Why Atoms Shrink Across a Period but Grow Down a Group

Sodium reacts violently with water, chlorine is a poisonous gas and argon hardly reacts at all — yet all three sit in the same row of the periodic table. Their behaviour changes in a regular way because the pull of the nucleus on the outer electrons changes steadily across periods and down groups.

This lesson covers the trends in radii, ionisation enthalpy, electron gain enthalpy and electronegativity, and how effective nuclear charge explains them.

Across a period, atomic radius decreases while ionisation enthalpy and electronegativity generally increase and electron gain enthalpy becomes more negative; down a group, atomic radius increases while ionisation enthalpy and electronegativity decrease.

Atomic radius:

- Across a period — decreases; sodium (186 pm) is larger than magnesium (160 pm)
- Down a group — increases as new shells are added

Ionic radius:

- A cation is smaller than its parent atom, and an anion is larger
- In an isoelectronic series, the ion with more protons is smaller: ${\mathrm{O}}^{2-}>{\mathrm{F}}^{-}>\mathrm{N}{\mathrm{a}}^{+}>\mathrm{M}{\mathrm{g}}^{2+}$

Ionisation enthalpy — the energy needed to remove the outermost electron from a gaseous atom:

- Across a period — generally increases
- Down a group — decreases; lithium 520, sodium 496 and potassium 419 kJ mol${}^{-1}$
- Exceptions — beryllium (899) is higher than boron (801), and nitrogen (1402) is higher than oxygen (1314) kJ mol${}^{-1}$

Electron gain enthalpy — the enthalpy change when a gaseous atom gains an electron:

- Generally becomes more negative across a period and less negative down a group
- Chlorine ($-349$ kJ mol${}^{-1}$) is more negative than fluorine ($-333$ kJ mol${}^{-1}$)
- Noble gases have positive values, because the added electron must enter a new shell

Electronegativity — the tendency of a bonded atom to attract the shared electron pair:

- Increases across a period and decreases down a group
- Fluorine is the most electronegative element, at 4.0 on the Pauling scale

An everyday example. Sodium stored under kerosene in a school laboratory reflects its low ionisation enthalpy — it gives up an electron so readily that it reacts with air and moisture.

The substance. Electron gain enthalpy and electronegativity are not the same — the first describes an isolated gaseous atom, the second an atom within a bond.

The trends arise because effective nuclear charge — the net pull felt by outer electrons after shielding by inner electrons — rises across a period while the number of shells stays the same, and down a group the added shells and extra shielding outweigh the rising nuclear charge.

Across a period:

- Nuclear charge rises by one at each step, but electrons join the same shell, where they shield one another poorly
- Effective nuclear charge rises and pulls electrons closer, so radius falls and ionisation enthalpy and electronegativity rise

Down a group:

- Each step adds a new shell, placing outer electrons farther from the nucleus
- Inner electrons shield outer electrons more, so they are held less tightly — radius rises and ionisation enthalpy falls

Explaining the exceptions:

- Be versus B — boron's outer electron is in 2p, higher in energy and better shielded than beryllium's 2s electrons, so it is easier to remove
- N versus O — nitrogen's half-filled $2p^3$ subshell is extra stable, while one of oxygen's $2p^4$ electrons is paired and repelled by its partner
- F versus Cl — fluorine's small 2p subshell is crowded, so an added electron is strongly repelled, making its electron gain enthalpy less negative than chlorine's

Successive ionisation enthalpies. For magnesium, $[\mathrm{Ne}]3s^2$, the third ionisation enthalpy is far higher than the second, because the third electron must come from the stable noble-gas core.

An everyday example. A student in the back row of a crowded classroom hears the teacher less clearly because the students in between block the sound — just as inner electrons shield outer electrons from the nucleus.

The substance. Nuclear charge does increase down a group — but the extra shells and shielding more than cancel its effect.