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Trends in physical properties of elements in the periodic table

Senior 4 Chemistry · 14 questions with answers. Try each question on paper, then tap it to see the answer.

Unit 5 Trends in physical properties of elements in the periodic table

Key unit competence: Use atomic structure and electronic configuration to explain the trends in the physical properties of elements in the periodic table.

Key ideas

  • History: Döbereiner's triads, Newlands' octaves, Mendeleev's table (by atomic mass, with gaps and predictions such as eka-silicon = germanium), Moseley's work on atomic number leading to the modern periodic law: properties are a periodic function of atomic number.
  • Blocks: s-block (groups 1 and 2, outer electrons in s), p-block (groups 13–18), d-block (transition elements), f-block (lanthanides and actinides). Period = highest principal quantum number; group from outer electrons.
  • Atomic radius decreases across a period (nuclear charge increases, shielding nearly constant) and increases down a group (more shells). Cations are smaller than their atoms; anions are larger.
  • Ionisation energy increases across a period and decreases down a group (with dips at groups 13 and 16). Electronegativity increases across a period and decreases down a group; fluorine is the most electronegative element.
  • Electron affinity: energy change when a gaseous atom gains an electron; generally more exothermic across a period (chlorine has the most exothermic value).
  • Melting points across period 3 rise from Na to Si (metallic then giant covalent) then fall sharply for P4, S8, Cl2 and Ar (simple molecular, London forces). Metallic character decreases across a period and increases down a group.

Common misconceptions

  • "Atoms get bigger across a period because they have more electrons." No: They get smaller: the increasing nuclear charge pulls the electrons closer, while shielding is nearly the same.
  • "Silicon has a high melting point because it is a metal." No: Silicon is a metalloid with a giant covalent structure.
  • "A negative ion is smaller than its atom." No: Adding electrons increases repulsion and the ion is larger.

Worked examples

Example 1. Explain the trend in atomic radius across period 3 and down group 2.

Solution

Across period 3 (Na to Cl), the nuclear charge increases by one each step while electrons enter the same shell, so shielding hardly changes.

The outer electrons are pulled closer: atomic radius decreases.

Down group 2 (Be to Ba), each element has an extra shell of electrons, further from the nucleus and better shielded, so radius increases.

Example 2. Explain the variation in melting points across period 3.

Solution

Na, Mg, Al: metallic bonding gets stronger (more delocalised electrons, smaller ions): m.p. rises.

Si: giant covalent structure with many strong bonds: highest m.p. (1410 °C).

P4, S8, Cl2: simple molecules with weak London forces: low m.p. (S8 highest as the largest molecule).

Ar: single atoms with very weak forces: lowest m.p.

Practice questions

  1. State the modern periodic law.

    Show answer

    The properties of elements are a periodic function of their atomic numbers.

    Common mistake: Saying atomic masses.

  2. Name the four blocks of the periodic table.

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    s, p, d and f blocks.

    Common mistake: Naming groups instead.

  3. Define electronegativity.

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    The ability of an atom in a covalent bond to attract the shared pair of electrons.

    Common mistake: Confusing it with electron affinity.

  4. Explain why Mg2+ is smaller than Mg.

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    It has lost its outer shell (3s electrons) and the same nuclear charge pulls fewer electrons closer.

    Common mistake: Saying it gained protons.

  5. Which period 3 element has the highest first ionisation energy?

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    Argon.

    Common mistake: Saying sodium.

  6. Give one difference between Mendeleev's table and the modern table.

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    Mendeleev arranged by atomic mass and left gaps; the modern table uses atomic number and includes noble gases.

    Common mistake: Saying Mendeleev used electron configurations.

  7. State the block, group and period of [Ar]3d10 4s2 4p5.

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    p-block, group 17, period 4 (bromine).

    Common mistake: Saying d-block.

  8. Why do noble gases have the highest ionisation energies in their periods?

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    They have full outer shells, high nuclear charge for their period and small radius.

    Common mistake: Saying they have no electrons.

  9. Explain why silicon is a poor conductor although it has a high melting point.

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    Giant covalent structure with all electrons fixed in bonds.

    Common mistake: Saying it is ionic.

  10. Arrange Na, Mg, Cl, S in order of increasing electronegativity.

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    Na < Mg < S < Cl.

    Common mistake: Putting S after Cl.

Matching and drag-and-drop

  1. Matching

    Match each item in column A with the correct answer in column B. Write the letter next to each number.

    Column AColumn B
    1. A chloride ion is larger than a chlorine atom because:a. p-block (He in s)
    2. Which period 3 element has the lowest melting point?b. added electron increases repulsion
    3. The modern periodic law states that properties of elements are a periodic function of their:c. Argon
    4. Which block contains the noble gases?d. atomic number
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    1 → b (added electron increases repulsion) · 2 → c (Argon) · 3 → d (atomic number) · 4 → a (p-block (He in s))

    • Same nuclear charge, more electrons.
    • Single atoms with weak London forces.
    • Moseley showed atomic number is fundamental.
    • Groups 13–18 are the p-block.

  2. Drag and drop

    Choose the correct word or phrase from the box for each gap. One word in the box is not needed.

    increases  ·  it has lost its outer shell  ·  lanthanides and actinides  ·  alkali metals
    1. The f-block elements are also called .
    2. Electronegativity across a period .
    3. A sodium ion is smaller than a sodium atom because .
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    1: lanthanides and actinides · 2: increases · 3: it has lost its outer shell

    • Inner transition elements.
    • Atoms become smaller with higher nuclear charge.
    • Na+ is 2,8 instead of 2,8,1.

  3. Matching

    Match each item in column A with the correct answer in column B. Write the letter next to each number.

    Column AColumn B
    1. Electron affinity is the energy change when:a. decreases
    2. Across a period, atomic radius:b. a gaseous atom gains an electron
    3. The most electronegative element is:c. fluorine
    4. Metallic character increases:d. down a group and from right to left
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    1 → b (a gaseous atom gains an electron) · 2 → a (decreases) · 3 → c (fluorine) · 4 → d (down a group and from right to left)

    • X(g) + e^− → X^−(g).
    • Nuclear charge increases with similar shielding.
    • It is small with a high effective nuclear charge.
    • Easier loss of electrons.

  4. Drag and drop

    Choose the correct word or phrase from the box for each gap. One word in the box is not needed.

    d-block  ·  decreases  ·  increases  ·  group 16, period 3
    1. Elements whose outer electrons are in d orbitals belong to the .
    2. Down a group, atomic radius .
    3. The element with configuration [Ne]3s2 3p4 is in .
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    1: d-block · 2: increases · 3: group 16, period 3

    • Transition elements.
    • Extra electron shells are added.
    • 6 outer electrons, n = 3.

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