Pearson Edexcel International AS Chemistry Topic 2: Atomic Structure and Periodic Table

Topic 2: Atomic Structure & The Periodic Table

1. The Atom & Isotopes (2.1 - 2.5)

Atoms consist of a dense nucleus containing protons and neutrons, with electrons orbiting in shells.

Particle	Relative Mass	Relative Charge
Proton	1	+1
Neutron	1	0
Electron	1/1840	-1

Key Definitions:

• Atomic Number: Number of protons in the nucleus.
• Mass Number: Total number of protons and neutrons.
• Isotopes: Atoms of the same element with the same number of protons but different numbers of neutrons.

2. Mass Spectrometry (2.6 - 2.7)

Mass spectrometry is used to determine the isotopic composition of elements and the relative molecular mass of compounds.

Formula: Relative Atomic Mass ($A_r$)

$$A_r=\frac{\sum (\text{isotopic mass}\times \text{abundance})}{\text{total abundance}}$$

Diatomic Molecules (e.g. Chlorine)

For ${\text{Cl}}_2$, with isotopes ${}^{35}\text{Cl}$ and ${}^{37}\text{Cl}$ in a 3:1 ratio, the mass spectrum shows peaks at 70, 72, and 74 in a 9:6:1 ratio.

3. Ionisation Energies (2.8 - 2.11)

First Ionisation Energy: The energy required to remove one mole of electrons from one mole of gaseous atoms.

$$\text{X}\left(\text{g}\right)\to {\text{X}}^{+}\left(\text{g}\right)+{\text{e}}^{-}$$

Factors Affecting Ionisation Energy:

• Nuclear Charge: More protons = stronger attraction.
• Atomic Radius: Greater distance = weaker attraction.
• Shielding: More inner shells = weaker attraction.

4. Electronic Configuration (2.12 - 2.16)

Electrons occupy orbitals (s, p, d). An orbital can hold two electrons with opposite spins.

• s-orbital: Spherical shape.
• p-orbital: Dumbbell shape.

Exam Tip: Remember the exceptions for Chromium (Z=24) and Copper (Z=29). They prefer half-filled or full d-subshells for stability:
Cr: $\left[\text{Ar}\right]4{\text{s}}^{1}3{\text{d}}^5$

5. Periodicity (2.17 - 2.18)

Trends in melting and boiling temperatures are determined by structure and bonding.

• Giant Metallic (Li, Be, Na, Mg, Al): High melting points.
• Giant Covalent (C, Si): Very high melting points.
• Simple Molecular (P, S, Cl): Low melting points (dependent on London forces).

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Practice Zone

Quick MCQ Practice

Q1: Which factor explains why the first ionisation energy decreases down Group 2?

• A) Increased nuclear charge
• B) Decreased shielding
• C) Increased atomic radius
• D) Decreased number of protons

Click for Answer

Correct Answer: C. As you go down a group, the outermost electron is further from the nucleus and experiences more shielding, making it easier to remove.

Exam-Style Calculation

Q2: A sample of Magnesium contains ${}^{24}\text{Mg}$ (79%), ${}^{25}\text{Mg}$ (10%), and ${}^{26}\text{Mg}$ (11%). Calculate the relative atomic mass.

Click for Worked Solution

$$A_r=\frac{(24\times 79)+(25\times 10)+(26\times 11)}{100}$$ $$A_r=\frac{1896+250+286}{100}=24.32$$

Successive Ionisation Energy Challenge

Q3: An element in Period 3 has the following successive ionisation energies (kJ/mol): 738, 1451, 7733, 10543. Identify the element.

Click for Explanation

Answer: Magnesium (Mg). There is a massive jump between the 2nd and 3rd ionisation energies. This indicates the 3rd electron is being removed from a inner shell, meaning the element has 2 valence electrons (Group 2).