Every atom has the same three ingredients: protons, neutrons and electrons. Count them and you know the element; arrange the electrons in shells and you know where it sits in the Periodic Table and how it will react.
An atom has a small central nucleus of protons and neutrons, surrounded by electrons in shells. The proton number (atomic number) is the number of protons; in a neutral atom it also equals the number of electrons. The mass number (nucleon number) is protons plus neutrons. Electrons fill shells in order: 2 in the first, then 8, then 8 for the first 20 elements. The number of outer-shell electrons gives the group (I to VII), and the number of occupied shells gives the period.
Build an atom in the simulator. Add protons and watch the element change; add neutrons and only the mass changes; add or remove electrons and the atom becomes a charged ion.
| Particle | Where it is | Relative charge | Relative mass |
|---|---|---|---|
| proton | nucleus | +1 | 1 |
| neutron | nucleus | 0 | 1 |
| electron | shells around the nucleus | −1 | 1/1836 |
The symbol 2311Na carries two numbers. The bottom one is the proton number, the top one the mass number.
The mass number is not the number of neutrons. It counts protons and neutrons together, so always subtract: neutrons = mass number − proton number. Also check which number is which: the mass number is always the larger one.
Four quick checks. Each correct answer earns XP and records this skill on your profile.
What is the relative mass of an electron?
Where are the neutrons in an atom?
How many neutrons are in an atom of 2311Na?
The proton number of an atom is the number of:
Fill the shells in order, 2 then 8 then 8, and the Periodic Table's layout appears on its own.
| Element | Proton number | Configuration | Group | Period |
|---|---|---|---|---|
| helium, He | 2 | 2 | VIII | 1 |
| carbon, C | 6 | 2,4 | IV | 2 |
| oxygen, O | 8 | 2,6 | VI | 2 |
| neon, Ne | 10 | 2,8 | VIII | 2 |
| sodium, Na | 11 | 2,8,1 | I | 3 |
| chlorine, Cl | 17 | 2,8,7 | VII | 3 |
| argon, Ar | 18 | 2,8,8 | VIII | 3 |
| potassium, K | 19 | 2,8,8,1 | I | 4 |
| calcium, Ca | 20 | 2,8,8,2 | II | 4 |
Unlocks once the four checks above are done. Worth more XP, written to IGCSE Paper 2 and Paper 4 standard.
How many protons, neutrons and electrons are in an atom of 2713Al?
An atom has the electronic configuration 2,8,6. Which element is it, and where is it in the Periodic Table?
What is the electronic configuration of calcium (proton number 20)?
Why are the noble gases unreactive?
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