Cambridge IGCSE • Year 11 • Chemistry
Atoms, Elements and Compounds
Elements, compounds, atomic structure, bonding and giant structures.
Chapter 2
Verified Curriculum Topic
What is Atoms, Elements and Compounds?
Elements, compounds, atomic structure, bonding and giant structures.
Atoms, Elements and Compounds matters because it links chemical ideas, reactions, and reasoning patterns that recur throughout the syllabus. At Year 11 level, students are often expected to define terms accurately, explain processes clearly, and connect theory to reactions, observations, or applications.
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Summary
The One Thing
The identity of an element is determined by its proton number, while the structure and bonding of its particles determine its chemical and physical properties. Chemical formulae represent composition, and balanced equations show how atoms are rearranged while conserving mass.
Reactions, Processes and Experiments
| What happens | Equation or process | What you observe | Type |
|---|---|---|---|
| Sodium and chlorine combine in a fixed ratio to form sodium chloride. | NaCl for sodium chloride | — | Compound formation and ionic bonding |
| Magnesium and oxygen combine to form magnesium oxide. | MgO for magnesium oxide | — | Compound formation and ionic bonding |
| Hydrogen and oxygen are represented as water. | H2O for water | — | Compound formula |
| Carbon and oxygen are represented as carbon dioxide. | CO2 for carbon dioxide | — | Compound formula |
| Carbon and hydrogen are represented as methane. | CH4 for methane | — | Compound formula |
| A magnesium ion combines with two chloride ions to produce an overall neutral ionic compound. | MgCl2 because Mg2+ requires two Cl- ions. | — | Ionic compound formula |
| Atoms form ions by losing or gaining electrons. Metals usually lose electrons and non-metals usually gain electrons. | Electron loss or gain | Positive ions form when electrons are lost; negative ions form when electrons are gained. | Ion formation |
| Atoms form compounds through transfer of electrons from a metal atom to a non-metal atom. | Ionic bonding: strong electrostatic attraction between oppositely charged ions | Ionic compounds form regular arrangements of oppositely charged ions. | Ionic bonding |
| Atoms share electrons to form molecules or giant covalent structures. | Covalent bonding: strong electrostatic attraction between a shared pair of electrons and the nuclei of the bonded atoms | Small molecules or extended networks of atoms form. | Covalent bonding |
| Positive metal ions are held together by mobile electrons. | Metallic bonding: attraction between positive metal ions and a sea of delocalised electrons | Metals conduct electricity and heat; they are malleable. | Metallic bonding |
| Oppositely charged ions are arranged in a regular three-dimensional structure. | Giant ionic lattice: a regular three-dimensional arrangement of oppositely charged ions held together by strong electrostatic attractions | High melting and boiling points; solid compounds do not conduct, but molten compounds and aqueous solutions conduct. | Giant ionic structure |
| Small molecules contain strong covalent bonds internally and weaker forces between molecules. | Simple molecular structure | Usually low melting and boiling points; generally no electrical conductivity. | Simple molecular structure |
| Many atoms are joined by strong covalent bonds in an extended network. | Giant covalent structure | Generally high melting points. | Giant covalent structure |
| Each carbon atom bonds to four others in a three-dimensional network. | Diamond is a giant covalent structure. | Diamond is very hard and does not conduct electricity. | Giant covalent structure |
| Each carbon atom bonds to three others in layers, with delocalised electrons present. | Graphite is a giant covalent structure. | Layers can slide, and graphite conducts electricity. | Giant covalent structure |
| A single layer of graphite forms a two-dimensional carbon structure. | Graphene is a single layer of graphite. | Graphene has strength, flexibility and electrical conductivity. | Giant covalent structure |
| Silicon and oxygen form an extended covalent network. | Silicon dioxide has a giant covalent structure similar in overall arrangement to diamond. | Silicon dioxide has a high melting point. | Giant covalent structure |
| Positive metal ions can move relative to one another while metallic bonding remains. | Metallic structure | Metals are malleable and conduct electricity and heat. | Metallic bonding and metallic properties |
| The periodic table arranges elements according to proton number. | The periodic table is arranged in order of increasing atomic number. | Elements appear in increasing atomic number. | Periodic classification |
| Elements in the same group have the same number of outer-shell electrons. | Group arrangement in the periodic table | Elements in the same group have similar chemical properties. | Periodic classification |
| The period number corresponds to the number of occupied electron shells. | Period number shows the number of occupied electron shells in an atom. | Elements in a period have the same number of occupied shells. | Electronic configuration |
| A chemical equation represents the rearrangement of atoms during a reaction. | Chemical equations must be balanced using coefficients, not by changing the formulae of substances. | The same number of atoms of each element appears on both sides. | Balanced chemical equation |
| Atoms are rearranged but not created or destroyed during a reaction. | The conservation of mass means that atoms are rearranged during a reaction but are not created or destroyed. | Total mass is conserved. | Conservation of mass |
Key Terms
- Atom: The smallest particle of an element that can take part in a chemical reaction.
- Element: A substance made of only one type of atom, identified by its proton number.
- Compound: A pure substance formed when two or more different elements chemically combine in fixed proportions.
- Mixture: Two or more substances present together without chemical bonding, so they can usually be separated by physical methods.
- Proton: A positively charged particle found in the nucleus; its number determines the element.
- Neutron: A particle with no charge found in the nucleus.
- Electron: A negatively charged particle found in electron shells or energy levels around the nucleus.
- Atomic number: The number of protons in the nucleus of an atom.
- Mass number: The total number of protons and neutrons in an atom.
- Isotope: Atoms of the same element with the same number of protons but different numbers of neutrons.
- Relative atomic mass: The weighted mean mass of an element's isotopes compared with one-twelfth of the mass of a carbon-12 atom.
- Ion: A charged particle formed when an atom or group of atoms gains or loses electrons.
- Electronic configuration: The arrangement of electrons in shells around the nucleus, such as 2,8,1.
- Ionic bonding: Strong electrostatic attraction between oppositely charged ions, usually formed when electrons transfer from a metal atom to a non-metal atom.
- Covalent bonding: The strong electrostatic attraction between a shared pair of electrons and the nuclei of the bonded atoms.
- Metallic bonding: The attraction between positive metal ions and a sea of delocalised electrons.
- Simple molecular structure: A structure containing small molecules with strong covalent bonds within each molecule but weaker forces between molecules.
- Giant ionic lattice: A regular three-dimensional arrangement of oppositely charged ions held together by strong electrostatic attractions.
- Giant covalent structure: A large network of atoms joined by many strong covalent bonds, such as diamond, graphite or silicon dioxide.
- Delocalised electron: An electron that is free to move through a structure rather than being attached to one particular atom or bond.
- Chemical formula: A notation showing the elements present in a substance and the ratio of their atoms or ions.
- Balanced equation: A chemical equation with the same number of atoms of each element on both sides, showing conservation of mass.
- Relative formula mass, Mr: The sum of the relative atomic masses of all atoms shown in a formula.
Easily Confused
- Element vs compound: An element contains one type of atom; a compound contains two or more different elements chemically combined in fixed proportions.
- Compound vs mixture: A compound contains chemically bonded elements and has a fixed composition; a mixture contains substances together without chemical bonding and can usually be separated physically.
- Atomic number vs mass number: Atomic number is the number of protons; mass number is the total number of protons and neutrons.
- Isotope vs ion: Isotopes differ in neutron number; ions differ in electron number and therefore carry a charge.
- Ionic bonding vs covalent bonding: Ionic bonding involves electrostatic attraction between oppositely charged ions; covalent bonding involves attraction between shared electrons and atomic nuclei.
- Covalent bonding vs intermolecular forces: Covalent bonds hold atoms together within molecules or networks; intermolecular forces act between molecules and are weaker.
- Giant ionic structures vs simple molecular structures: Giant ionic structures contain strong attractions throughout a lattice; simple molecular substances have strong bonds within molecules but weaker forces between molecules.
- Diamond vs graphite: Each carbon atom bonds to four others in diamond, making it hard and non-conductive; each bonds to three others in graphite, whose layers slide and whose delocalised electrons conduct electricity.
- Graphite vs graphene: Graphite consists of many layers; graphene is a single layer of graphite.
- Solid ionic compounds vs molten or aqueous ionic compounds: Ions cannot move in a solid ionic compound, but they are mobile when molten or dissolved in water.
- Formulae vs balanced equations: A formula shows the composition of one substance; a balanced equation shows how atoms are rearranged in a reaction.
- Coefficients vs subscripts: Coefficients balance an equation; changing subscripts changes the substance's formula and is not permitted when balancing.
What Gets Asked
- Questions may require identification of an element from its proton number, or calculation of neutrons using
number of neutrons = mass number - atomic number. The common error is confusing atomic number with mass number. - Questions may ask for an atom's electronic configuration, such as
2,8,1, or explain chemical behaviour using outer-shell electrons. The mark-losing slip is failing to distinguish occupied shells from outer-shell electrons. - Questions may require calculation of relative formula mass, Mr, by adding the relative atomic masses of all atoms in a formula. Every atom shown in the formula must be included.
- Questions may ask for the formula of an ionic compound, such as
MgCl2. The formula must give an overall neutral charge;Mg2+requires twoCl-ions. - Questions may ask students to explain melting point, hardness, solubility or electrical conductivity using structure and bonding. The explanation must refer to the relevant particles, forces and whether charged particles or electrons can move.
- Questions may require a balanced chemical equation. The equation must contain the same number of atoms of each element on both sides, using coefficients rather than changing the formulae of substances.
Flashcards
Quick quiz
What determines the identity of an element?
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Sign up free — save & unlock everythingLearning objectives
- 2.1Describe the structure of an atom in terms of protons, neutrons, and electrons, and their relative charges and masses.
- 2.2Define proton (atomic) number and nucleon (mass) number, and use them to calculate the number of neutrons in an atom.
- 2.3Define isotopes as atoms of the same element with the same proton number but different nucleon numbers.
- 2.4Describe the arrangement of electrons in shells and relate this to an element's position in the Periodic Table.
- 2.5Explain the formation of ionic bonds by the transfer of electrons between metal and non-metal atoms.extended
- 2.6Explain the formation of covalent bonds by the sharing of electron pairs between non-metal atoms.extended
Practice questions
Q1. An atom of sodium has a proton number of 11 and a nucleon number of 23. How many neutrons does it contain?1 mark · core
- A. 11
- B. 12
- C. 23
- D. 34
Answer: B
- • 1 mark for selecting B
Number of neutrons = nucleon number − proton number = 23 − 11 = 12.
Q2. Sodium (2,8,1) reacts with chlorine (2,8,7) to form sodium chloride. Explain, in terms of electron transfer, how the ionic bond in sodium chloride is formed.4 marks · extended
Answer: Each sodium atom transfers its one outer-shell electron to a chlorine atom. This gives the sodium atom a full outer shell and an overall charge of +1 (Na+), and gives the chlorine atom a full outer shell and an overall charge of −1 (Cl−). The oppositely charged ions are then held together by strong electrostatic forces of attraction, forming the ionic bond.
- • 1 mark: sodium atom transfers its outer electron to a chlorine atom
- • 1 mark: sodium becomes a positive ion (Na+) with a full outer shell
- • 1 mark: chlorine becomes a negative ion (Cl−) with a full outer shell
- • 1 mark: oppositely charged ions attract electrostatically, forming the ionic bond
Q3. Chlorine has two common isotopes, chlorine-35 and chlorine-37. State what is the same and what is different about these two isotopes.2 marks · core
Answer: They have the same number of protons (and electrons) but a different number of neutrons (different nucleon number).
- • 1 mark: same proton number/number of electrons
- • 1 mark: different number of neutrons / different nucleon number
Q4. An atom has the electron arrangement 2,8,3. State the group and period this element is found in the Periodic Table.2 marks · core
Answer: Group 13 (III), Period 3 — because the outer shell has 3 electrons (giving the group) and there are 3 occupied shells (giving the period).
- • 1 mark: Group 13/III (outer shell has 3 electrons)
- • 1 mark: Period 3 (three occupied electron shells)
Key ideas to master
- Learn the precise terms, laws, and reaction patterns associated with Atoms, Elements and Compounds.
- Understand why each step or change happens instead of memorising the result only.
- Practise writing balanced equations, comparisons, or structured explanations where relevant.
- Revise common exceptions, observations, and applications that examiners often test.
Common exam prompts
- Define the main idea in Atoms, Elements and Compounds using correct chemical terminology.
- Write or interpret the reactions, observations, or comparisons that belong to this topic.
- Explain why a process happens, not just what happens.
- Summarise the high-yield facts and exceptions examiners often choose from this chapter.
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What is Atoms, Elements and Compounds in Cambridge IGCSE Year 11 Chemistry?
Elements, compounds, atomic structure, bonding and giant structures.
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