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

	Created by Oliver H over 11 years ago

KS4 (GCSE) Year 7 (Chemistry) Note on Chem Notes, created by Oliver H on 30/04/2013.