How does the atomic radius change from left to right across a period of the periodic table?

The atomic radius is the distance from the centre of the nucleus of an atom to its outermost shell of electrons. Since atoms do not have sharp boundaries, it is usually measured as half the distance between the nuclei of two bonded atoms of the same element (covalent radius) or of neighbouring atoms in a metal (metallic radius). It is measured in picometres (pm) or angstroms.

Across a period (from left to right):

• The atomic number increases by one at each step, so the positive charge on the nucleus increases.
• Electrons are added to the same outermost shell, so the number of shells stays the same.

The increasing nuclear charge pulls the electrons more strongly towards the nucleus. The shielding effect of inner electrons does not increase much, because the new electrons are in the same shell.

As a result, the atomic radius decreases. For example, in the third period, sodium (Na) has the largest atom and chlorine (Cl) has the smallest (noble gases are usually treated separately, since their radii are measured differently, as van der Waals radii).

Down a group (from top to bottom):

• A new electron shell is added with each period.
• The outer electrons are farther from the nucleus and are shielded by more inner shells.

So the atomic radius increases. For example, among alkali metals, lithium is the smallest and caesium (or francium) the largest.

These trends explain other periodic properties:

• Ionisation energy and electronegativity generally increase across a period and decrease down a group, because smaller atoms hold their electrons more tightly.
• Metallic character decreases across a period and increases down a group.
• Ionic radii follow similar trends; a cation is smaller than its parent atom, and an anion is larger.

The modern periodic law, proposed by Henry Moseley in 1913, states that the properties of elements are a periodic function of their atomic numbers. Earlier, Mendeleev's periodic law (1869) was based on atomic masses.

Why not the other options?

• ✗It increases: this is true down a group, not across a period.
• ✗It remains the same: incorrect.
• ✗It first increases and then decreases: incorrect.