An atomic symbol gives you two numbers, and a third follows from them. The bottom number is the proton number. The top number is the nucleon number, and the difference between them is the number of neutrons.
This lesson is the first in SPM Science elements and materials. The next, comparing elements, compounds and mixtures, relies on the same reading habit.
What does each number in the symbol mean?
In the symbol ²⁷₁₃Al, the top number 27 is the nucleon number, the count of protons plus neutrons. The bottom number 13 is the proton number, which also identifies the element.
The chemical symbol Al tells you which element it is. The proton number and the symbol always agree, so the number 13 is aluminium in every case.
Worked example: one symbol, every count
An invented question gives the symbol ³¹₁₅P for a neutral phosphorus atom and asks for protons, neutrons, electrons and electron arrangement.
Protons. The proton number is 15.
Neutrons. 31 − 15 = 16.
Electrons. The atom is neutral, so electrons = protons = 15.
Electron arrangement. Fill from the inside: 2 in the first shell, 8 in the second, leaving 15 − 10 = 5 for the third. The arrangement is 2.8.5. The 5 outer electrons place phosphorus in Group 15.
Check the total: 2 + 8 + 5 = 15, which matches the electron count.
The mistake that costs marks
The slip is to subtract the wrong way round, or to write the nucleon number as the neutron count. A student writes “neutrons = 31” for the phosphorus atom.
| Step | Wrong | Right |
|---|---|---|
| Neutron count | 31 (nucleon number) | 31 − 15 = 16 |
| Check | Protons + neutrons = 46 | 15 + 16 = 31, matching the top number |
The check is simple. Protons plus neutrons must equal the nucleon number. If they do not, the subtraction has gone wrong.
How does this link to the periodic table?
The electron arrangement tells you where an element sits in the periodic table. The number of shells gives the period. The number of outer electrons gives the group: the same number for Groups 1 and 2, and 10 more from Group 13 onwards.
For ³¹₁₅P: the arrangement 2.8.5 has 3 shells, so it is in Period 3, and 5 outer electrons make it Group 15 (10 + 5).
Check yourself
An invented atom has the symbol ⁴⁰₂₀Ca. State the numbers of protons, neutrons and electrons, and write the electron arrangement.
Answer
Protons = 20. Neutrons = 40 − 20 = 20. Electrons in a neutral atom = 20.
Electron arrangement: 2 in the first shell, 8 in the second, 8 in the third, and the remaining 2 in the fourth, giving 2.8.8.2. Check: 2 + 8 + 8 + 2 = 20.
What to study next
Go on to comparing elements, compounds and mixtures. For more symbol drills, use the atomic structure and periodic position practice tool, and log slips in the mistake log.
If you want a teacher to drill this with you, see online one-to-one Science tuition.