WHAT ARE WE LEARNING IN THIS UNIT?
• How different people have thought about
matter over time
• How we believe the atom is structured
today
• How radioactivity works and how it is used
• How we classify atoms
• How the periodic table works and why it’s
useful
• Using the periodic table to make
predictions about elements.
WHAT IS AN ATOM?
• An atom is the basic building block for all matter
• But what is matter?
– Matter is anything that takes up space and has mass
– Mass is a measure of how much stuff (matter) something
has
• Atoms make up everything you see (and can’t see) around
you
• Atoms have different properties depending on what
element they are
WHAT’S INSIDE AN ATOM?
• An atom is mostly (99.99%) empty
space
• In the centre of the atom is a
nucleus that is orbited by
electrons
• The nucleus is very dense and
contains the vast majority of the
matter of the atom
SUBATOMIC PARTICLES
• The atom is made up of three types of particles;
– Protons Proton
Charge
– Neutrons+1
Neutron
Electron
0
-1
Mass
– Electrons1 amu
1 amu
0 (~1/1840) amu
Symbol
n
e
p
• These have different properties
• We measure the masses of atoms and subatomic particles in amus or atomic
mass units because a kilogram is much larger than we need
• 1 amu = 1.66 x 10-27 kg = 0.00000000000000000000000000166 kg
ELEMENTS
• Every atom is classified by what element it is
• Each element has unique properties and are differentiated by how many
protons they have
– We call this number the atomic number
• This means, every atom with 6 protons is carbon, and every atom with 1
proton is hydrogen
• Elements also have a mass number which can be calculated by adding the
number of protons and the number of neutrons in an atom
• In order to balance the charge of the protons, atoms usually have equal
numbers of protons and electrons, so the atomic number also tells you how
many electrons the element normally has
ELECTRON SHELL DIAGRAMS
• An electron shell diagram shows how many protons, neutrons and electrons are contained in
an atom, and also shows how electrons are arranged
• The nucleus is in the middle, and shows the number of protons and neutrons while the
electrons are drawn around in shells
• Electrons are always arranged (for first 20 elements) 2, 8, 8, 2
WHAT’S WITH THE SHELLS?
• The electrons distribute themselves so neatly because each of the shells has a different energy
associated with it
• Like everything else, electrons love to be in a low energy state, and that happens to be close to the
nucleus, so they all fall in line above that
• Because higher shells have higher energy, when an atom absorbs energy, one or more of its electrons
might jump up to a higher energy state before relaxing
• When these electrons relax, they release energy, usually in the form of light
FLAME TESTS
https://www.youtube.com/watch?v=1EXr_L7Ojqg
• When we perform a flame test, we see this transition occurring very quickly in lots of different
atoms
• The exact colour of the flame depends on the energy difference between the electron shells which
is unique to each atom and so can be used to identify metals