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