Atomic Number, Mass Number and Isotopes

Every element has its own atoms, and what makes one element different from another is simply the number of protons in its atoms. Two numbers — the atomic number and the mass number — let us work out exactly how many protons, neutrons and electrons any atom contains. They also let us understand isotopes: atoms of the same element that have different numbers of neutrons. This lesson, part of Topic C1 of OCR Gateway Science A, shows how to use atomic and mass numbers, how to read isotope notation, and why isotopes of an element behave the same chemically.

By the end of this lesson you should be able to define atomic number and mass number, use isotope notation to find the numbers of protons, neutrons and electrons, define isotopes, explain why isotopes share chemical properties, and describe briefly how ions differ from atoms.

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Atomic Number and Mass Number

Two whole numbers describe every atom:

• the atomic number (also called the proton number), symbol $Z$Z, is the number of protons in the nucleus;
• the mass number, symbol $A$A, is the total number of protons and neutrons in the nucleus.

The atomic number identifies the element. Every atom of a particular element has the same number of protons — for example, every carbon atom has 6 protons, and any atom with 6 protons is a carbon atom. The atomic number is also what puts the elements in order in the periodic table (which is arranged by increasing atomic number).

It is the proton number, not the number of neutrons or electrons, that fixes which element an atom is. This is an important point. An atom can gain or lose electrons to become an ion, or have a different number of neutrons (an isotope), and yet remain the same element throughout — because the proton number has not changed. Change the number of protons, however, and you have a completely different element. This is why the atomic number is sometimes called the "fingerprint" of an element: it never changes for a given element, and no two elements share the same value. It is also why the periodic table can be laid out so neatly in order of atomic number, with each successive element having exactly one more proton than the last.

From these two numbers you can find all three subatomic particle counts for a neutral atom:

Particle	How to find it
Protons	= atomic number, $Z$Z
Electrons	= atomic number, $Z$Z (because a neutral atom has equal protons and electrons)
Neutrons	= mass number − atomic number, $A - Z$A−Z

Exam Tip: The number of neutrons is not given directly — you must subtract: neutrons $= A - Z$=A−Z (mass number minus atomic number). Forgetting to subtract, and giving the mass number as the neutron count, is one of the most common errors in this topic.

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Isotope Notation

An atom is often written using isotope notation, which shows both numbers next to the element's symbol:

$^{A}_{Z}\text{X}$ZA​X

Here X is the element symbol, the top number is the mass number ($A$A) and the bottom number is the atomic number ($Z$Z). For example, sodium is written:

$^{23}_{11}\text{Na}$1123​Na

This tells you that a sodium atom has a mass number of 23 and an atomic number of 11, so it contains: 11 protons, 11 electrons, and $23 - 11 = 12$23−11=12 neutrons.

Worked Example 1 — Reading isotope notation

How many protons, neutrons and electrons are in an atom of $^{27}_{13}\text{Al}$1327​Al?

Step 1 — protons $= Z = 13$=Z=13.

Step 2 — electrons $= Z = 13$=Z=13 (neutral atom).

Step 3 — neutrons $= A - Z = 27 - 13 = 14$=A−Z=27−13=14.

Answer: 13 protons, 13 electrons, 14 neutrons.

Worked Example 2 — A larger atom

Find the numbers of protons, neutrons and electrons in $^{40}_{20}\text{Ca}$2040​Ca.

Step 1 — protons $= 20$=20; electrons $= 20$=20.

Step 2 — neutrons $= 40 - 20 = 20$=40−20=20.

Answer: 20 protons, 20 electrons, 20 neutrons.

Here is a short table of common atoms to check your method against:

Atom	$Z$Z	$A$A	Protons	Electrons	Neutrons ($A-Z$A−Z)
$^{1}_{1}\text{H}$11​H	1	1	1	1	0
$^{12}_{6}\text{C}$612​C	6	12	6	6	6
$^{16}_{8}\text{O}$816​O	8	16	8	8	8
$^{23}_{11}\text{Na}$1123​Na	11	23	11	11	12
$^{35}_{17}\text{Cl}$1735​Cl	17	35	17	17	18

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What Are Isotopes?

Isotopes are atoms of the same element (so the same number of protons, the same atomic number) that have different numbers of neutrons (and therefore different mass numbers).

Because the number of protons is the same, they are the same element; but because the number of neutrons differs, they have different masses. The classic examples are the three isotopes of carbon:

Isotope	Protons	Neutrons	Mass number
Carbon-12, $^{12}_{6}\text{C}$612​C	6	6	12
Carbon-13, $^{13}_{6}\text{C}$613​C	6	7	13
Carbon-14, $^{14}_{6}\text{C}$614​C	6	8	14

All three are carbon (6 protons each), but they have 6, 7 and 8 neutrons respectively. Other important examples are the two isotopes of chlorine ($^{35}_{17}\text{Cl}$1735​Cl and $^{37}_{17}\text{Cl}$1737​Cl) and the isotopes of hydrogen ($^{1}_{1}\text{H}$11​H, $^{2}_{1}\text{H}$12​H "deuterium", and $^{3}_{1}\text{H}$13​H "tritium").

Exam Tip: The definition examiners want is precise: isotopes are atoms of the same element with the same number of protons but different numbers of neutrons (and so different mass numbers). Saying they have "different atomic numbers" is wrong — that would make them different elements.

Worked Example 3 — Spotting isotopes

Which two of these are isotopes of each other? Atom P: 17 protons, 18 neutrons. Atom Q: 18 protons, 18 neutrons. Atom R: 17 protons, 20 neutrons.

Step 1 — isotopes must have the same number of protons. P and R both have 17 protons; Q has 18.

Step 2 — check they differ in neutrons: P has 18, R has 20 — different, as isotopes should be.

Answer: P and R are isotopes of each other (both are chlorine, with 17 protons); Q is a different element (argon).

Worked Example 4 — Building an atom from its numbers

An atom has 19 protons, 20 neutrons and 19 electrons. Write its isotope notation and name the element.

Step 1 — the atomic number is the number of protons, so $Z = 19$Z=19.

Step 2 — the mass number is protons + neutrons, so $A = 19 + 20 = 39$A=19+20=39.

Step 3 — element 19 is potassium (symbol K), and the atom is neutral (19 protons = 19 electrons).

Answer: the notation is $^{39}_{19}\text{K}$1939​K — the atom is potassium. Working in this direction (from particle counts back to the notation) is just the reverse of reading the notation, and it is a good way to check you understand what each number means.

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Why Isotopes Have the Same Chemical Properties