Yttrium, atomic number 39, isn’t a lanthanide but is included in the rare earths because it often occurs with them in nature – it has similar chemical properties.

So, the mass number of this potassium isotope is 39. Atomic Number and Mass Number of First 30 Elements. Students are generally advised to memorise the first 30 elements from the periodic table.

Carbon atoms have six protons, so its atomic number is six. Elements also each have a unique atomic mass. The atom's mass primarily comes from the protons and neutrons in the atom's nucleus.