Atomic number and mass number

What makes a gold atom different from an iron atom? The number of protons.

Atomic number (Z) = the number of protons in the nucleus. It decides which element the atom is and how it behaves in reactions. In a neutral atom, the number of electrons is the same as the number of protons.

• Hydrogen: Z = 1 (1 proton, 1 electron).
• Helium: Z = 2.

Mass number (A) = protons + neutrons. Protons and neutrons together are called nucleons (particles of the nucleus). Electrons are too light to count.

• Hydrogen: 1 + 0 = 1.
• Helium: 2 + 2 = 4.
• Lithium: 3 + 4 = 7.

How to write it. Put the mass number at the top left of the symbol and the atomic number at the bottom left: ¹²₆C is carbon with 6 protons and 12 nucleons, so 6 neutrons.

Finding the missing number:

• Protons = Z.
• Neutrons = A – Z.
• Electrons (neutral atom) = Z.

Example: iron has Z = 26 and A = 56, so it has 26 protons, 56 – 26 = 30 neutrons, and 26 electrons.