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Periodic trends: why the biggest atoms hold on the weakest

Your guide: Miss SanaCounts atoms the way you count rupees: carefully.

The problem

Caesium is one of the biggest atoms in the table. You would expect a big atom to be strong. Yet caesium gives away its outer electron more easily than almost any other element.

Think of mobile signal. Next to the tower it is strong; far away, behind a few walls, it drops. The nucleus is the tower, distance is the size of the atom, and the inner electrons are the walls.

The two trends: atomic radius decreases across a period and increases down a group. Ionization energy does the opposite: it increases across a period and decreases down a group.

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Notes, short questions and MCQs

Read the full notes: key terms, model answers and MCQs with answers

The problem

Caesium is one of the biggest atoms in the table. You would expect a big atom to be strong. Yet caesium gives away its outer electron more easily than almost any other element.

Think of mobile signal. Next to the tower it is strong; far away, behind a few walls, it drops. The nucleus is the tower, distance is the size of the atom, and the inner electrons are the walls.

The two trends: atomic radius decreases across a period and increases down a group. Ionization energy does the opposite: it increases across a period and decreases down a group.

Key terms

Atomic radius
A single atom cannot be measured directly. So we measure the distance between the centres of two bonded atoms and take half of it.
Ionic radius
A positive ion is always smaller than its atom: it loses its outer shell. A negative ion is always bigger: the extra electron adds repulsion and the shell expands.
Ionization energy
The minimum energy needed to remove an electron from the outermost shell of an isolated gaseous atom. Removing a second electron always costs more.
Shielding effect
Inner electrons repel the outer electron and block part of the nucleus's pull. More shells means more shielding.

Short questions with model answers

  1. Q1. Arrange sodium, chlorine, magnesium and sulphur in order of increasing atomic radius.

    • Na, Mg, S and Cl are all in period 3.
    • Protons: Na 11 < Mg 12 < S 16 < Cl 17
    • Cl (99) < S (104) < Mg (136) < Na (157) pm

    Cl < S < Mg < Na

  2. Q2. The radius of a sodium atom is 157 pm, but a sodium ion is only 95 pm. Explain why.

    • Na (2, 8, 1) → Na+ (2, 8) + e−
    • Now 11 protons pull only 10 electrons.

    Na+ is smaller: one shell fewer, and a stronger pull on the rest.

  3. Q3. Explain why the ionization energy of caesium is lower than that of lithium, although caesium has far more protons.

    • Li has 2 shells; Cs has 6 shells.
    • More inner electrons mean more shielding.
    • IE: Li 520 > Cs 376 kJ mol−1

    Distance and shielding win over the extra protons.

Common mistakes

  • ✗ Writing that atoms get bigger across a period because they have more electrons.

    ✓ The new electrons go into the same shell, while the nuclear charge grows. The pull wins, so atoms shrink.

  • ✗ Saying a negative ion is smaller than its atom.

    ✓ It is bigger: F is 72 pm but F− is 136 pm. The extra electron increases repulsion and the shell expands.

  • ✗ Assuming ionization energy rises smoothly at every step across a period.

    ✓ The trend is general, not perfect. Al (578) is lower than Mg (738), and S (1000) is lower than P (1012).

MCQs

  1. 1. Which atom has the largest radius?

    1. (a) Li
    2. (b) Na
    3. (c) K
    4. (d) Cs
    Show answer

    (d) They are all in group 1, and caesium is lowest, with the most shells.

  2. 2. Across period 3 from sodium to chlorine, the atomic radius:

    1. (a) increases
    2. (b) decreases
    3. (c) stays the same
    4. (d) first rises, then falls
    Show answer

    (b) Nuclear charge rises while electrons fill the same shell, so the shell is pulled in.

  3. 3. Which particle is bigger than the atom it comes from?

    1. (a) Na+
    2. (b) F−
    3. (c) Mg2+
    4. (d) Li+
    Show answer

    (b) Only the negative ion grows: F is 72 pm but F− is 136 pm. Positive ions always shrink.

  4. 4. The second ionization energy of magnesium (1451 kJ mol−1) is higher than the first (738) because:

    1. (a) the electron is removed from a positive ion
    2. (b) magnesium gains a proton
    3. (c) the atom becomes bigger
    4. (d) shielding increases
    Show answer

    (a) The second electron must be pulled away from Mg+, which already attracts its electrons more strongly.

  5. 5. Going down a group, ionization energy decreases mainly because of:

    1. (a) fewer protons
    2. (b) larger size and more shielding
    3. (c) smaller nuclear charge
    4. (d) fewer electrons
    Show answer

    (b) Protons do increase, but the outer electron is further away and better shielded, so it is held weakly.

Quick revision

  • Atomic radius decreases across a period and increases down a group.
  • A positive ion is smaller than its atom; a negative ion is bigger.
  • Ionization energy runs opposite to size: it rises across a period and falls down a group, because of nuclear charge, distance and shielding.