2: Atoms, elements and compounds

Element

A substance containing only one type of atom
A substance that cannot be further divided into simpler substances by chemical methods
Made of one type of atom
E.g. Gold (Au) is an element as it is made of only gold atoms

Compound

A substance containing atoms of two or more elements chemically bonded together
A substance made up of two or more elements, chemically bonded together
Compounds cannot be separated physically
E.g. H20 (water) is a compound made of one oxygen atom and two hydrogen atoms

Molecule

Two or more atoms chemically bonded together
Atoms in a molecule may or may not be the same element

Mixture

A combination of two or more substances (elements or compounds) physically mixed together but not chemically bonded
E.g. salt water is a mixture of sodium chloride (NaCl) and water (H2O)
The components in a mixture keep their individual properties
Can be separated by physical method (e.g. filtration, distillation, evaporation)

Isotope

Different atoms of the same element the same number of protons, different different number of neutrons

Relative atomic mass

(mass no. A x percentage A)+ (mass no. B x percentage B)/100.The average mass of the isotopes of an element (compared to 1/12th of the mass of a Carbon 12 atom)

Ionic bond

An ionic bond is a strong electrostatic attraction between oppositely charged ions. It occurs between metal and non-metals atoms.

Ionic Compounds

giant lattice structure, a regular arrangement of alternating positive and negative ions. High melting/boiling points because strong electrostatic bonds between oppositely charged ions. Good electrical conductivity when aqueous or molten and poor when solid: In their solid form, ions in ionic compounds are stuck in place, so they can't conduct electricity. But when you dissolve them in water or melt them, the ions can move around and carry electrical charge, so they become good conductors in these states.

Covalent bonds

Pair of electrons shared between two atoms
Occurs in elements and compounds containing non-metallic elements only. Covalently bonded atoms are referred to as molecules
All the atoms in a molecule have a full outer shell of electrons.

Simple molecular compounds

low melting points and boiling points due to weak intermolecular forces of attraction between molecules. poor electrical conductivity because Simple molecular compounds are made of uncharged molecules

C2H4

O2

CO2

N2

H2 bond

Cl2 bond

H2O bond

CH4 bond

NH3 bond

HCl bond

Diamond

Tetrahedral network, Each carbon atom forms covalent bond with 4 other carbon atoms. Cannot conduct electricity because no free electrons. Extremely hard, used in cutting tools.

Graphite

Layers of hexagonal rings, each carbon atom bonded ton3 other carbon atoms. Can conduct electricity because of delocalized electrons within layers. Electrode, allows electricity to flow. Lubricant, layers can slide over each other.

Silicon (IV) Oxide or SiO2

each silicon atom bonds to 4 other oxyten atoms, each oxygen atom bonds to 2 silicon atoms. Tetrahedral network.

Similarities between diamond and SiO2

High hardness, High melting and boiling points,Non-conductivity.

Metallic bonding

electrostatic attraction between the positive ions in a giant metallic lattice and a ‘sea of delocalised electrons’. Malleable and ductile because metal ions slide past each other while the metallic bond remains intact.’