Counting particles: atomic number and mass number
Every element has its own atomic number Z, the number of protons in its nucleus. It is written at the lower left of the symbol. In a neutral atom the number of electrons equals the number of protons. The mass number A is the number of protons plus the number of neutrons, so it counts all the nucleons. It is written at the upper left of the symbol. It is always a whole number. To find the neutrons, subtract: neutrons = A − Z. Example: sodium is written ²³₁₁Na. Z = 11, so it has 11 protons and 11 electrons. A = 23, so the neutrons are 23 − 11 = 12. Do not mix up the mass number with the “atomic mass” printed on some tables of elements: the mass number is a simple count.
| Number | Written | It counts |
|---|---|---|
| Z | lower left | protons |
| A | upper left | protons + neutrons |
| A − Z | neutrons |
| Atom | Z | A | p | n | e |
|---|---|---|---|---|---|
| ¹²₆C | 6 | 12 | 6 | 6 | 6 |
| ¹⁶₈O | 8 | 16 | 8 | 8 | 8 |
Build an atom. Cut paper circles in three colours or use coloured beads: one colour for protons, one for neutrons and one for electrons. Build ¹⁹₉F: place the protons and neutrons in the middle, then put the electrons around them. Count each colour and check against Z and A. Then ask a friend to read out Z and A for another atom while you build it.