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AS CHEMISTRY UNIT 1 DEFINITIONS

Atomic Number, Z The number of protons in the nucleus of an atom

Mass Number, A The total number of protons plus neutrons in the nucleus of an atom

IsotopesAtoms of the same element (same atomic number) but different mass

numbers (different numbers of neutrons)

Abundance (of

isotopes)

Often quoted as % values, these are the proportions of each stable

isotope in the naturally occurring elements

Relative Atomic

Mass, Ar  

The average mass of an atom of an element compared with 1/12 t  the

mass of a carbon-12 atom (exactly 12 units)

Relative Molecular

Mass, Mr  

The average mass of a molecule compared with 1/12 t  the mass of a

carbon-12 atom (exactly 12 units)

Avogadro Constant,

L

The number of C atoms in exactly 12 grams of carbon-12

(Can be thought of as the number of particles in 1 mole of a susbstance)

The mole (mol)

The strict definition is: that amount of substance which contains the

 same number of “entities” (atoms, molecules or “formula units”) as

there are atoms of 12C in exactly 12 grams of that isotope. 

Think of it as Avogadro’s Number of particles. 

DO NOT CONFUSE IT WITH MOLARITY!

Molarity (M)The concentration of a solution expressed in moles per cubic decimetre

(litre). Units are mol.dm-3, often written as M

Empirical FormulaThe simplest (whole number) ratio of atoms of each element in a

compound

Molecular Formula

The actual number of atoms of each element in a molecule of the

compound. (The molecular formula is always a whole number times theempirical formula, e.g., C3H6 is 3 times CH2)

Percentage Yield

The actual mass of product obtained in a reaction expressed as a % of

the theoretical mass calculated from the balanced equation:-

% yield = actual mass of product  100

theoretical mass

Percentage Atom

Economy

Calculated from the balanced equation for the reaction:-

% atom economy = mass of desired product  100

total mass of reactants

Ideal Gas Equation

Can also be known as “the equation of state for an ideal gas” and it

relates the volume, pressure, temperature and number of moles of gas:

PV = nRT (R is the Molar Gas Constant)Remember that SI units must be used in calculations (Pa for pressure,

m3 for volume and K for temperature –  add 273 onto OC)

Metallic bonding

A giant lattice of close-packed metal ions (+ve) surrounded by a “sea”

of delocalised valence electrons. Electrostatic attraction of the cations

for the negatively charged “sea” of electrons is strong. 

Ionic bonding

Ions are formed when atoms lose or gain electrons (+ve and – ve ions

respectively). The strong electrostatic force of attraction between +ve

and – ve ions is called ionic bonding.

Covalent bond A pair of electrons shared between two atoms

Dative covalent bond Sometimes called a co-ordinate bond, this occurs when both electronsfor the covalent bond are provided by only one of the pair of atoms

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ElectronegativityThe power of an atom to withdraw electron density from a covalent

 bond

Polar bond (dipole)

This is when electrons are not shared equally between two atoms with

different electronegativity, so that one end of the bond has a permanent

slight+ charge and one end has a permanent slight

- charge

Polar molecule

(permanent dipole)

The molecule as a whole has a permanent uneven distribution of

electrons so that one end of it is always + and one end is always -.

 Note that molecules with polar bonds can be non-polar overall, if they

are symmetrical (e.g., CO2 is non-polar because it is linear, but H2O is

 polar because it is V-shaped)

Fractional

Distillation

The physical separation of a mixture of liquids which have different

 boiling points.

FractionA mixture of compounds which have similar boiling points. (For

hydrocarbons in crude oil, molecules with a similar number of C atoms.)

Hydrocarbon A compound containing only the elements carbon and hydrogen.

Homologous Series

A family of carbon compounds with similar chemical properties becausethey have the same functional group. Each member of the series differs

from the previous one by a CH2 unit and they all have the same general

formula. There is a gradual trend in physical properties (e.g., boiling

 points) down the series.

Functional Group

The part of a molecule of a (carbon) compound which is responsible for

the typical reactions of the homologous series to which the compound

 belongs, e.g., alkenes have a C=C double bond.

Structural isomers

Compounds with the same molecular formula, but with different

structural formulae, because the atoms are joined up in a different order.

There are three types of structural isomerism: chain, positional and

functional group.

Chain isomers

Compounds with the same molecular formula, but where the carbon

“backbone” of the molecule is of different lengths with side branches

(e.g. butane and methylpropane).

Positional isomers

Compounds with the same molecular formula and the same length

carbon chain, but the substituent or functional group is in a different

 place along the chain (e.g., 1-chloropropane and 2-chloropropane).

Functional group

isomers

Compounds with the same molecular formula, but the molecules have a

different functional group, e.g., C4H8 could be butene (alkene) or

cyclobutane (cyclic alkane).

CrackingThe breaking up of larger (hydrocarbon) molecules into smaller, moreuseful ones. It is an example of decomposition and can be done either

 by high temperatures or using a catalyst.

Homolysis (or

homolytic fission)

The equal splitting of a covalent bond where each atom in the bond

receives one electron. This leads to the formation of free radicals.

Heterolysis (or

heterolytic fission)

The unequal splitting of a covalent bond where one atom in the bond

takes both electrons. This leads to the formation of ions.

Free radical A species (atom, molecule or ion) with an unpaired electron.