The Periodic Table
Middle SchoolOrder from electron structure
The periodic table arranges elements by increasing atomic number into periods (rows) and groups (columns). Elements in the same group share the same number of valence electrons and therefore similar chemistry. The table's blocks (s, p, d, f) mirror the orbital being filled.
- Group 1 — alkali metals, 1 valence electron, very reactive, form +1 ions.
- Group 17 — halogens, 7 valence electrons, reactive non-metals, form −1 ions.
- Group 18 — noble gases, full outer shells, almost inert.
Four trends recur predictably across the table:
| Property | Across a period (→) | Down a group (↓) | Cause |
|---|---|---|---|
| Atomic radius | Decreases | Increases | Nuclear charge vs. added shells |
| Ionisation energy | Increases | Decreases | Attraction to valence electron |
| Electronegativity | Increases | Decreases | Pull on bonding electrons |
| Metallic character | Decreases | Increases | Ease of losing electrons |
Two competing forces
Nearly every periodic trend is a tug-of-war between two effects. First, effective nuclear charge (Zeff) — the net positive pull felt by a valence electron after inner electrons "shield" it. Across a period, protons are added but electrons enter the same shell and shield poorly, so Zeff rises, pulling electrons in tighter: the atom shrinks and holds its electrons more strongly. Second, shell number — going down a group adds a whole new shell, placing valence electrons farther out and shielding them from the nucleus, so the atom grows and loses electrons more easily.
Mendeleev's genius in 1869 was to leave gaps for undiscovered elements and predict their properties from the pattern. When gallium and germanium were found with almost exactly the predicted properties, the periodic law was vindicated — order in chemistry is real and predictive, not merely descriptive.
Worked example — ranking by trend
Arrange Na, Mg, and K in order of increasing first ionisation energy, and explain.
- Locate them: Na and Mg are in Period 3 (Na in Group 1, Mg in Group 2); K is in Period 4, Group 1.
- Compare Na and Mg (same period): ionisation energy increases across a period, so Mg > Na.
- Compare Na and K (same group): ionisation energy decreases down a group, so Na > K.
- Combine: K < Na < Mg.
- Answer: increasing order is K < Na < Mg. K's valence electron is farthest from the nucleus and best shielded, so it is easiest to remove; Mg has the highest Zeff of the three.
Why is the atomic radius of a cation smaller than its parent atom?
Forming a cation removes one or more valence electrons — often an entire outer shell. The remaining electrons feel the same nuclear charge shared among fewer electrons, so they are pulled in tighter. Na⁺, for example, has lost its whole 3rd shell, so it is much smaller than Na.
Which has the higher electronegativity, oxygen or sulfur? Why?
Oxygen. Both are in Group 16, but oxygen is above sulfur. Its valence electrons are in the n = 2 shell, closer to the nucleus and less shielded, so oxygen pulls bonding electrons more strongly. Electronegativity decreases down a group.
Explain why noble gases have extremely high ionisation energies.
Noble gases have full outer shells (a stable octet, or duet for He). This configuration is energetically very stable, and Zeff is at a maximum across the period, so removing an electron requires a large energy input. They also have little tendency to gain electrons, making them chemically inert.
Where is the metal–non-metal dividing line, and what are metalloids?
A "staircase" line runs diagonally from around boron down to astatine. Metals lie to the lower-left, non-metals to the upper-right. Elements straddling the line — B, Si, Ge, As, Sb, Te — are metalloids, showing intermediate properties (e.g. silicon's semiconducting behaviour, central to electronics).
How the ideas connect
Every key idea in this chapter, branching from the core concept — use it to see the whole picture at a glance.
The key facts, visualised
Worked problems, step by step
Follow each solution line by line, then try to reproduce it on paper before moving on.
Example 1Rank Na, Mg and Cl by atomic radius, largest first.
- All are in period 3, so radius decreases left to right.
- Order left to right is Na, Mg, then Cl.
- So radius is largest at Na and smallest at Cl.
Example 2Which loses an electron more easily, Na or Cl?
- Ionisation energy rises across a period.
- Na is on the left (low ionisation energy); Cl is on the right (high).
- Lower ionisation energy means the electron leaves more easily.
Now you try
Work each one out first, then tap to reveal the worked answer.