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A substance that is made up of two or more different elements combined together chemically.

Octet Rule

States that when bonding occurs, atoms tend to reach an electron arrangement with eight electrons in the outermost shell.

Exceptions to the octet rule

1. Transition metals can have more or less than eight electrons in their outermost energy level.
2. The elements near helium in the Periodic Table tend to achieve the electronic arrangement of helium with two electrons in the outer shell rather than eight.


A charged atom or group of atoms.


Another name for positive ions


Another name for negative ions

Ionic bond

The force of electrostatic attraction between oppositely charged ions in a compound.

Ionic substances

1. Contains a network of ions in the crystal.
2. Usually hard and brittle.
3. Have high melting points and boiling points.
4. Usually solid at room temperature.
5. Conduct electricity in molten state or when dissolved in water

Crystal lattice

The three dimensional structural arrangement of ions

Chemical formula

A way of representing a compound using symbols for the atoms present and numbers to show how many atoms of each element are present


Ending for compounds which contain just two elements


Ending for compounds which contain oxygen as well as the other two elements

Transition metal

An element that forms at least one ion with a partially filled d sublevel


A group of atoms joined together. It is the smallest particle of an element or compound that can exist independently.


The number of atoms of hydrogen or any other monovalent element with which each atom of the element combines

Covalent bond

The chemical bond formed by sharing a pair of electrons

Single bond

Formed when one pair of electrons is shared

Double bond

Formed when two pairs of electrons are shared

Triple bond

Formed when three pairs of electrons are shared

Sigma bond

Formed by the head on overlap of the two atomic orbitals (s or p)

Pi bond

Formed by the sideways overlap of two p orbitals

Covalent substances

1. Contains individual molecules.
2. Usually soft.
3. Have low melting and boiling points.
4. Usually liquids, gases or soft solids at room temperatures.
5. Do not conduct electricity


The relative attraction that an atom in a molecule has for the shared pair of electrons in a covalent bond.


Scientist who studied the amounts of energy needed to break certain bonds and so set up a scale of relative values of electronegativity.

Non-polar covalent

Bond where the electrons are equally shared. Electronegativity difference less than 0.4

Polar covalent

Bond where the electrons are not equally shared. Electronegativity difference between 0.4 and 1.7 inclusive


Bond with an electronegativity difference greater than 1.7


Types of bonds hold atoms or ions together within the molecule or crystal lattice.


Types of bonds that exist between one molecule and another.

Van der Waals forces

Weak attractive forces between molecules resulting from the formation of temporary dipoles. (The only forces of attraction that exist between non polar molecules. Their strength increases as the molecules get bigger due to the greater number of electrons in the electron clouds allowing the temporary dipoles to form more easily.)

Dipole dipole forces

Forces of attraction between the negative pole of one molecule and the positive pole of another. (Give rise to higher boiling points than those of similar non polar molecules.)

Hydrogen bonds

Particular types of dipole-dipole attractions between molecules in which hydrogen atoms are bonded to nitrogen, oxygen or fluorine. (The hydrogen atom acts as a bridge between two electronegative atoms. The boiling points of H2O, HF and NH3 are much higher than other hydrogen compounds.)

N O and F

Elements which will undergo hydrogen bonding with hydrogen

VSEPR Theory

Valence Shell Electron Pair Repulsion Theory - used to determine the shapes of molecules


2 bond pairs. Bond angle = 180°

Triangular planar

3 bond pairs. Bond angle = 120°


4 bond pairs. Bond angle = 109.5°


3 bond pairs and 1 lone pair. Bond angle = 107°


2 bond pairs and 2 lone pairs. Bond angle = 104.5°

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