Atoms, Elements & Compounds – Core Revision

Matter & Atoms

Matter is anything that has mass and occupies space. All matter is composed of extremely small units called atoms.

Structure of the Atom

An atom has a dense central nucleus (containing protons and neutrons) surrounded by electrons that occupy energy levels (shells) around the nucleus.

Sub-Atomic Particles

• Proton – positive charge, mass ≈ $1\,\text{amu}$, located in nucleus.
• Neutron – no charge, mass ≈ $1\,\text{amu}$, located in nucleus.
• Electron – negative charge, mass ≈ $1/2000\,\text{amu}$, orbits nucleus.

Reading the Periodic Table

Each element is shown in a box that lists:
• Atomic number = $\text{number of protons}$.
• Symbol = one- or two-letter abbreviation.
• Name of the element.
• Average atomic mass (weighted average of all isotopes).
The table’s arrangement allows prediction of physical/chemical properties and bonding behavior.

Atomic Mass (Mass Number)

Atomic mass (mass number) = $\text{protons}+\text{neutrons}$.
Example: $3$ protons + $4$ neutrons ⇒ $7\,\text{amu}$.

Isotopes

Atoms of the same element (same proton count) with different neutron numbers are isotopes; they therefore have different mass numbers.

Ions

Loss or gain of electrons produces ions.
• Gain electrons ⇒ negative ion (anion).
• Lose electrons ⇒ positive ion (cation).
Proton count (atomic number) remains unchanged.

Pure Substances vs Mixtures

• Pure substance – only one type of particle: an element (identical atoms) or a compound (identical molecules).
• Mixture – two or more different particles (elements and/or compounds) physically combined.

Atoms, Molecules & Diatomic Species

• Atom – single particle of an element.
• Molecule – two or more atoms bonded together.
• Diatomic molecule – molecule with exactly two atoms of the same element (e.g. $\text{H}_2$).

Compounds & Chemical Formulae

When atoms of different elements chemically bond, they form compounds. The chemical formula indicates:
• Elements present.
• Number of atoms of each element within one molecule.
Atoms in a compound are always in a fixed ratio; e.g. water is always $2:1$ hydrogen to oxygen ($\text{H}_2\text{O}$).

Chemical Bonds & Decomposition Reactions

• Chemical bond – strong force holding atoms in a molecule.
• Decomposition reaction – breaks a compound into simpler substances.
– Electrolysis: uses electrical energy.
– Thermal decomposition: uses heat energy.

Quick Reference Equations

$\text{Mass number}=p+n$
$\text{Charge of ion}=p-e$

Use these fundamentals for predicting properties, calculating masses, and understanding the behavior of matter.