Atomic structure and the Periodic Table
Protons, neutrons and electrons, proton and nucleon number, electronic configuration, and its link to group and period.
Review these first
Learning objectives
What you need to be able to do
Teacher-mapped phrasing — check against the official Cambridge syllabus for exact wording.
- 2.2.1State the relative charges and masses of protons, neutrons and electrons.
- 2.2.2Define proton number and nucleon number and use them to determine the number of each particle.
- 2.2.3Write the electronic configuration of the first 20 elements and relate it to group and period.
7 minute read
Inside the atom
The three particles
| Particle | Relative mass | Relative charge | Where |
|---|---|---|---|
| Proton | 1 | +1 | nucleus |
| Neutron | 1 | 0 | nucleus |
| Electron | 1/1836 (≈0) | −1 | shells around the nucleus |
Proton number (Z) = number of protons = the element's identity, and its position in the Periodic Table. Nucleon number (A) = protons + neutrons. So neutrons = A − Z, and in a neutral atom electrons = protons.
Electronic configuration
Electrons occupy shells, filling from the inside out. For the first 20 elements the capacities are 2, 8, 8, 2.
Examples:
- Sodium (Z = 11): 2,8,1
- Chlorine (Z = 17): 2,8,7
- Calcium (Z = 20): 2,8,8,2
The pattern that makes the Periodic Table make sense
- The number of outer-shell electrons = the group number (for groups I–VII).
- The number of occupied shells = the period number.
Sodium has 1 outer electron and 3 shells → Group I, Period 3. That single rule lets you place any of the first 20 elements without memorising the table.
Why elements react the way they do
Atoms react to achieve a full outer shell — the stable arrangement of the noble gases. Metals (few outer electrons) lose them; non-metals (nearly full shells) gain or share them. Every bonding topic that follows is a consequence of this one idea.
Isotopes
Isotopes are atoms of the same element with the same proton number but different numbers of neutrons. They have identical chemical properties because chemistry depends on the electrons, which are unchanged.
Think of it like this
The shells are like rows of seats in a theatre that fill from the front. An atom is "comfortable" only when the row it is working on is completely full.
Worked examples
Method, step by step
An atom has nucleon number 39 and proton number 19. State the number of protons, neutrons and electrons, and give its electronic configuration.
- 1Protons = proton number = 19
- 2Neutrons = 39 − 19 = 20
- 3Electrons = protons = 19 (neutral atom)
- 4Fill shells 2, 8, 8, then 1 left over
19 protons, 20 neutrons, 19 electrons; configuration 2,8,8,1 (potassium, Group I, Period 4)
Chlorine has two isotopes: 75% ³⁵Cl and 25% ³⁷Cl. Calculate its relative atomic mass.
- 1Ar = (75 × 35 + 25 × 37) / 100
- 2Ar = (2625 + 925) / 100
- 3Ar = 3550 / 100
Ar = 35.5
Common misconceptions
- Thinking isotopes react differently. They have the same electronic configuration, so their chemistry is the same.
- Writing 2,8,9 for potassium. The third shell holds 8 before the fourth starts, so potassium is 2,8,8,1.
- Confusing nucleon number with the number of neutrons — neutrons are nucleon number minus proton number.
In the exam
- Write electronic configurations with commas: 2,8,7 — not 287.
- For "explain why X is in Group II", say it has 2 electrons in its outer shell. That is the whole answer.
- Relative atomic mass questions with isotopes need a weighted average, not a simple mean.