Atomic number, mass number and isotopes

IB Chemistry SL· Structure 1.2 - The nuclear atom· 15 min read

1. Atomic Number and Mass Number★☆☆☆☆⏱ 5 min

📘 Definition

Atomic Number

$Z$

The number of protons in the nucleus of an atom of an element, which uniquely identifies the element. All atoms of the same element have the same atomic number, and in neutral atoms, this equals the number of electrons.

Example:

Carbon always has $Z = 6$

📘 Definition

Mass Number

$A$

The total number of protons and neutrons in the nucleus of a specific atom. Since protons and neutrons each have ~1 atomic mass unit, mass number is always a whole number.

Example:

A carbon-12 atom has $A = 12$ (6 protons + 6 neutrons)

Standard nuclide notation combines this information as $_{Z}^{A} X$ , where $X$ is the chemical symbol of the element. To find the number of neutrons in any atom, use the formula $neutrons = A - Z$ .

📐 Worked Example

How many protons, neutrons and electrons are there in a neutral atom of $_{12}^{24} Mg$ ?

1
Step 1: Identify $Z$ (lower number) and $A$ (upper number) from the notation: $Z = 12$ , $A = 24$
2
Step 2: Atomic number = number of protons, so number of protons = 12
3
Step 3: The atom is neutral, so number of electrons = number of protons = 12
4
Step 4: Calculate number of neutrons using $A - Z$
5
$24 - 12 = 12$
6
Final answer: 12 protons, 12 neutrons, 12 electrons

2. Isotopes★★☆☆☆⏱ 5 min

📘 Definition

Isotopes

Atoms of the same element that share the same atomic number (same number of protons) but have different mass numbers (different number of neutrons).

Example:

Hydrogen has three naturally occurring isotopes: $_{1}^{1} H$ (protium), $_{1}^{2} H$ (deuterium), $_{1}^{3} H$ (tritium)

Because isotopes have the same number of protons, they also have identical electron configurations. This means they have the same chemical properties, only differing in physical properties such as mass, density, and boiling point.

📐 Worked Example

Identify which of the following species are isotopes of the same element: $_{6}^{14} X$ , $_{7}^{12} Y$ , $_{7}^{14} Z$ , $_{6}^{12} W$

1
Step 1: Recall that isotopes of the same element must have the same atomic number ( $Z$ )
2
Step 2: Group species by their $Z$ value: $Z = 6$ : X and W; $Z = 7$ : Y and Z
3
Step 3: X and W have the same $Z$ (same element) but different mass numbers, so they are isotopes. Y and Z also share the same $Z$ so they are isotopes of a second element.

3. Calculating Relative Atomic Mass★★☆☆☆⏱ 6 min

📘 Definition

Relative Atomic Mass

$A_{r}$

The weighted average of the mass numbers of all naturally occurring isotopes of an element, weighted by their relative abundance. This is the value shown for each element on the IB periodic table.

For percentage abundance data, the formula for relative atomic mass is:

A_r = \frac{(A_1 \times \%_1) + (A_2 \times \%_2) + ...}{100}

Where $A_{1}$ and $A_{2}$ are the mass numbers of each isotope. If abundances are given as decimal fractions, you do not need to divide by 100.

📐 Worked Example

Chlorine has two naturally occurring isotopes: 75.77% $^{35} Cl$ and 24.23% $^{37} Cl$ . Calculate the relative atomic mass of chlorine.

1
Step 1: Substitute values into the formula:
2
$A_r = \frac{(35 \times 75.77) + (37 \times 24.23)}{100}$
3
Step 2: Calculate the numerator:
4
$(35 \times 75.77) = 2651.95; \quad (37 \times 24.23) = 896.51; \quad \text{Total} = 3548.46$
5
Step 3: Divide by 100 to get the final result:
6
$A_r = 35.48 \approx 35.5$

✓ Quick check

Check your understanding:

Neon has two isotopes: 90% $^{20} Ne$ and 10% $^{22} Ne$ . What is the relative atomic mass of neon?
- 20.0
- 20.2
- 21.0
- 22.0
Reveal answer
1 —
Correct calculation: $(20 \times 0.9) + (22 \times 0.1) = 18 + 2.2 = 20.2$

4. Common Pitfalls

Wrong move:

Using the relative atomic mass from the periodic table to calculate neutrons

Why:

Relative atomic mass is a weighted average of all isotopes, not the mass number of an individual atom, so it is usually a decimal and cannot be used directly

Correct move:

Always use the mass number given in the nuclide notation to calculate neutrons as $A - Z$

Wrong move:

Swapping the positions of $Z$ and $A$ in nuclide notation

Why:

Mixing up the lower and upper positions leads to wrong counts of protons and neutrons

Correct move:

Remember: A for mass number is Above the element symbol, Z for atomic number is at the Zero (bottom)

Wrong move:

Claiming isotopes have different chemical properties

Why:

Chemical reactivity depends on electron configuration, which is identical for all isotopes of an element

Correct move:

Only physical properties (mass, density, boiling point) differ between isotopes

Wrong move:

Forgetting to divide by 100 when using percentage abundances

Why:

Percentages add up to 100, so the sum of mass × percentage will be 100 times larger than the correct result

Correct move:

Always divide by 100 for percentage abundances, or convert percentages to decimals before adding

5. Quick Reference Cheatsheet

Term	Symbol	Key Definition	Formula
Atomic Number	$Z$	Number of protons in nucleus
Mass Number	$A$	Total protons + neutrons	$Neutrons = A - Z$
Isotopes		Same $Z$ , different $A$
Relative Atomic Mass	$A_{r}$	Weighted average of isotopes	$A_{r} = \frac{\sum ( A _{i} \times % _{i} )}{100}$
Nuclide Notation	$_{Z}^{A} X$	Standard notation for atoms

6. Frequently Asked

Do isotopes have the same chemical properties?

Yes. Chemical properties depend on electron configuration, which is identical for isotopes (same number of protons = same number of electrons). Only physical properties such as mass and density differ.

Why is relative atomic mass not a whole number?

It is a weighted average of the mass numbers of all naturally occurring isotopes of the element, weighted by their relative abundance, so it is rarely a whole number.

When this came up on past exams

AI-estimated based on syllabus patterns — cross-check with official past papers for accuracy. Use only as revision-focus signals.

2025 · 1
Calculate relative atomic mass of chlorine
2024 · 1
Identify isotopes from particle count data

Going deeper

Official SyllabusIB Chemistry SL Subject GuideReference for Structure 1.2 learning outcomes

What's Next

This sub-topic is the foundation for all further study of atomic structure and the periodic table in IB Chemistry SL. Understanding the properties of isotopes and how to calculate relative atomic mass is essential for upcoming topics including mass spectrometry, stoichiometric calculations, and nuclear chemistry. Mastery of these basic definitions and simple calculations will help you secure easy marks in both Paper 1 and Paper 2 of your final exam, and builds the core knowledge you need for more advanced concepts.