2. Nucleus contains protons and neutrons The electron are found in shells or energy levels. The maximum of 2 electrons in the first shell, 8 electrons in the 2 nd shell and 8 in the 3 rd shell. The way the electrons are arranged is known as the electron configuration. A quick way of showing how the electrons are arranged in potassium is writing 2,8,8,1.

3. Sub atomic particles 0 (nearly) -1 Electron 1 0 Neutron 1 +1 Proton Mass (in mass units – v. small) Charge Particle

4. The atomic number or proton number tells us how many protons are in the atom. In an atom this always equals the number of electrons. The mass number is equal to the number of protons + the number of neutrons The number of neutrons = mass number – atomic number

5. What is an isotope?? Some isotopes are radioactive – some are not. Radioisotopes can be useful e.g. power stations, tracing pollution and dating objects. Isotopes have the same number of electrons & protons but a different number of neutrons. Cl Cl 35 37 17 17 This atom has 18 neutrons This atom has 20 neutrons

6. RAM You may find that chlorine’s mass number is 35.5 – how do you get ½ a neutron? The answer is you don’t – that is called the Relative Atomic Mass or RAM Relative Atomic Mass or RAM is the average mass of the isotopes. Ahh! I hear you cry, but the average of 35 & 37 is 36!!! TRUE – AREN’ T YOU CLEVER BUT you must take account of how much of each isotope there is: Any sample of Cl contains 75% 35Cl and 25% 37Cl. SO – ((75x35)+(25x37))/100=35.5

7. Now try this!!! Write the following words in a list and write down what they mean – check the spelling carefully. Atomic number, mass number, relative atomic mass and isotope.