What you'll learn
This topic forms the foundation of CIE IGCSE Chemistry, covering the building blocks of all matter. You need to understand how atoms combine to form elements and compounds, how to write chemical formulae, and how to distinguish between pure substances and mixtures. These concepts appear in multiple exam papers, both as standalone questions and as essential knowledge for answering questions on reactions, bonding, and quantitative chemistry.
Key terms and definitions
Atom — the smallest particle of an element that can exist, consisting of a nucleus containing protons and neutrons, surrounded by electrons in shells.
Element — a pure substance made of only one type of atom, which cannot be broken down into simpler substances by chemical means.
Compound — a substance containing two or more elements chemically bonded together in fixed proportions.
Molecule — two or more atoms chemically bonded together; can be atoms of the same element (O₂) or different elements (H₂O).
Ion — an atom or group of atoms that has gained or lost electrons, giving it an overall positive or negative charge.
Mixture — two or more substances (elements or compounds) present together but not chemically bonded, which can be separated by physical methods.
Chemical formula — a representation showing the elements present in a compound and the ratio of atoms of each element.
Diatomic molecule — a molecule consisting of two atoms bonded together, such as H₂, N₂, O₂, F₂, Cl₂, Br₂, or I₂.
Core concepts
Atomic structure
Atoms consist of three subatomic particles:
- Protons: positively charged particles located in the nucleus; mass = 1; charge = +1
- Neutrons: neutral particles located in the nucleus; mass = 1; charge = 0
- Electrons: negatively charged particles orbiting the nucleus in shells; mass = 1/1840 (approximately 0); charge = -1
The atomic number (proton number) equals the number of protons in an atom. This defines which element the atom is. In a neutral atom, the number of electrons equals the number of protons, making the overall charge zero.
The mass number (nucleon number) equals the total number of protons plus neutrons in the nucleus. The number of neutrons can be calculated using: neutrons = mass number - atomic number.
Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons. For example, carbon-12 and carbon-14 are both carbon atoms (6 protons each) but carbon-12 has 6 neutrons while carbon-14 has 8 neutrons. Isotopes have identical chemical properties because they have the same electron arrangement, but different physical properties due to different masses.
Electron arrangement
Electrons occupy shells (energy levels) around the nucleus. CIE IGCSE Chemistry requires knowledge of the first three shells:
- First shell: holds up to 2 electrons
- Second shell: holds up to 8 electrons
- Third shell: holds up to 8 electrons (for elements 1-20)
Electron arrangement is written as a series of numbers separated by commas or full stops. For example:
- Sodium (atomic number 11): 2,8,1
- Chlorine (atomic number 17): 2,8,7
- Calcium (atomic number 20): 2,8,8,2
The number of electrons in the outer shell determines an element's chemical properties and reactivity. Elements with the same number of outer shell electrons belong to the same group in the periodic table and have similar chemical properties.
Elements and their properties
Elements are pure substances that appear in the periodic table. Each element is represented by a chemical symbol — either one capital letter (C for carbon, N for nitrogen) or one capital letter followed by a lowercase letter (Ca for calcium, Cl for chlorine).
Elements can be classified as:
Metals (found on the left and centre of the periodic table):
- Good conductors of heat and electricity
- Malleable (can be hammered into shape) and ductile (can be drawn into wires)
- Shiny when freshly cut
- High melting and boiling points (generally)
- Form positive ions by losing electrons
Non-metals (found on the right of the periodic table):
- Poor conductors of heat and electricity (except graphite)
- Brittle when solid
- Dull appearance
- Low melting and boiling points (many are gases at room temperature)
- Form negative ions by gaining electrons, or share electrons in covalent bonds
Seven elements exist as diatomic molecules at room temperature: H₂, N₂, O₂, F₂, Cl₂, Br₂, I₂. This must be remembered for writing equations correctly.
Compounds and chemical formulae
Compounds form when two or more elements chemically bond together. The properties of a compound differ completely from the properties of the elements that formed it. For example, sodium (a reactive metal) and chlorine (a toxic gas) form sodium chloride (table salt, safe to eat).
Chemical formulae show:
- Which elements are present (using chemical symbols)
- How many atoms of each element are present (using subscript numbers)
For example, H₂O contains 2 hydrogen atoms and 1 oxygen atom; CaCO₃ contains 1 calcium atom, 1 carbon atom, and 3 oxygen atoms.
Writing formulae from names:
- Compounds ending in -ide usually contain just two elements: copper oxide (CuO), magnesium chloride (MgCl₂)
- Compounds ending in -ate contain oxygen plus at least two other elements: copper sulfate (CuSO₄), calcium carbonate (CaCO₃)
- Compounds ending in -ite also contain oxygen but less than the -ate version: sulfite (SO₃²⁻) versus sulfate (SO₄²⁻)
Common ions to memorize for CIE IGCSE:
Positive ions (cations):
- Ammonium: NH₄⁺
- Metal ions: Na⁺, K⁺, Ca²⁺, Mg²⁺, Al³⁺, Cu²⁺, Fe²⁺, Fe³⁺, Zn²⁺, Ag⁺
Negative ions (anions):
- Hydroxide: OH⁻
- Nitrate: NO₃⁻
- Carbonate: CO₃²⁻
- Sulfate: SO₄²⁻
- Chloride: Cl⁻
- Bromide: Br⁻
- Iodide: I⁻
Balancing charges in formulae:
The total positive charge must equal the total negative charge. Use the swap-and-drop method:
- Write the ions with their charges: Mg²⁺ and Cl⁻
- Swap the numbers (ignore the signs): Mg₁Cl₂
- Simplify if possible: MgCl₂
For calcium carbonate: Ca²⁺ and CO₃²⁻ → CaCO₃ (charges already balanced)
For aluminum oxide: Al³⁺ and O²⁻ → Al₂O₃ (swap 3 and 2)
Mixtures versus compounds
Understanding the difference between mixtures and compounds is frequently tested:
Compounds:
- Elements chemically bonded together
- Fixed composition (always the same ratio of elements)
- Cannot be separated by physical methods
- Properties completely different from constituent elements
- Formed in chemical reactions with energy changes
- Represented by a single chemical formula
Mixtures:
- Two or more substances present together but not bonded
- Variable composition (proportions can vary)
- Can be separated by physical methods (filtration, distillation, chromatography)
- Each substance retains its own properties
- No chemical reaction occurs when mixing
- No single formula
Examples of mixtures: air (nitrogen, oxygen, carbon dioxide, etc.), seawater (water, sodium chloride, other dissolved substances), crude oil (mixture of hydrocarbons).
Relative atomic mass and relative formula mass
Relative atomic mass (Aᵣ) is the average mass of atoms of an element compared to 1/12 of the mass of a carbon-12 atom. For elements with isotopes, it's a weighted average. Values are given in the CIE periodic table.
Relative formula mass (Mᵣ) is calculated by adding up the relative atomic masses of all atoms in the formula.
For example, calculate Mᵣ of Ca(OH)₂:
- Ca: 1 × 40 = 40
- O: 2 × 16 = 32
- H: 2 × 1 = 2
- Total Mᵣ = 74
Note that the subscript outside brackets multiplies everything inside: Ca(OH)₂ contains 1 Ca, 2 O, and 2 H atoms.
Worked examples
Example 1: Determining atomic structure
Question: An atom of chlorine can be represented as ³⁵₁₇Cl. (a) State the number of protons, neutrons and electrons in this atom. (b) Draw the electron arrangement of this chlorine atom. [4 marks]
Solution:
(a)
- Number of protons = 17 (the atomic number) [1]
- Number of neutrons = 35 - 17 = 18 (mass number minus atomic number) [1]
- Number of electrons = 17 (same as protons in a neutral atom) [1]
(b) Electron arrangement: 2,8,7 [1]
Example 2: Writing chemical formulae
Question: Write the chemical formula for: (a) magnesium nitrate, (b) iron(III) sulfate, (c) ammonium carbonate. [3 marks]
Solution:
(a) Magnesium ion: Mg²⁺; Nitrate ion: NO₃⁻ To balance charges, need 2 nitrate ions for each magnesium ion. Formula: Mg(NO₃)₂ [1]
(b) Iron(III) ion: Fe³⁺; Sulfate ion: SO₄²⁻ Swap and drop: Fe₂(SO₄)₃ [1]
(c) Ammonium ion: NH₄⁺; Carbonate ion: CO₃²⁻ Need 2 ammonium ions to balance one carbonate ion. Formula: (NH₄)₂CO₃ [1]
Example 3: Distinguishing mixtures and compounds
Question: Iron and sulfur can be mixed together or heated to form iron sulfide. Describe two differences between a mixture of iron and sulfur and the compound iron sulfide. [2 marks]
Solution:
The mixture can be separated using a magnet (iron is magnetic) but the compound cannot / The iron retains its magnetic properties in the mixture but not in the compound [1]
The mixture can have any proportion of iron to sulfur, but the compound has a fixed ratio / The compound is formed by a chemical reaction with energy change, but the mixture is not [1]
Common mistakes and how to avoid them
Mistake: Confusing mass number with atomic number. Correction: Atomic number = protons only (bottom number in notation); mass number = protons + neutrons (top number in notation).
Mistake: Writing electron arrangements incorrectly for ions. Correction: Positive ions have lost electrons (fewer electrons than protons); negative ions have gained electrons (more electrons than protons). For Cl⁻ (17 protons): electron arrangement is 2,8,8 (18 electrons), not 2,8,7.
Mistake: Forgetting brackets in formulae with polyatomic ions. Correction: When you need more than one polyatomic ion, use brackets: Ca(OH)₂ not CaOH₂; (NH₄)₂SO₄ not NH₄₂SO₄.
Mistake: Treating elements as single atoms when they're diatomic. Correction: Remember H₂, N₂, O₂, F₂, Cl₂, Br₂, I₂ exist as molecules. Write "O₂" not "O" when referring to oxygen gas.
Mistake: Thinking compounds can be separated by physical methods. Correction: Compounds require chemical reactions to break them down; only mixtures can be separated physically.
Mistake: Calculating relative formula mass incorrectly for brackets. Correction: Multiply the subscript outside the bracket by each subscript inside. In Ca(NO₃)₂, there are 2 N atoms and 6 O atoms (2 × 3), not 3 O atoms.
Exam technique for Atoms, Elements and Compounds
Command word "State": Give a brief answer without explanation. For "State the number of protons in ²⁴₁₂Mg", simply write "12" — no calculation shown needed (1 mark).
Command word "Describe": Requires more detail than "state" but less than "explain". When describing differences between mixtures and compounds, give observable or measurable differences (2-3 marks typically, 1 mark per distinct point).
Drawing electron arrangements: Some exam questions provide circles for shells, others expect you to draw them. Draw circles around a central point (nucleus), place electrons as crosses or dots, and ensure the correct number in each shell. Label shells if the question asks.
Formula questions: Always check your formula balances charges. Show working for complex formulae to gain method marks even if the final answer is wrong. When Mᵣ calculations are required, show each step to maximize marks.
Quick revision summary
Atoms contain protons and neutrons in the nucleus, with electrons in shells around it. Atomic number = protons; mass number = protons + neutrons. Isotopes have the same protons but different neutrons. Elements contain one type of atom; compounds contain different elements chemically bonded in fixed ratios. Mixtures contain substances not chemically bonded, separable by physical methods. Write formulae by balancing ionic charges. Calculate Mᵣ by adding Aᵣ values for all atoms present. Remember the seven diatomic elements and common ion charges.