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Basic Ideas About Atoms

Ionisation energy and its variation between elements

  • 3 things to know
  • Components 1 and 2

Overview

First ionisation energy of an element, second ionisation energy of an element and general rise in first ionisation energy across a period.

What you need to know

Answer each one before you reveal it.

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Key term

Why is the second ionisation energy of any element always greater than its first?

Key term

Write the equation that defines the first ionisation energy of sodium.

Key term

Why does the definition of ionisation energy insist that the atoms are gaseous?

Questions from this subtopic

Each one with the model answer underneath.

  1. 1

    Define the first ionisation energy of an element.

    Model answer

    It is the energy needed to remove one electron from each atom in one mole of gaseous atoms, forming one mole of gaseous singly positively charged ions.

  2. 2

    Write the equation that defines the first ionisation energy of sodium.

    Model answer

    The equation is Na(g) → Na⁺(g) + e⁻.

  3. 3

    In what units is a standard molar ionisation energy quoted?

    Model answer

    It is quoted in kJ mol⁻¹.

  4. 4

    Why does the definition of ionisation energy insist that the atoms are gaseous?

    Model answer

    In a solid or a liquid, extra energy would be needed to separate the particles as well, so the measured value would not refer to an isolated atom on its own.

  5. 5

    Define the second ionisation energy of an element.

    Model answer

    It is the energy needed to remove one electron from each ion in one mole of gaseous 1+ ions to give one mole of gaseous 2+ ions.

  6. 6

    Write the equation that defines the second ionisation energy of magnesium.

    Model answer

    The equation is Mg⁺(g) → Mg²⁺(g) + e⁻.

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