1.1 Atomic Structure

Atom

simplest particle of an element that can exist on its own in stable environment

Ancient greek model

Atoms are tiny solid spheres which cannot be divided

JJ Thompson

Discovered electron and developed "plum-pudding" model

Plum pudding model

sphere of positive charge, with negatively charged electrons embedded in it

Rutherford

Carried out alpha particle scattering experiment and developed nuclear model

Alpha particle scattering experiment

directed beam of alpha particles at thin sheet of gold foil suspended in vacuum, tiny flash is emitted if it hits screen

Observations from experiment

most particles passed straight through foil but small number were deflected by large angles or straight back

Conclusions from experiment

mass of atom is concentrated at the centre (nucleus) that had a positive charge

Nuclear model of the atom

atom is mostly empty space with positively charged centre containing most the mass and electrons orbiting

Niels Bohr

suggested electrons orbit nucleus at specific distances

James Chadwick

discovered the neutron leading to todays model

Why was the neutron hard to discover?

has no charge

Proton, charge and mass

positively charged particle found in the nucleus which defines the atom

+1, 1

Neutron, charge and mass

neutral particle found in the nucleus

0, 1

Electron, charge and mass

negatively charged particle found orbiting the nucleus in shells

-1, 1/1840

Why are atoms neutral?

same number of protons and electrons that cancel each other out

Atomic number (Z)

number of protons in (the nucleus of) an atom

Mass number (A)

total number of protons and neutrons in (the nucleus of) an atom

How to calculate neutrons

mass number - atomic number

Average atomic radius

0.1 nm (1 × 10^-10 m)

Average radius of an atomic nucleus

10,000 times smaller than an atom (1 x 10^-14 m)

Shells can hold

2, 8, 8, 2

Number of shells =

Period on the periodic table

What are valence electrons?

Outer shell electrons which are involved in reactions

Number of valence electrons =

Group number on periodic table

Drawing electronic configuration

nucleus in middle, drawing circle around for each shell and electrons inside

Isotopes

atoms of same element with same number protons but different number of neutrons

Relative atomic mass formula

isotope * abundance/ total percentage abundance

Why we use relative atomic mass

to accurately represent average mass of all isotopes in typical sample