Chem Notes

Page 1

Reverse sublimation

Elements and atomic structure

·

An element is a pure substance made up of only one type

of atom.

·

An atom is the smallest particle of an element that can

exist.

·

An

atom contains the sub-atomic particles protons, neutrons and electrons

sub-atomic

particle

relative

charge

relative

mass

location

proton

+1

1

nucleus

neutron

0

1

nucleus

electron

-1

1/2000

shells around the nucleus

·

Atomic

(proton) number = number of protons

(this is also equal to the number of electrons

for atoms)

·

Mass

number = number of protons + number

of neutrons

·

Therefore

the number of neutrons = mass number – atomic number

·

Isotopes are atoms with the same number of protons

but a different number of neutrons

e.g. chlorine has atoms with mass numbers 35

and 37 in a ratio of 3:1 (75% 35Cl and only 25% 37Cl)

The relative atomic mass of chlorine is

taken as the average of these two types of atom BUT the average must be

weighted to take into account the fact that the isotopes are present in

different proportions i.e. there are more 35Cl atoms than 37Cl.

To calculate the weighted average mass: (75 x 35)

+ (25 x 37) = 35.5

100

·

Atomic

(proton) number = number of protons

(this is also equal to the number of electrons

for atoms)

·

Mass

number = number of protons + number

of neutrons

·

Therefore

the number of neutrons = mass number – atomic number

·

Isotopes are atoms with the same number of protons

but a different number of neutrons

e.g. chlorine has atoms with mass numbers 35

and 37 in a ratio of 3:1 (75% 35Cl and only 25% 37Cl)

The relative atomic mass of chlorine is

taken as the average of these two types of atom BUT the average must be

weighted to take into account the fact that the isotopes are present in

different proportions i.e. there are more 35Cl atoms than 37Cl.

To calculate the weighted average mass: (75 x 35)

+ (25 x 37) = 35.5

100

·

An

ionic bond is formed when a metal reacts with a non-metal

·

Ionic

substances are always compounds

·

The

metal atom gives up electrons from its outer shell and becomes a positively

charged ion (CATION); the non-metal atom gains these electrons into its outer

shell and becomes a negatively charged ion (ANION)

·

An ion is a charged particle formed by the loss or gain

of electrons from an atom.

·

Ions

are charged because they either have more electrons than protons (this makes

them negatively charged) or fewer electrons than protons (this makes them

positively charged)

·

An

ionic bond is the electrostatic

attraction between oppositely charged ions

·

Ionic

bonding can be shown using dot and cross diagrams:

·

Covalent

bonding occurs between two non-metals

·

Both

atoms achieve a full outer shell by SHARING

PAIRS of electrons

·

Each

shared pair of electrons is a single

covalent bond

·

This

type of bonding does not involve the

transfer of electrons and so ions are NOT formed.

·

The

electrostatic attraction is between the nuclei (of both atoms) and the shared pairs

of electrons

·

A

covalent substance could either be an element e.g. H2 or a compound

e.g. H2O

·

Many

covalent substances are simple molecules

·

Dot

and cross diagrams can be used to show covalent bonding

Particle Theory

Elements and atomic structure

Ionic Bonding

Covalent Bonding