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Mass number vs atomic number: what's the difference?

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University Course Reader · STEM

Carbon-14 has the same atomic number as carbon-12 but a mass number of 14, so it is heavier and radioactive.

The reader highlighted one word in a textbook passage. Clicked broke down the chemistry term “atomic number” into plain English:

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Overview

Atomic number is how many protons an atom has, and that determines which element the atom is. An atom with six protons is carbon, every time. Mass number is protons plus neutrons, and it measures that one atom's mass. Neutrons can vary without changing the element. Carbon-12 has six protons and six neutrons, carbon-14 has six protons and eight, and both are carbon. Atomic number says what it is; mass number says how much mass it has.
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Overview

Atomic number is the proton count, and it tells you which element you have. Six of them and it is carbon, no exceptions. Mass number counts protons and neutrons together, and it says how much matter that one atom holds. Add a neutron and it is still the same element, only heavier. Carbon-12 and carbon-14 are both carbon; the second carries two extra neutrons and that is the whole difference. Same element at different masses is what an isotope is. 😎

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Detail

Atomic number is the number of protons in an atom, and that determines which element the atom is. An atom with six protons is carbon; give it a seventh and it is nitrogen. Mass number is protons plus neutrons, and it measures that particular atom's mass. The neutron count plays no part in which element an atom is, and it can vary. Carbon-12 has six protons and six neutrons, carbon-14 has six protons and eight, and both are carbon; such atoms are called isotopes. Subtract the atomic number from the mass number and you get the neutrons, so for carbon-14 that is eight. The two numbers matter for different things. Protons determine how many electrons an atom carries, and it is the electrons that take part in chemical reactions, so atomic number governs the chemistry. Carbon-12 and carbon-14 behave the same way in a chemical reaction. Neutrons change what the nucleus itself does, so mass number is the number that separates carbon-14, which decays, from carbon-12, which is stable. The number with decimals on the periodic table, 12.011 for carbon, is a third quantity. It is not any atom's mass number but the average of the mass numbers of the carbon atoms found in nature, weighted by how common each isotope is. A mass number is a whole number, because it counts particles in one atom.
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Detail

Atomic number is the proton count and mass number is protons plus neutrons. Six protons make carbon and seven make nitrogen, and one extra proton turns the first into the second. Neutrons have no vote on the element, but they do two things: they add mass, and they set whether the nucleus stays put or falls apart. An atom can gain or lose a neutron and stay what it was. Carbon comes as carbon-12 and carbon-14. Both have six protons, one has six neutrons and the other eight, and both count as carbon. Isotopes is the word for that. Mass number minus atomic number gives the neutrons, which is where carbon-14's eight comes from. Why keep two numbers? Because they do different jobs. Chemistry runs on electrons, and an atom with no charge carries one electron per proton, so the proton count sets how the atom reacts. Carbon-12 and carbon-14 burn the same and bond the same. Neutrons work on the nucleus instead, and carbon-14's nucleus falls apart over thousands of years while carbon-12's never does, which is the basis of carbon dating. And the periodic table's 12.011? It is no atom's mass number. That figure averages all the carbon in the world, most of it carbon-12 with a smidge of the heavier kinds. Whole numbers count particles; the decimal is an average. 😎

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Analogy

A Golf is a Golf whichever variant it is, small engine or big, spare wheel or none, and the variants weigh different amounts. The model name stands for the atomic number, because it settles what the car is and every Golf shares it, just as every carbon atom shares six protons. The weight of one particular variant stands for the mass number. The variants themselves stand for the isotopes, one car at several weights, just as carbon-12 and carbon-14 are one element at two weights. Average the weight of every Golf on the road and you get a figure that no single Golf weighs, and that figure is the number with decimals on the periodic table.
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Analogy

The same pair of jeans comes in size 30 and size 40, and the two weigh different amounts on the shop's scale. Every pair with that product number is the same jeans, so the number on the label is the atomic number here, the way six protons stands in for carbon. One pair's weight is a mass number; the run of sizes on the rail is the run of isotopes, one pair of jeans at several weights. Weigh every pair on the rail and average them, and no single pair weighs that; the periodic table's decimal is that kind of average. 😎
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Formal definition — The same term, explained the usual way

Atomic number (Z) is the number of protons in the nucleus of an atom; it identifies the element and, in a neutral atom, equals the number of electrons. Mass number (A) is the total number of protons and neutrons in the nucleus, so the neutron count is A minus Z. Atoms with the same atomic number but different mass numbers are isotopes of the same element; they share chemical behaviour, which is governed by electron configuration, but differ in nuclear properties such as stability. In nuclide notation the mass number is written as a superscript and the atomic number as a subscript to the left of the element symbol. Mass number is an integer for a given nuclide and is distinct from the relative atomic mass shown on the periodic table, which is a weighted average over the naturally occurring isotopes.

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