Edexcel A Level (IAL) Physics-4.31 Nucleon & Proton Number- Study Notes- New Syllabus

Nucleon Number (Mass Number) and Proton Number (Atomic Number)

Atoms consist of a small, dense nucleus surrounded by electrons. The composition of the nucleus is described using the proton number and the nucleon number.

 Proton Number (Atomic Number)

Definition: The proton number is the number of protons in the nucleus of an atom.

Proton number \( Z \)

• Determines the identity of the element.
• All atoms of the same element have the same proton number.
• In a neutral atom, proton number = number of electrons.

Example:

• Carbon has proton number 6 → it always has 6 protons.
• Oxygen has proton number 8 → it always has 8 protons.

Nucleon Number (Mass Number)

Definition: The nucleon number is the total number of nucleons (protons + neutrons) in the nucleus.

Nucleon number \( A \)

\( A = \text{number of protons} + \text{number of neutrons} \)

• Represents the total mass of the nucleus (approximately).
• Atoms of the same element can have different nucleon numbers.

 Neutron Number

The number of neutrons in the nucleus is found using:

\( \text{Number of neutrons} = A – Z \)

Nuclide Notation

An atom or nucleus is commonly written using the notation:

\( ^A_Z X \)

• \( X \) = chemical symbol
• \( Z \) = proton number
• \( A \) = nucleon number

Example:

\( ^{14}_{6}\text{C} \)

• Protons = 6
• Neutrons = \( 14 – 6 = 8 \)

Isotopes

• Isotopes have the same proton number but different nucleon numbers.
• They are the same element but have different numbers of neutrons.
• They have similar chemical properties but different nuclear properties.

Key Differences Summary