cosmic rays (a special case, e.g., the three lowest entries in Table 1) or because they themselves are daughters of some longer-lived parent such as uranium.
  5. Hugh Ross, Creation and Time, (Colorado Springs, CO: NavPress, 1994).
  6. Holden, 941-58; see also geochronology textbooks such as Dickin; Faure.
  7. Half lives taken from Holden, 941-58; see also geochronology textbooks such as Dickin; Faure. 

    Figure Captions

Table I.

Parent-daughter pairs and half-lives for some of the most commonly used radiometric dating methods.7




Decay Product




Samarium-147 Neodymium-143 106 billion
Rubidium-87 Strontium-87 48.8 billion
Rhenium-187 Osmium-187 42 billion
Lutetium-176 Hafnium-176 38 billion
Thorium-232 Lead-208 14 billion
Uranium-238 Lead-206 4.5 billion
Potassium-40 Argon-40 1.26 billion
Uranium-235 Lead-207 0.7 billion
Beryllium-10 Boron-10 1.52 million
Chlorine-36 Argon-36 300,000
Carbon-14 Nitrogen-14 5,715

Sidebar: Atoms, Isotopes, and Radioactive Decay

By Fazale Rana, Ph.D.

Atoms, the smallest, chemically distinct units of matter, are roughly 0.1 to 0.2 nm in size (one nanometer is one-billionth of a meter). Three elementary particles make up atoms. Two of them, protons and neutrons, interact to form the atom’s nucleus. A cloud of electrons surrounds the nucleus. Essentially all of an atom’s volume comes from its electron cloud, whereas nearly all of the atom’s weight resides in the protons and neutrons in its nucleus.

Protons possess a unit positive charge, while neutrons contain no charge. This renders the nucleus with a positive charge equal to the number of protons resident in the nucleus. Electrons possess a negative charge. For an atom to maintain electrical neutrality, the number of its electrons must equal the number of its protons.

The number and arrangement of electrons surrounding the nucleus determines the atom’s chemistry. Since the electronic structure of an atom depends on the number of its protons, that proton number (atomic number) defines the atom. Any atom with 19 protons, for example, is a potassium atom; any atom with 37 protons is a rubidium atom, and any atom with 38 protons is a strontium atom. The number of protons and neutrons determines the atom’s mass (atomic mass).

While the proton number must remain fixed for a particular type of atom, the number of neutrons may vary. Variation in neutron number does not change the chemistry of the atom, but does change the atomic weight. For example, potassium-39 has 19 protons and 20 neutrons; potassium-40 has 19 protons and 21 neutrons. Both potassium-39 and potassium-40 display identical chemical properties, since they both have 19 protons. Potassium-40 weighs more than potassium-39 by 1 atomic mass unit (amu), since it has one more neutron in its nucleus than potassium-39.

Atoms with the identical number of protons but possessing a differing number of neutrons are called isotopes. Potassium-39 and potassium-40 are both isotopes of potassium.

Certain proton and neutron number combinations are unstable. When this instability occurs, the nucleus breaks down through the process of radioactive decay to a stable combination of protons and neutrons. In this decay process, the (parent) atom’s nucleus either gains or loses protons. This results in the formation of a new (daughter) atom. For example, potassium-40’s nucleus is unstable. As a result, the potassium-40 nucleus picks up an electron from the surrounding electron cloud. This electron combines with a proton to form a neutron. The resulting nucleus gains a neutron and loses a proton. Since the total number of protons plus neutrons defines the atom’s mass, the atomic mass remains unchanged, but the atomic number decreases by one. The newly formed daughter atom possesses 18 protons, 18 electrons and 22 neutrons. Any atom with 18 protons is an argon atom. This transformation, or “radioactive decay’” process, alters the chemical properties of the parent potassium atom producing a daughter atom of argon, a gas.