Which one of the following properties decreases across the periodic table from left to right and increases from top to bottom?

Correct — D, Atomic radius. Move left to right along a period and electrons enter the same shell while protons keep being added, so the growing nuclear charge pulls the whole electron cloud inward and the atoms get smaller. Move down a group and each element has one more occupied shell than the one above, and the inner shells screen the outer electrons from the nucleus, so the atoms get larger. Atomic radius is therefore the property that decreases across and increases down. The other three all run the opposite way.

• (a)Ionization energy — Ionisation energy increases across a period, because a smaller atom holds its outer electron more tightly, and decreases down a group as that electron moves further from the nucleus.
• (b)Electron affinity — Electron affinity broadly increases across a period, since a smaller atom with a higher nuclear charge attracts an incoming electron more strongly, and it falls down a group.
• (c)Electronegativity — Electronegativity increases across a period — fluorine is the most electronegative element — and decreases down a group. It follows the same pattern as ionisation energy, not the reverse one.

Periodic trends follow from two competing effects. Effective nuclear charge rises across a period because protons are added while the shielding stays much the same, which pulls electrons in. Down a group a new shell is added each time and inner electrons shield the outer ones, which pushes them out. Size responds to both in one direction; ionisation energy, electron affinity and electronegativity all respond in the other, because they measure how tightly the atom holds or attracts electrons.

This is really one trend asked four ways. Fix atomic radius in mind — smaller across, larger down — and the other three follow as its inverse, since a smaller atom holds its electrons more tightly. That single anchor is worth more than four memorised arrows, and it survives the exceptions, which are what the harder versions of this question exploit. The most quoted of those is that noble gas radii are not directly comparable to the rest, being measured as van der Waals radii rather than covalent ones.

• Atomic radius decreases from left to right across a period and increases from top to bottom down a group.
• Ionisation energy, electron affinity and electronegativity all increase across a period and decrease down a group.
• Effective nuclear charge rises across a period because protons are added without a new shell.
• Adding a shell down a group increases size and increases the shielding of the outer electrons.
• Fluorine is the most electronegative element on the Pauling scale.

• Assuming all periodic properties move in the same direction.
• Reversing the group trend, since a larger atomic number is easy to associate with a smaller size.
• Treating electron affinity as perfectly regular; chlorine has a higher electron affinity than fluorine.

A trend-direction item; the sibling papers ask the same content through a size ordering of named elements or through an assertion-and-reason pair on the group trend.

• practice — not a real PYQ

  Which one of the following has the largest atomic radius?

  • (a)Na
  • (b)Mg
  • (c)K
  • (d)Ca

  Answer(c) K — it is furthest down and furthest left of the four, and both directions increase size.

• practice — not a real PYQ

  Ionisation energy generally

  • (a)increases across a period and increases down a group
  • (b)increases across a period and decreases down a group
  • (c)decreases across a period and increases down a group
  • (d)decreases across a period and decreases down a group

  Answer(b) increases across a period and decreases down a group — the opposite of atomic radius.