Edexcel International A Level (IAL) Chemistry (YCH11) - Unit 1 - 2.3–2.4 Atomic number, mass number and ions-Study Notes - New Syllabus

2.3 Atomic (Proton) Number and Mass Number

Each element is defined by the number of protons present in the nucleus of its atoms. Two important numbers are used to describe the composition of an atom: the atomic (proton) number and the mass number.

Atomic (Proton) Number

The atomic number (also called the proton number) is the number of protons in the nucleus of an atom.

The atomic number determines the identity of an element. All atoms of the same element have the same atomic number.

In a neutral atom, the number of electrons is equal to the number of protons.

Examples

• Hydrogen has atomic number \(1\), so it contains 1 proton.
• Carbon has atomic number \(6\), so it contains 6 protons.
• Sodium has atomic number \(11\), so it contains 11 protons.

Mass Number

The mass number is the total number of protons and neutrons in the nucleus of an atom.

Since electrons have a very small mass, they are not included when calculating the mass number.

The relationship can be written as:

Mass number = number of protons + number of neutrons

\( A = Z + N \)

• \(A\) = mass number
• \(Z\) = atomic number (protons)
• \(N\) = number of neutrons

Nuclear Notation

Atoms are often written using nuclear notation:

\( ^A_Z X \)

Where:

• \(A\) = mass number
• \(Z\) = atomic number
• \(X\) = chemical symbol

Example:

\( ^{23}_{11}\mathrm{Na} \)

This shows that sodium has:

• 11 protons
• 23 − 11 = 12 neutrons