# Trends in the Periodic Table

> How atomic size, reactivity, metallic character and electronegativity change across periods and down groups.

From HSC Chemistry on WhizzNotes, free study notes for NSW students: https://whizznotes.com/chemistry/periodic-trends/. Updated 3 November 2026.
Year 11 and Year 12 boxes go deeper than the Year 10 text around them.

### Story time: The tug of war
Imagine the nucleus as a team pulling on a rope, and the outer electrons on the other end.

In a small atom with a strong team (lots of protons) and the electrons close by, the team holds on tight. The electrons are hard to take away.

In a big atom, the outer electrons are far away, and lots of inner electrons stand in between, blocking the pull. The grip is weak. A passing atom can easily steal an electron.

Almost every trend in the periodic table comes down to this tug of war: **how many protons are pulling**, and **how far away the outer electrons are**.

**In short:** Properties change in patterns across the periodic table. Atoms get bigger down a group and smaller across a period, group 1 metals get more reactive down the group while halogens get less reactive, and electronegativity rises across a period and falls down a group.

## The two big ideas

1. **Across a period** (left to right): the number of protons goes up, but the electrons are added to the **same shell**. The pull on the outer electrons gets **stronger**.
2. **Down a group**: a **new shell** is added each time. The outer electrons are further away, and the inner shells **shield** them from the nucleus. The pull gets **weaker**.

## Atomic radius (size)

- **Down a group**: atoms get **bigger**, because there are more shells.
- **Across a period**: atoms get **smaller**, because more protons pull the same shell in tighter.

Sodium is bigger than chlorine, even though chlorine has more electrons.

*Diagram: Trend arrows on a grid: atomic radius increases down a group and decreases across a period, so the largest atoms are bottom left*
*Atomic radius grows down and to the left.*

## Ionic radius

When an atom becomes an ion, its size changes.

- **Positive ions are smaller** than their atoms. $\mathrm{Na}$ loses its only outer electron, so its whole outer shell disappears. $\mathrm{Na^+}$ is much smaller than $\mathrm{Na}$.
- **Negative ions are bigger** than their atoms. $\mathrm{Cl}$ gains an electron. The extra electron repels the others, and the same number of protons now holds more electrons, so the cloud spreads out. $\mathrm{Cl^-}$ is bigger than $\mathrm{Cl}$.

### Exam tip: Lose and shrink, gain and grow
Lose electrons: smaller. Gain electrons: bigger. "If you are negative you act bigger, if you are positive you act smaller."

## The octet rule

Atoms tend to gain, lose or share electrons until they have **eight** electrons in their outer shell (two for hydrogen and helium), like a noble gas. This is the **octet rule**. It explains why metals on the left lose electrons (they have only a few to give away) and non-metals on the right gain them (they are only a few short of eight).

## Reactivity of metals (group 1)

Metals react by **losing** electrons. The easier it is to lose the outer electron, the more reactive the metal.

**Down group 1, reactivity increases.** Lithium fizzes gently on water. Sodium whizzes around and melts. Potassium bursts into a lilac flame. Caesium explodes. The outer electron is further away and easier to lose each time.

$$\text{lithium} + \text{water} \rightarrow \text{lithium hydroxide} + \text{hydrogen}$$

## Reactivity of non-metals (group 17)

Non-metals react by **gaining** electrons. The stronger the pull on an incoming electron, the more reactive.

**Down group 17, reactivity decreases.** Fluorine is the most reactive element of all. Iodine is much calmer. In bigger atoms the incoming electron is further from the nucleus and feels a weaker pull.

A more reactive halogen **displaces** a less reactive one from its salt:

$$\text{chlorine} + \text{potassium bromide} \rightarrow \text{potassium chloride} + \text{bromine}$$

### Key idea: Opposite trends
- Group 1 metals: **more** reactive going **down**.
- Group 17 non-metals: **less** reactive going **down**.

Both come from the same cause: bigger atoms hold their outer electrons less tightly.

## Metallic character

**Metallic character** is how much an element behaves like a metal.

- It **increases down** a group. Carbon at the top of group 14 is a non-metal, silicon and germanium are metalloids, tin and lead are metals.
- It **decreases across** a period. Period 3 goes from metals (Na, Mg, Al) to a metalloid (Si) to non-metals (P, S, Cl, Ar).

## Other trends to know

| Property | Across a period | Down a group |
|---|---|---|
| Atomic radius | Decreases | Increases |
| Metallic character | Decreases | Increases |
| Electronegativity | Increases | Decreases |
| First ionisation energy | Generally increases | Decreases |
| Electron affinity | Generally increases | Decreases |
| Ionic radius (same charge) | | Increases |
| Group 1 melting point | | Decreases |
| Group 17 melting and boiling points | | Increase (gas, gas, liquid, solid) |

**Electronegativity** is how strongly an atom pulls on shared electrons in a bond. Fluorine is the most electronegative element. Noble gases are usually left out because they rarely bond.

**Ionisation energy** is the energy needed to **remove** an electron from an atom. Atoms on the right of a period hold their electrons tightly, so their ionisation energy is high. Atoms lower down hold them loosely, so it is low.

**Electron affinity** is the energy released when an atom **gains** an electron. It shows how much an atom "wants" an extra electron. Halogens, one electron short of a full shell, have the highest electron affinities. Chlorine has a higher electron affinity than sulfur.

### Exam tip: Corner to corner
Many trends run diagonally. **Bottom left** (caesium, francium): biggest, most metallic, lowest electronegativity. **Top right** (fluorine): smallest non-noble atom, highest electronegativity.

### Watch out: More electrons does not mean bigger across a period
Chlorine has more electrons than sodium, but it is smaller. Its extra electrons are in the same shell and its extra protons pull everything closer.

**Mind map**

```text
Periodic trends
  Cause
    Protons pulling
    Distance and shielding
  Atomic radius
    Bigger down
    Smaller across
  Ions
    Positive ions smaller
    Negative ions bigger
    Octet rule
  Reactivity
    Group 1: more reactive down
    Group 17: less reactive down
    Displacement of halogens
  Metallic character
    More down
    Less across
  Electronegativity
    Higher across
    Lower down
    Fluorine highest
    Electron affinity: halogens highest
```

### Year 11: Ionisation energy and nuclear charge
**First ionisation energy** is the energy needed to remove one electron from each atom in a mole of gaseous atoms:

$$\mathrm{Na(g) \rightarrow Na^+(g) + e^-}$$

It **decreases down a group** (outer electron further away and more shielded) and **generally increases across a period** (more protons, same shielding).

Chemists explain this with **core charge**: the protons minus the inner (core) electrons. Sodium has 11 protons and 10 inner electrons, so a core charge of $+1$. Chlorine has 17 protons and 10 inner electrons, so a core charge of $+7$. Chlorine's outer electrons are held much more strongly.

### Year 12: Bumps in the ionisation energy trend
Across period 3 the ionisation energy rises, but with two dips.

- **Mg to Al drops**: aluminium's outer electron is in a 3p subshell, which is higher in energy and slightly further out than 3s.
- **P to S drops**: in sulfur, two electrons share one 3p orbital. They repel each other, so one is easier to remove.

**Successive ionisation energies** reveal the group. For magnesium, the first two electrons come off fairly easily, then the third needs a huge jump in energy because it comes from a full inner shell. A big jump after the second electron means 2 valence electrons, so group 2.

## Quick summary

- Trends depend on the number of protons and on how far the outer electrons are from the nucleus.
- Atoms get **bigger down** a group and **smaller across** a period.
- Group 1 metals get **more reactive down** the group. Group 17 halogens get **less reactive down** the group.
- Metallic character increases down and decreases across.
- Electronegativity increases across and decreases down. Fluorine is the highest.

## Quiz

Try these questions from the chapter quiz.

1. Going down a group, atomic radius:
   - A. Increases
   - B. Decreases
   - C. Stays the same
   - D. First increases then decreases
   - Answer: A. Increases. Each new period adds a shell, so atoms get bigger.

2. Going across a period from left to right, atomic radius:
   - A. Increases
   - B. Decreases
   - C. Stays the same
   - D. Doubles
   - Answer: B. Decreases. More protons pull the same shell in closer.

3. Which atom is the largest?
   - A. Lithium
   - B. Sodium
   - C. Potassium
   - D. Hydrogen
   - Answer: C. Potassium. Potassium is lowest in group 1, so it has the most shells.

The whole chapter quiz is on the page, for signed-in students: https://whizznotes.com/chemistry/periodic-trends/#quiz