Chapter 2: Atoms, elements and compounds — Part 1

01 Elements, compounds and mixtures

Substances can be classified according to whether they are pure or mixed, and whether their composition is fixed or variable.

Element

A substance made of only one type of atom. Elements are pure substances that cannot be broken down into simpler substances by chemical means. Each element is represented by a chemical symbol (e.g. O for oxygen, Fe for iron). Some elements exist as diatomic molecules (e.g. O₂, H₂, N₂).

Compound

A substance formed when two or more different elements chemically combine in a fixed ratio. Compounds are pure substances with properties different from the elements that form them. They are represented by chemical formulas (e.g. H₂O, CO₂, NaCl) and can only be separated into elements by chemical methods.

Mixture

A combination of two or more substances (elements and/or compounds) that are not chemically bonded. The composition can vary (e.g. air, salt water, brass). Components retain their individual properties and can be separated by physical methods such as filtration, distillation, or evaporation.

02 Atomic structure

Atoms are the smallest particles of an element that take part in chemical reactions. Every atom has a small, dense nucleus surrounded by electrons in shells (energy levels).

Proton

A positively charged particle found in the nucleus. Each proton has a relative charge of +1 and a relative mass of 1. The number of protons defines the identity of the element.

Neutron

A neutral particle found in the nucleus. Each neutron has no charge and a relative mass of 1. Neutrons contribute to the mass of the atom but do not affect its chemical identity.

Electron

A negatively charged particle with a relative charge of -1 and a negligible mass (about 1/1840 of a proton). Electrons are arranged in shells around the nucleus and are involved in bonding and chemical reactions.

03 Definitions

Atomic number (proton number), Z

The number of protons in the nucleus of an atom. It determines the identity of the element and its position in the periodic table. In a neutral atom, the number of electrons equals the atomic number.

Mass number (nucleon number), A

The total number of protons and neutrons in the nucleus. It is written as a superscript before the element symbol (e.g. ²³₁₁Na). The number of neutrons = A - Z.

04 Electronic configuration and the periodic table

Electrons occupy shells around the nucleus. The first shell holds a maximum of 2 electrons; the second shell holds a maximum of 8 electrons; the third shell holds a maximum of 8 electrons (for the first 20 elements).

Groups

Vertical columns in the periodic table. Elements in the same group have the same number of outer-shell electrons and therefore show similar chemical properties. Group numbers (I–VII) correspond to the number of valence electrons (Group I = 1, Group VII = 7).

Periods

Horizontal rows in the periodic table. The period number equals the number of occupied electron shells in the atoms of elements in that row.

Group VIII (noble gases)

Elements with a full outer shell of electrons (except helium, which has 2). They are very unreactive because they already have a stable electron arrangement.

Metallic character

Increases down a group (outer electrons are further from the nucleus and easier to lose). Decreases across a period from left to right (atoms hold onto electrons more strongly as nuclear charge increases).