UK K12 GCSE/A-Level: Year 12 A-Level Lower Sixth Chemistry - Atomic Structure, Ionisation Energy

Learning Objectives

By the end of this topic, students will be able to: - Explain the concept of ionisation energy and its significance in atomic structure. - Describe the factors affecting ionisation energy, including atomic number, atomic radius, and electron shielding. - Calculate ionisation energy using the formula IE = 13.6Z^2/n^2 eV. - Analyze the relationship between ionisation energy and the periodic table. - Evaluate the importance of ionisation energy in chemical bonding and reactivity.

Core Concepts

Ionisation energy (IE) is the energy required to remove an electron from a neutral atom in its ground state. It is a fundamental property of atoms and is influenced by several factors:

• Atomic number (Z): Ionisation energy increases with atomic number due to the increasing nuclear charge and the decreasing atomic radius.
• Atomic radius: Ionisation energy decreases with decreasing atomic radius due to the increasing effective nuclear charge.
• Electron shielding: Ionisation energy is affected by the presence of inner electrons, which shield the outer electrons from the nuclear charge. This effect is more significant for larger atoms.
• Energy levels (n): Ionisation energy is inversely proportional to the square of the principal quantum number (n). This means that ionisation energy decreases as the energy level increases.

Worked Examples

Example 1

Calculate the ionisation energy of a magnesium atom (Z = 12) with an electron in the 3rd energy level (n = 3).

IE = 13.6Z^2/n^2 eV = 13.6 x 12^2 / 3^2 = 13.6 x 144 / 9 = 13.6 x 16 = 217.6 eV

Example 2

Compare the ionisation energies of sodium (Z = 11, n = 3) and magnesium (Z = 12, n = 3).

IE (sodium) = 13.6 x 11^2 / 3^2 = 13.6 x 121 / 9 = 13.6 x 13.44 = 182.0 eV IE (magnesium) = 13.6 x 12^2 / 3^2 = 13.6 x 144 / 9 = 13.6 x 16 = 217.6 eV

The ionisation energy of magnesium is higher than that of sodium due to the increasing atomic number.

Common Misconceptions

• Ionisation energy is directly proportional to the atomic number (Z).
• Ionisation energy is only affected by the atomic radius.
• Electron shielding has no effect on ionisation energy.

Exam Tips

• Make sure to use the correct formula for calculating ionisation energy: IE = 13.6Z^2/n^2 eV.
• Consider the factors affecting ionisation energy, including atomic number, atomic radius, and electron shielding.
• Analyze the relationship between ionisation energy and the periodic table.
• Evaluate the importance of ionisation energy in chemical bonding and reactivity.

MCQs

MCQ 1 [F]

What is the formula for calculating ionisation energy? A) IE = 13.6Z/n^2 eV B) IE = 13.6Z^2/n eV C) IE = 13.6Z^2/n^2 eV D) IE = 13.6Z/n eV

Correct answer: C) IE = 13.6Z^2/n^2 eV Why the distractors fail: - A) Omitting the squared term (Z^2) results in an incorrect formula. - B) Omitting the squared term (n) results in an incorrect formula. - D) Omitting the squared term (Z^2) and the squared term (n) results in an incorrect formula.

MCQ 2 [H]

Which of the following factors affects ionisation energy? A) Atomic number only B) Atomic radius only C) Electron shielding only D) Atomic number, atomic radius, and electron shielding

Correct answer: D) Atomic number, atomic radius, and electron shielding Why the distractors fail: - A) Omitting atomic radius and electron shielding results in an incomplete answer. - B) Omitting atomic number and electron shielding results in an incomplete answer. - C) Omitting atomic number and atomic radius results in an incomplete answer.

MCQ 3 [F]

What happens to ionisation energy as the atomic number increases? A) It decreases B) It remains the same C) It increases D) It becomes negative

Correct answer: C) It increases Why the distractors fail: - A) Ionisation energy actually increases with increasing atomic number. - B) Ionisation energy does not remain the same with increasing atomic number. - D) Ionisation energy is always positive and does not become negative.

MCQ 4 [H]

Compare the ionisation energies of sodium (Z = 11, n = 3) and magnesium (Z = 12, n = 3). A) Sodium has a higher ionisation energy B) Magnesium has a higher ionisation energy C) Both have the same ionisation energy D) It is impossible to determine which has a higher ionisation energy

Correct answer: B) Magnesium has a higher ionisation energy Why the distractors fail: - A) Magnesium actually has a higher ionisation energy than sodium. - C) The ionisation energies are not the same. - D) It is possible to determine which has a higher ionisation energy by comparing the calculated values.

MCQ 5 [H]

What is the significance of ionisation energy in chemical bonding and reactivity? A) It determines the type of bond formed B) It determines the strength of the bond C) It determines the reactivity of an element D) It has no significance in chemical bonding and reactivity

Correct answer: C) It determines the reactivity of an element Why the distractors fail: - A) Ionisation energy does not determine the type of bond formed. - B) Ionisation energy does not determine the strength of the bond. - D) Ionisation energy actually has a significant impact on chemical bonding and reactivity.

Short-answer questions

1. Describe the factors affecting ionisation energy. (10 marks)
2. Calculate the ionisation energy of a chlorine atom (Z = 17) with an electron in the 3rd energy level (n = 3). (10 marks)
3. Compare the ionisation energies of sodium (Z = 11, n = 3) and magnesium (Z = 12, n = 3). (10 marks)
4. Evaluate the importance of ionisation energy in chemical bonding and reactivity. (15 marks)
5. Analyze the relationship between ionisation energy and the periodic table. (15 marks)