OCR A-Level Chemistry: Atoms, Ions & Compounds Revision Notes

Subatomic Particles and Isotopes

You've probably wondered what makes one element different from another - it's all about the tiny particles inside atoms! Protons carry a +1 charge, neutrons are neutral, and electrons have a -1 charge with almost no mass.

Isotopes are simply atoms of the same element that have different numbers of neutrons. Think of them as different versions of the same element - like different models of the same car. They have different masses but behave identically in chemical reactions.

Here's the key point: isotopes react in exactly the same way because they have the same number of electrons. The number of neutrons doesn't affect how atoms bond or react with other substances.

Quick Tip: Remember that isotopes are like twins with different weights - they look different but behave the same way!

Ions and Atomic Mass

When atoms gain or lose electrons, they become ions - charged particles that are crucial in chemistry. Cations are positive ions (fewer electrons than protons), while anions are negative ions (more electrons than protons).

Relative isotopic mass compares the mass of an isotope to one-twelfth of a carbon-12 atom. It's like using a standard ruler to measure everything else. Relative atomic mass (Ar) is the weighted average mass of all isotopes of an element.

Think of relative atomic mass as finding the average height of students in your year - you'd consider both how tall each student is and how many students there are of each height.

Memory Trick: Cations are "paws-itive" (they've lost electrons), anions are "negative" (they've gained electrons)!

Mass Spectrometry

Mass spectrometry is like a sophisticated sorting machine that separates isotopes by their mass. The process is surprisingly straightforward once you break it down.

First, the sample gets vaporised and ionised to create positive ions. These ions are then accelerated through the machine. Heavier ions move more slowly and are harder to deflect than lighter ones, causing separation.

The detector measures ions as a mass-to-charge ratio (m/z). The more abundant an isotope is, the stronger the signal it produces. This creates a mass spectrum - essentially a fingerprint showing which isotopes are present and in what amounts.

Real-World Connection: Mass spectrometry is used in forensic science to identify substances at crime scenes!

Calculating Relative Atomic Mass

Let's work through a real example with chlorine isotopes to show how straightforward these calculations can be. Chlorine has two main isotopes: Cl-35 (75.78% abundant) and Cl-37 (24.22% abundant).

The formula is: Ar = (abundance₁ × mass₁) + (abundance₂ × mass₂) / 100

For chlorine: Ar = (75.78 × 35) + (24.22 × 37) / 100 = 35.5

Common ions follow predictable patterns. Group 1 elements form +1 ions, Group 2 form +2 ions, and so on. Transition metals can form multiple different ions, which makes them more complex but also more versatile.

Exam Success: Always check your calculated atomic mass lies between the masses of the isotopes involved!

Important Ion Names

Memorising common ion names might seem tedious, but they're the vocabulary of chemistry - you'll use them constantly. Start with the most common ones you'll encounter in practicals and exams.

Key polyatomic ions include nitrate (NO₃⁻), carbonate (CO₃²⁻), sulfate (SO₄²⁻), hydroxide (OH⁻), and ammonium (NH₄⁺). These appear in countless compounds and reactions.

Simple metal ions like zinc (Zn²⁺) and silver (Ag⁺) are equally important. Notice that silver is unusual - it forms a +1 ion rather than the +2 you might expect.

Study Strategy: Create flashcards for ion names and formulae - 10 minutes daily practice makes them automatic!

Balancing Chemical Equations

Balancing equations is like solving a puzzle - you need the same number of each type of atom on both sides. Let's walk through aluminium reacting with oxygen to form aluminium oxide.

Start by working out the correct formulae: Al³⁺ and O²⁻ ions combine to give Al₂O₃. Write the unbalanced equation: Al + O₂ → Al₂O₃.

Now balance systematically: 4Al + 3O₂ → 2Al₂O₃. Check your work - you should have 4 Al atoms and 6 O atoms on each side. Finally, add state symbols: 4Al$s$ + 3O₂$g$ → 2Al₂O₃$s$.

Pro Tip: Always balance the most complex molecule first, then work through the simpler ones!