1.3 Atomic Structure

REVISION NOTES

IGCSE Edexcel Chemistry

1.3.1 Know what is meant by the terms atom and molecule

1.3.2 Know the structure of an atom in terms of the positions, relative masses and relative charges of subatomic particles

1.3.3 Know what is meant by the terms atomic number, mass number, isotopes and relative atomic mass (Ar)

Calculation of the number of neutrons, protons and electrons

Number of protons = atomic number
Number of electrons =
- For atoms: number of protons = number of electrons (Because atom has overall zero charge)

- For ions: number of protons ≠ number of electrons
Number of neutrons = mass number – atomic number

ISOTOPES

Atoms of same element; same number of protons; different number of neutrons
They have identical chemical properties, as they have same number of electrons
Slightly different physical properties

1.3.4 Be able to calculate the relative atomic mass of an element (Ar) from isotopic abundances

ISOTOPIC ABUNDANCE

Isotopic abundance: percentage of the isotope found in a naturally occurring sample

The relative atomic mass of each element is calculated from the mass number and relative abundances of all the isotopes of a particular element

EXAMPLE

A sample of chlorine gas is a mixture of 2 isotopes, chlorine-35 and chlorine-37. These isotopes occur in specific proportions in the sample i.e. 75% chlorine-35 and 25% chlorine-37. Calculate the relative atomic mass of chlorine in the sample.