Isotopes

Atomic number (Z)

The atomic number is the number of protons in the nucleus of an atom. It is also equal to the number of electrons in a neutral atom. The atomic number is written at the bottom left side of an element symbol.

Example:

• Carbon: ₆C
• Oxygen: ₈O
• Chlorine: ₁₇Cl
• Calcium: ₂₀Ca

Atomic number = Number of protons = Number of electrons

Mass number / Atomic mass (A)

The mass number is the total number of protons and neutrons in an atom. It is written at the top left side of the element symbol.

Examples:

• Carbon: ¹²C
• Chlorine: ³⁵Cl

Mass number = Number of protons + Number of neutrons Neutron number = Mass number − Atomic number

Example Problem:

An atom R has a mass number of 40 and an atomic number of 20. (a) Neutron number = 40 − 20 = 20 (b) Number of electrons = Atomic number = 20

Isotopes

Isotopes are atoms of the same element that have the same number of protons but different numbers of neutrons.

1. They have the same atomic number but different mass numbers.
2. Isotopes have the same chemical properties.
3. They have slightly different physical properties due to the different number of neutrons.

Examples of elements with isotopes:

• Carbon: ¹²C, ¹³C, ¹⁴C
• Uranium: ²³⁵U, ²³⁸U

Note: In isotope notation:

• The number above the element is the mass number.
• The number below the element is the atomic number.