1.2 Atomic number and mass number; isotopes

Atomic Number

Mass Number and Isotopes

The nucleus is where the major mass of an atom is concentrated (the electronic mass is almost negligible).

• Atoms are electrically neutral, so the number of protons must = the number of electrons
• A list of the elements in atomic number order is found in the periodic table.
• The atomic number defines the element, no two elements can have the same atomic number.
• Atomic number is often written as a subscript in front of the symbol of an element.

E.g. ₆C, ₉₂U or ₅₃I etc

Protons and Neutrons are found in the nucleus, a term is needed to define the number of protons and neutrons (nucleons) in an element.

Important: Atomic number = no. of protons and no. of electrons

Mass number = no. of particles in nucleus

No. of neutrons = Mass number – Atomic number

Some elements contain different numbers of neutrons, but have the same number of protons (these are what specify an element remember). These are called Isotopes.

1p 0n	1p 1n	1p 2n
Hydrogen	Deuterium (heavy hydrogen)	Tritium (radioactive)

Isotopes of hydrogen

Try the atomic number and isotopes quick quiz