High School Chemistry - Quick Notes
Important topics, rules and equations for fast revision
1. Atomic Structure
• Atom = nucleus (protons + neutrons) with electrons in shells. Atomic number (Z) = protons. Mass
number (A) = protons + neutrons.
• Electrons fill shells 2, 8, 8 (first 20 elements). Valence electrons decide chemical behaviour. Isotopes:
same protons, different neutrons (e.g. C-12 and C-14).
• Relative atomic mass is the average mass of isotopes compared with 1/12 of carbon-12.
Term / Concept Key Point
Proton Mass 1, charge +1
Neutron Mass 1, charge 0
Electron Mass 1/1840, charge -1
Example: sodium Z = 11, A = 23, electrons 2,8,1
2. Periodic Table
• Elements are arranged by increasing atomic number. Groups (columns) have the same valence
electrons; periods (rows) have the same number of shells.
• Across a period: metallic character decreases, atomic size decreases, electronegativity increases.
Down a group: atomic size increases, reactivity of metals increases, reactivity of non-metals decreases.
Term / Concept Key Point
Soft, reactive, react with water to give hydroxide + hydrogen;
Group 1: Alkali metals
reactivity Li < Na < K
Diatomic, reactivity decreases down; F2 and Cl2 gases, Br2
Group 7: Halogens
liquid, I2 solid
Group 0: Noble gases Unreactive; full outer shell; used in lights and balloons
Transition metals Coloured compounds, catalysts, form ions of different charges
3. Chemical Bonding
• Ionic bond: transfer of electrons between metal and non-metal (NaCl). High melting point, conducts
when molten or dissolved.
• Covalent bond: sharing of electrons between non-metals (H2O, CO2, CH4). Low melting point, usually
non-conductors.
• Metallic bond: positive ions in a sea of delocalised electrons. Conducts electricity, malleable, ductile.
• Giant covalent: diamond (hard) and graphite (conducts, slippery, layers). Hydrogen bond: strong
attraction between H and N, O or F (explains water's high boiling point).
4. Moles & Stoichiometry
• One mole contains 6.02 x 1023 particles (Avogadro number). Molar volume of any gas at STP = 22.4
dm3 (24 dm3 at room temperature).
High School Chemistry - Quick Notes | Page 1
Important topics, rules and equations for fast revision
1. Atomic Structure
• Atom = nucleus (protons + neutrons) with electrons in shells. Atomic number (Z) = protons. Mass
number (A) = protons + neutrons.
• Electrons fill shells 2, 8, 8 (first 20 elements). Valence electrons decide chemical behaviour. Isotopes:
same protons, different neutrons (e.g. C-12 and C-14).
• Relative atomic mass is the average mass of isotopes compared with 1/12 of carbon-12.
Term / Concept Key Point
Proton Mass 1, charge +1
Neutron Mass 1, charge 0
Electron Mass 1/1840, charge -1
Example: sodium Z = 11, A = 23, electrons 2,8,1
2. Periodic Table
• Elements are arranged by increasing atomic number. Groups (columns) have the same valence
electrons; periods (rows) have the same number of shells.
• Across a period: metallic character decreases, atomic size decreases, electronegativity increases.
Down a group: atomic size increases, reactivity of metals increases, reactivity of non-metals decreases.
Term / Concept Key Point
Soft, reactive, react with water to give hydroxide + hydrogen;
Group 1: Alkali metals
reactivity Li < Na < K
Diatomic, reactivity decreases down; F2 and Cl2 gases, Br2
Group 7: Halogens
liquid, I2 solid
Group 0: Noble gases Unreactive; full outer shell; used in lights and balloons
Transition metals Coloured compounds, catalysts, form ions of different charges
3. Chemical Bonding
• Ionic bond: transfer of electrons between metal and non-metal (NaCl). High melting point, conducts
when molten or dissolved.
• Covalent bond: sharing of electrons between non-metals (H2O, CO2, CH4). Low melting point, usually
non-conductors.
• Metallic bond: positive ions in a sea of delocalised electrons. Conducts electricity, malleable, ductile.
• Giant covalent: diamond (hard) and graphite (conducts, slippery, layers). Hydrogen bond: strong
attraction between H and N, O or F (explains water's high boiling point).
4. Moles & Stoichiometry
• One mole contains 6.02 x 1023 particles (Avogadro number). Molar volume of any gas at STP = 22.4
dm3 (24 dm3 at room temperature).
High School Chemistry - Quick Notes | Page 1
