Chemistry AS level definitions

Element

a substance made up of 1 type of atom

Compound

a substance made up of more than 1 type of atom - chemically bonded

Atom

the smallest electrically neutral particle of an element that can take part in a chmeical change

Molecule

the smallest electrically neutral particle of an element or compound that can exist on its own

The mole

the amount of substance which contains the same number of particles as there are carbon atoms in 12g of carbon-12. This number is 6.02x10(23), it's called Avogadro's number.

Molar mass

the mass in grams of 1 mole

Relative atomic mass (Ar)

the mass of a single atom on a scale of which the mass of an atom of carbon-12 is 12 atomic mass units

Amount of moles

mass(g) / molar mass

Standard enthalpy change of a reaction

the energy transferred when the molar quantities of reactants as stated in the equation react under standard conditions

Standard enthalpy change of combustion

the enthalpy change when 1 mole of the substance burns completely in oxygen under standard conditions

Standard enthalpy change of neutralisation

the enthalpy change when the amounts of acid and alkali in the equation for the reaction neutralise each other under standard conditions

Standard enthalpy of formation

the enthalpy change when 1 mole of the compound forms from its elements under standard conditions with the elements and the compound in their standard (stable) state

Standard enthalpy change of atomisation

the enthalpy change when 1 mole of gaseous atoms is formed from the element under standard conditions

Hess's Law

says that the enthalpy change in converting reactants to products is the same regardless of the route taken, provided the initial and final conditions are the same

Specific heat capacity

the energy needed to raise the temperature of 1g of the material by 1K

Bond enthalpy

the energy required to break 1 mole of the bonds in a substance in the gaseous state

Atomic number

the number of protons in the nucleus of an atom

Mass number

the number of protons and neutrons in the nucleus of an atom

Molar mass

the mass of 1 mole of a chemical - unit is g mol-1

Molecular formula

shows the numbers of atoms of the different elements in 1 molecule of a compound

Empirical formula

shows the simplest whole ratio of the atoms of different elements in a compound

Isotope

are atoms of the same element with the same atomic number, but different mass numbers

Relative isotopic mass

the mass of 1 atom of an isotope relative to the mass of 1 carbon atom of carbon-12 for which the relative mass is exactly 12

Relative atomic mass

the weighted average mass of an atom of an element relative to the mass of 1 atom of carbon-12, for which the relative mass is defined as exactly 12

Relative molecular mass (Mr)

the sum of the relative atomic masses of all the atoms in its molecular formula

Relative formula mass

the sum of the relative atomic masses of all the atoms in its formula

1st electron affinity

ther energy change when each atom in 1 mole of gaseous atoms gains 1 electron to form 1 mole of gaseous ions with a single negative charge
the gain of the first e- is exothermic, but the gain of the second e- is endothermic

Lattice energy

the standard enthalpy change when 1 mole of the compound forms from free gaseous atoms

Polarisation

the distortion of the electron cloud in a molecule or ion by a nearby charge

Dative covalent bond

a bond in which 2 atoms share a pair of electrons, both the electrons being donated by 1 atom

Functional group

the group of atoms which gives an organic compound its characteristic properties and reactions

Structural formula

shows in minimal detail which atoms, or groups of atoms, are attached to each other in 1 molecule of a compound

Displayed formula

shows all the atoms and all the bonds between them in 1 molecule of a compound

Skeletal formula

shows the functional groups fully, but the hydrocarbon part of a molecule simply as lines between carbon atoms, omitting the symbols for carbon and hydrogen atoms

Structural isomers

compounds with the same molecular formula but different strutural formulae

Free radicals

highly reactive single atoms, or groups of atoms, with unpaired electrons

Nucleophiles

ions or molecules with a lone pair of electrons that attack positive ions or positive centres in molecules

Electrophiles

reactive ions and molecules which attack negative ions or negative centres in molecules

Aliphatic

straight-chain and branched hydrocarbons with no rings of carbon atoms

Alicyclic

hydrocarbons with at least 1 ring of carbon atoms

Arenes

hydrocarbons with a ring or rings of carbon atoms in which there are delocalised electrons

Substitution reaction

a reaction where an atom, or group of atoms, is replaced (substituted) by another atom, or group of atoms

Initiation

the step which produces free radicals

Propagation

steps which form products and more free radicals

Termination

steps which remove free radicals by turning them into molecules

Sustainable lifestyle

involves living within the means of the environment in order that the Earth's natural resources are available for future generations

Unsaturated

compounds contain 1 or more double or triple bonds between atoms in their molecules

Cis and trans isomers

molecules with the same molecular formula, the same structural formula but different displayed formula

Addition reaction

a reaction in which 2 molecules add together to form a single product

Intermediates

atoms, molecules, ions or free-radicals which do not appear in the overall equation for a reaction, but which are formed during 1 step of a reaction and then used up in the next

Inductive effect

describes the way in which electrons are either pushed towards or pulled away from a carbon atom by the atoms or groups which it is bonded

Addition polymerisation

an addition reaction in which small molecules, called monomers, join together forming a giant molecule, called a polymer