Cambridge AS & A Level Chemistry — Topics 1–6: General Chemistry
Cambridge AS & A Level Chemistry
General Chemistry
Topics 1–6: Atomic Structure | Electrons in Atoms | Stoichiometry | Chemical Bonding | States of Matter | Enthalpy Changes
Topic 1 — Atomic Structure
1.1 Elements and Atoms
Definitions: An ELEMENT is a pure substance made of only one type of atom and cannot be broken down into
simpler substances by chemical means. An ATOM is the smallest particle of an element that retains the chemical
properties of that element.
Term Definition Example
Pure substance; one type of atom; cannot be
Element Oxygen O, Carbon C, Iron Fe
chemically decomposed
Two or more elements chemically bonded in
Compound Water H2O, Sodium chloride NaCl
fixed ratio; can be decomposed chemically
Two or more substances physically combined;
Mixture variable composition; can be separated by Air, seawater, crude oil
physical means
Smallest particle of an element retaining its
Atom H atom, O atom, Fe atom
chemical properties; electrically neutral
Atom or group of atoms with net charge due to
Ion Na+ (lost 1e-); Cl- (gained 1e-); SO42-
loss or gain of electrons
Two or more atoms covalently bonded; can be
Molecule O2, H2O, CO2, CH4
same or different elements
Atoms of the SAME element with the SAME 12C and 13C and 14C; 35Cl and 37Cl;
Isotopes number of protons but DIFFERENT numbers 1H (protium) and 2H (deuterium) and
of neutrons 3H (tritium)
fjtdlsg. AS & A Level Chemistry (9701). Page 1 of 37
,Cambridge AS & A Level Chemistry — Topics 1–6: General Chemistry
1.2 Inside the Atom — Subatomic Particles
Subatomic Particles
Relative Relative Actual Charge
Particle Symbol Location Actual Mass (kg)
Mass Charge (C)
+ 1.602 ×
Proton p+ Nucleus 1 +1 1.673 × 10-27
10-19
Neutron n0 Nucleus 1 0 1.675 × 10-27 0
Shells / orbitals 1/1836 - 1.602 ×
Electron e- -1 9.109 × 10-31
around nucleus (~0.0005) 10-19
Feature Detail
Size of atom Approximately 1 × 10-10 m (0.1 nm) diameter
Approximately 1 × 10-15 m — about 100,000 times smaller than the atom; contains
Size of nucleus
almost all the mass
Number of protons in the nucleus; defines the element; same as number of
Atomic number (Z)
electrons in neutral atom
Mass number (A) Total number of protons + neutrons (nucleons) in the nucleus
Neutron number (N) N = A - Z (number of neutrons = mass number - atomic number)
AZX where A = mass number, Z = atomic number, X = element symbol. e.g. 126C
Nuclide notation
(carbon-12)
Charge of nucleus = +Z (due to Z protons; each with charge +1)
Neutral atom Number of electrons = number of protons; overall charge = zero
A* Note: The mass of an electron is so small (~1/1836 of a proton) that electrons contribute negligible mass to
the atom. Almost all atomic mass is in the nucleus (protons + neutrons). The atom is mostly empty space.
fjtdlsg. AS & A Level Chemistry (9701). Page 2 of 37
,Cambridge AS & A Level Chemistry — Topics 1–6: General Chemistry
1.3 Numbers of Nucleons — Isotopes
Isotopes: Isotopes are atoms of the same element (same Z, same number of protons) with different mass
numbers (different numbers of neutrons). Isotopes have identical chemical properties (same electron
configuration) but different physical properties (different mass → different density, rate of diffusion, melting point).
Isotope Symbol Protons Neutrons Electrons Common Use / Notes
Defines atomic mass unit (1/12 its mass =
Carbon-12 12C 6 6 6
1 u); used as reference standard
NMR spectroscopy (13C NMR); stable
Carbon-13 13C 6 7 6
isotope
Radioactive; used in radiocarbon dating
Carbon-14 14C 6 8 6
(half-life 5730 years)
Chlorine-35 35Cl 17 18 17 ~75% natural abundance
~25% natural abundance; explains Ar(Cl)
Chlorine-37 37Cl 17 20 17
= 35.5
Fissile; used in nuclear reactors and
Uranium-235 235U 92 143 92
weapons
~99.3% natural abundance; not fissile;
Uranium-238 238U 92 146 92
used in fast breeder reactors
Hydrogen-1
1H 1 0 1 ~99.98% of hydrogen; normal hydrogen
(protium)
Hydrogen-2
2H or D 1 1 1 ~0.015%; heavy water D2O
(deuterium)
Hydrogen-3 Radioactive; extremely rare; used in
3H or T 1 2 1
(tritium) nuclear weapons
Why isotopes have the same chemical properties: Chemical properties are determined by the NUMBER AND
ARRANGEMENT OF ELECTRONS. Isotopes have the same number of protons → same number of electrons in
neutral atom → same electron configuration → same chemistry.
fjtdlsg. AS & A Level Chemistry (9701). Page 3 of 37
, Cambridge AS & A Level Chemistry — Topics 1–6: General Chemistry
Topic 2 — Electrons in Atoms
2.1 Simple Electronic Structure
Definition: Electrons occupy energy levels (shells) around the nucleus. Shell 1 is closest to and most strongly
attracted by the nucleus (lowest energy). The maximum number of electrons in shell n = 2n2.
Shell (n) Max electrons (2n2) Energy relative to nucleus Elements that fill this shell
1 2 Lowest (closest, most attracted) H, He
2 8 Medium Li to Ne
8 (first 8); up to 18 with Na to Ar (first 8); then transition
3 Higher
d-orbitals metals
4 32 (2×42) Highest (furthest) K onwards
Element Z Simple Electronic Structure Shorthand
Hydrogen 1 1 1
Helium 2 2 2
Lithium 3 2,1 2.1
Carbon 6 2,4 2.4
Nitrogen 7 2,5 2.5
Oxygen 8 2,6 2.6
Neon 10 2,8 2.8
Sodium 11 2,8,1 2.8.1
Chlorine 17 2,8,7 2.8.7
Argon 18 2,8,8 2.8.8
Calcium 20 2,8,8,2 2.8.8.2
fjtdlsg. AS & A Level Chemistry (9701). Page 4 of 37
Cambridge AS & A Level Chemistry
General Chemistry
Topics 1–6: Atomic Structure | Electrons in Atoms | Stoichiometry | Chemical Bonding | States of Matter | Enthalpy Changes
Topic 1 — Atomic Structure
1.1 Elements and Atoms
Definitions: An ELEMENT is a pure substance made of only one type of atom and cannot be broken down into
simpler substances by chemical means. An ATOM is the smallest particle of an element that retains the chemical
properties of that element.
Term Definition Example
Pure substance; one type of atom; cannot be
Element Oxygen O, Carbon C, Iron Fe
chemically decomposed
Two or more elements chemically bonded in
Compound Water H2O, Sodium chloride NaCl
fixed ratio; can be decomposed chemically
Two or more substances physically combined;
Mixture variable composition; can be separated by Air, seawater, crude oil
physical means
Smallest particle of an element retaining its
Atom H atom, O atom, Fe atom
chemical properties; electrically neutral
Atom or group of atoms with net charge due to
Ion Na+ (lost 1e-); Cl- (gained 1e-); SO42-
loss or gain of electrons
Two or more atoms covalently bonded; can be
Molecule O2, H2O, CO2, CH4
same or different elements
Atoms of the SAME element with the SAME 12C and 13C and 14C; 35Cl and 37Cl;
Isotopes number of protons but DIFFERENT numbers 1H (protium) and 2H (deuterium) and
of neutrons 3H (tritium)
fjtdlsg. AS & A Level Chemistry (9701). Page 1 of 37
,Cambridge AS & A Level Chemistry — Topics 1–6: General Chemistry
1.2 Inside the Atom — Subatomic Particles
Subatomic Particles
Relative Relative Actual Charge
Particle Symbol Location Actual Mass (kg)
Mass Charge (C)
+ 1.602 ×
Proton p+ Nucleus 1 +1 1.673 × 10-27
10-19
Neutron n0 Nucleus 1 0 1.675 × 10-27 0
Shells / orbitals 1/1836 - 1.602 ×
Electron e- -1 9.109 × 10-31
around nucleus (~0.0005) 10-19
Feature Detail
Size of atom Approximately 1 × 10-10 m (0.1 nm) diameter
Approximately 1 × 10-15 m — about 100,000 times smaller than the atom; contains
Size of nucleus
almost all the mass
Number of protons in the nucleus; defines the element; same as number of
Atomic number (Z)
electrons in neutral atom
Mass number (A) Total number of protons + neutrons (nucleons) in the nucleus
Neutron number (N) N = A - Z (number of neutrons = mass number - atomic number)
AZX where A = mass number, Z = atomic number, X = element symbol. e.g. 126C
Nuclide notation
(carbon-12)
Charge of nucleus = +Z (due to Z protons; each with charge +1)
Neutral atom Number of electrons = number of protons; overall charge = zero
A* Note: The mass of an electron is so small (~1/1836 of a proton) that electrons contribute negligible mass to
the atom. Almost all atomic mass is in the nucleus (protons + neutrons). The atom is mostly empty space.
fjtdlsg. AS & A Level Chemistry (9701). Page 2 of 37
,Cambridge AS & A Level Chemistry — Topics 1–6: General Chemistry
1.3 Numbers of Nucleons — Isotopes
Isotopes: Isotopes are atoms of the same element (same Z, same number of protons) with different mass
numbers (different numbers of neutrons). Isotopes have identical chemical properties (same electron
configuration) but different physical properties (different mass → different density, rate of diffusion, melting point).
Isotope Symbol Protons Neutrons Electrons Common Use / Notes
Defines atomic mass unit (1/12 its mass =
Carbon-12 12C 6 6 6
1 u); used as reference standard
NMR spectroscopy (13C NMR); stable
Carbon-13 13C 6 7 6
isotope
Radioactive; used in radiocarbon dating
Carbon-14 14C 6 8 6
(half-life 5730 years)
Chlorine-35 35Cl 17 18 17 ~75% natural abundance
~25% natural abundance; explains Ar(Cl)
Chlorine-37 37Cl 17 20 17
= 35.5
Fissile; used in nuclear reactors and
Uranium-235 235U 92 143 92
weapons
~99.3% natural abundance; not fissile;
Uranium-238 238U 92 146 92
used in fast breeder reactors
Hydrogen-1
1H 1 0 1 ~99.98% of hydrogen; normal hydrogen
(protium)
Hydrogen-2
2H or D 1 1 1 ~0.015%; heavy water D2O
(deuterium)
Hydrogen-3 Radioactive; extremely rare; used in
3H or T 1 2 1
(tritium) nuclear weapons
Why isotopes have the same chemical properties: Chemical properties are determined by the NUMBER AND
ARRANGEMENT OF ELECTRONS. Isotopes have the same number of protons → same number of electrons in
neutral atom → same electron configuration → same chemistry.
fjtdlsg. AS & A Level Chemistry (9701). Page 3 of 37
, Cambridge AS & A Level Chemistry — Topics 1–6: General Chemistry
Topic 2 — Electrons in Atoms
2.1 Simple Electronic Structure
Definition: Electrons occupy energy levels (shells) around the nucleus. Shell 1 is closest to and most strongly
attracted by the nucleus (lowest energy). The maximum number of electrons in shell n = 2n2.
Shell (n) Max electrons (2n2) Energy relative to nucleus Elements that fill this shell
1 2 Lowest (closest, most attracted) H, He
2 8 Medium Li to Ne
8 (first 8); up to 18 with Na to Ar (first 8); then transition
3 Higher
d-orbitals metals
4 32 (2×42) Highest (furthest) K onwards
Element Z Simple Electronic Structure Shorthand
Hydrogen 1 1 1
Helium 2 2 2
Lithium 3 2,1 2.1
Carbon 6 2,4 2.4
Nitrogen 7 2,5 2.5
Oxygen 8 2,6 2.6
Neon 10 2,8 2.8
Sodium 11 2,8,1 2.8.1
Chlorine 17 2,8,7 2.8.7
Argon 18 2,8,8 2.8.8
Calcium 20 2,8,8,2 2.8.8.2
fjtdlsg. AS & A Level Chemistry (9701). Page 4 of 37
