AQA GCSE Chemistry Paper 1

Atom

The atom is the fundamental building block of all matter, meaning everything around you is made from atoms.

Size of Atoms

Atoms are extremely small — about 0.1 nanometers (that's 1 x 10⁻¹⁰ meters).

Nucleus

Located at the centre of the atom; contains protons and neutrons tightly packed together.

Mass of Nucleus

The nucleus carries most of the atom's mass but occupies a very small volume.

Protons

Positively charged particles (+1 charge) with a relative mass of 1 atomic mass unit (amu).

Atomic Number (Z)

The number of protons in an atom, which defines the element.

Neutrons

Neutral particles with no electric charge (0 charge) and similar mass to protons (~1 amu).

Electrons

Negatively charged particles (-1 charge) with mass about 1/1836 of a proton.

Electron Shells

Electrons move around the nucleus in electron shells or energy levels.

Mass Number (A)

Total number of protons and neutrons in the nucleus.

Isotopes

Atoms of the same element that have different numbers of neutrons.

Example of Isotopes

Carbon-12 has 6 protons and 6 neutrons; Carbon-14 has 6 protons and 8 neutrons.

Relative Atomic Mass (Ar)

The weighted average mass of all isotopes, considering their abundance.

Calculation of Ar

Ar is calculated by multiplying the mass of each isotope by its relative abundance, adding these together, then dividing by 100.

Dalton's Model

Atoms are tiny, indivisible spheres.

Thomson's Model

The 'Plum pudding' model, atom as a positive 'pudding' with electrons embedded like raisins.

Rutherford's Model

Gold foil experiment showed atom mostly empty space, with a dense positive nucleus.

Bohr's Model

Electrons orbit the nucleus in fixed shells or energy levels, not randomly.

Electron Shell Capacity

1st shell: up to 2 electrons; 2nd shell: up to 8 electrons; 3rd shell: up to 8 electrons.

Periodic Table

Arranges elements in order of increasing atomic number.

Periods in Periodic Table

Each new period means a new electron shell is started.

Groups in Periodic Table

Elements in the same group have the same number of electrons in their outer shell.

Group 0: Noble Gases

Elements: Helium (He), Neon (Ne), Argon (Ar), Krypton (Kr), Xenon (Xe), Radon (Rn) with full outer shells.

Group 1: Alkali Metals

Elements: Lithium (Li), Sodium (Na), Potassium (K), Rubidium (Rb), Cesium (Cs), Francium (Fr) with one electron in their outer shell.

Alkali Metals

They have one electron in their outer shell.

Alkali Metals Ion Formation

They lose this outer electron easily to form +1 ions (e.g., Na⁺).

Alkali Metals Reactivity

React vigorously with water, producing hydrogen gas and alkaline solutions (e.g., sodium hydroxide).

Alkali Metals Reactivity Trend

Reactivity increases down the group because outer electron is further from nucleus and less tightly held.

Alkali Metals Physical Properties

Alkali metals are soft and have low melting and boiling points compared to other metals.

Halogens

Elements: Fluorine (F), Chlorine (Cl), Bromine (Br), Iodine (I), Astatine (At).

Halogens Electron Configuration

They have seven electrons in their outer shell.

Halogens Ion Formation

They tend to gain one electron to complete their outer shell and form -1 ions (halide ions).

Halogens Molecular Form

They form diatomic molecules (e.g., Cl₂, F₂) in elemental form.

Halogens Reactivity

React with metals to form ionic salts (e.g., NaCl).

Halogens Reactivity Trend

Reactivity decreases down the group because it becomes harder to gain an electron as the outer shell is further from the nucleus.

Halogens Physical States

Physical states change down the group: gases (F, Cl), liquid (Br), solid (I).

Transition Metals

Located in the centre of the periodic table (Groups 3 to 12).

Transition Metals Properties

Hard, dense metals with high melting and boiling points.

Transition Metals Reactivity

Less reactive than alkali metals.

Transition Metals Oxidation States

Can form various oxidation states, meaning they can form ions with different charges.

Transition Metals Compounds

Often form coloured compounds.

Transition Metals Catalysts

Used as catalysts in chemical reactions (e.g., iron in ammonia production).

Periodic Table Function

The periodic table works because of the electron configuration of atoms.

Periodic Table Group Properties

Elements in the same group have similar chemical properties because they have the same number of electrons in their outer shell.

Periodicity

The pattern of repeating properties is called periodicity.

Chemical Bonds

Chemical bonds are the strong forces of attraction that hold atoms or ions together in compounds or metals.

Octet Rule

Atoms want to have a full outer electron shell, usually 8 electrons, making them more stable.

Ionic Bonding

Happens between metal atoms (which tend to lose electrons) and non-metal atoms (which tend to gain electrons).

Cation Formation

Metals lose electrons to form positive ions (cations); non-metals gain electrons to form negative ions (anions).

Ionic Bond Example

Example: Sodium (Na) has 1 electron in its outer shell and loses it → Na⁺ ion; Chlorine (Cl) has 7 electrons and gains 1 → Cl⁻ ion.

Ionic Lattice

The oppositely charged ions are held together by strong electrostatic forces of attraction in all directions, called ionic bonds.

Ionic Compounds Properties

Ionic compounds have high melting and boiling points because it requires lots of energy to break these bonds.

Ionic Compounds Conductivity

Ionic compounds conduct electricity when molten or dissolved in water because the ions are free to move and carry charge.

Covalent Bonding

Occurs when two or more non-metal atoms share pairs of electrons to complete their outer shells.

Covalent Bond Formation

Each pair of shared electrons creates a covalent bond.

Small Molecules from Covalent Bonding

Small molecules formed by covalent bonding include: H₂, Cl₂, O₂, N₂, HCl, H₂O, NH₃, CH₄.

Larger Covalent Structures

Larger covalent structures include polymers (long chains of repeating units) and giant covalent structures (diamond, graphite).

Covalent Bond

Use dot and cross diagrams or lines to show shared pairs of electrons.

Single Bond

Represents one shared pair of electrons.

Double Bond

Exists where atoms share 2 pairs of electrons.

Triple Bond

Exists where atoms share 3 pairs of electrons.

Polymers

Made of many monomers joined by covalent bonds forming long chains.

Small Molecules

Have strong covalent bonds inside molecules but weak intermolecular forces.

Melting and Boiling Points of Small Molecules

Low melting and boiling points because only weak intermolecular forces need to be broken.

Electrical Conductivity of Small Molecules

Do not conduct electricity because there are no free charged particles.

Properties of Polymers

Solids with high melting points because of strong covalent bonds in the chain.

Giant Covalent Structures

Have very high melting points as strong covalent bonds throughout must be broken.

Metallic Bonding

The strong electrostatic attraction between positive ions and delocalised electrons.

Structure of Metals

Metals consist of atoms arranged in a lattice.

Delocalised Electrons

Outer electrons of metal atoms become delocalised, meaning they are free to move.

Properties of Metals

High melting and boiling points, malleability, and good electrical and thermal conductivity.

States of Matter

Matter exists as solid, liquid, or gas depending on how strongly particles attract each other and their kinetic energy.

Melting Process

Turns solids to liquids by breaking bonds or forces holding particles in fixed positions.

Boiling Process

Turns liquids to gases by breaking forces holding particles close.

Ionic Compounds

High melting and boiling points due to strong ionic bonds in the giant lattice.

Electrical Conductivity of Ionic Compounds

Conduct electricity when molten or dissolved in water; do not conduct when solid.

Giant Covalent Structures Characteristics

Very high melting and boiling points because all covalent bonds must be broken to melt.

Diamond Structure

Each carbon bonded to 4 others in a tetrahedral structure; very hard, no free electrons.

Graphite Structure

Carbon atoms bonded to 3 others in layers; layers held by weak forces so they slide.

Graphene

A single layer of graphite—strong, light, conducts electricity, flexible.

Fullerenes

Carbon atoms arranged in hollow spheres or tubes.

Carbon nanotubes

A type of fullerene with unique properties like high strength and electrical conductivity, useful in nanotechnology, electronics, and materials science.

Nanoparticles

Particles ranging from 1 to 100 nanometers (nm) in size.

Fine particles

Particles larger than 100 nm.

Coarse particles

Particles even larger than fine particles, commonly referred to as dust.

Surface Area to Volume Ratio

As particles get smaller, their surface area to volume ratio increases significantly, making nanoparticles much more reactive.

Uses of Nanoparticles

Used in medicine (e.g., targeted drug delivery), cosmetics, catalysts, and electronics due to their large surface area improving performance.

Potential risks of Nanoparticles

Include unknown health effects if inhaled or absorbed and environmental harm due to their tiny size and reactivity.

Conservation of Mass

Means that mass is never lost or gained in a chemical reaction; the total mass of reactants equals the total mass of products.

Example of Conservation of Mass

If 10 g of hydrogen reacts with 80 g of oxygen, the total mass of water formed will be 90 g.

Balancing Chemical Equations

A chemical equation must be balanced so that there are the same number of atoms of each element on both sides.

Example of Balancing Chemical Equations

Unbalanced: H2 + O2 → H2O; Balanced: 2 H2 + O2 → 2 H2O.

Relative Formula Mass (Mr)

The sum of the relative atomic masses (Ar) of all atoms in a compound.

Example of Relative Formula Mass

For water (H2O), Mr = (2 × 1) + 16 = 18.

Percentage by Mass of an Element

Calculated using the formula: Percentage by mass = (Mass of element in formula ÷ Mr of compound) × 100.

Example of Percentage by Mass

Percentage oxygen in water = (16 ÷ 18) × 100 = 88.9%.

Mass Changes in Reactions

Mass can seem to change during a reaction if gases escape or enter if the system is not closed.

Example of Mass Changes

Burning magnesium in air might increase mass if the container is open, but stays the same in a closed system.

The Mole

A unit to count atoms, molecules, or ions; 1 mole contains 6.02 × 10²³ particles (Avogadro's number).

Mass of 1 mole

The mass of 1 mole of a substance (in grams) equals its relative formula mass (Mr).