TOPIC 2: ATOMS, ELEMENTS & COMPOUNDS

Atom

Smallest particle of an element that retains its chemical properties

Element

Pure substance made of one type of atom

Compound

Substance made of two or more elements chemically combined in fixed ratios

Mixture

Two or more substances physically combined, not chemically bonded

Structure of atom

Nucleus contains protons and neutrons, electrons in shells around nucleus

Proton

Charge +1, relative mass 1, found in nucleus

Neutron

No charge, relative mass 1, found in nucleus

Electron

Charge -1, negligible mass, found in shells

Atomic number

Number of protons in an atom

Mass number

Total number of protons and neutrons

Isotopes

Atoms of same element with same number of protons but different neutrons

Electronic configuration

Arrangement of electrons in shells (e.g. 2,8,1)

Ionic bonding

Electrostatic attraction between oppositely charged ions formed by electron transfer

Formation of cations

Metals lose electrons to form positive ions

Formation of anions

Non-metals gain electrons to form negative ions

Properties of ionic compounds

High melting points, conduct electricity when molten or dissolved, brittle

Covalent bonding

Sharing of electron pairs between atoms

Single covalent bond

One shared pair of electrons

Double covalent bond

Two shared pairs of electrons

Simple molecular substances

Low melting/boiling points, weak intermolecular forces, do not conduct electricity

Giant covalent structures

Large network of atoms bonded covalently, very high melting points

Examples of giant covalent structures

Diamond, graphite, silicon dioxide

Diamond structure

Each carbon bonded to four others, very hard, does not conduct electricity

Graphite structure

Layers of carbon atoms, each bonded to three others, conducts electricity

Metallic bonding

Lattice of positive ions surrounded by delocalised electrons

Properties of metals

Good conductors, malleable, ductile, high melting points