1. Structure of Atoms. Originally scientists thought that atoms could not be split into anything smaller. We now know that atoms are made up of smaller particles called: Protons Electrons Neutrons

2. Properties of protons, electrons & neutrons Protons have a charge of 1+ (one positive), and a mass of 1 a.m.u. (one atomic mass unit). Electrons have a charge of 1- (one negative) and are so light we give them a mass of 0 amu. Neutrons have no charge (neutral) and have a mass of 1 a.m.u.

3. Where in the atom are the protons, neutrons and electrons? The protons and the neutrons are found in the centre of the atom – in a place called the nucleus. The nucleus is very small. The electrons spin round the nucleus.

4. Summary so far. ParticleChargeMass (amu) Where in atom? Proton Electron Neutron 1+ 1 amuInside nucleus 1- 0 amuCircling nucleus 0 1 amuInside nucleus

5. Atoms are neutral. Atoms do not have an overall charge – they are neutral. This means that atoms must have the same number of protons (positive charges) and electrons (negative charges). nucleus with 4 protons 4 electrons spinning round the nucleus

6. Atomic Number. The atomic number tells us the number of protons in the nucleus of an atom. As atoms are neutral, the atomic number also gives the number of electrons in an atom. It is the atomic number which identifies which element is present e.g. Hydrogen atoms have an atomic number of 1 Helium atoms have an atomic number of 2

7. Mass number. The mass number tells us the number of protons + the number of neutrons in an atom. Showing atomic and mass numbers. Chemists can show the atomic and mass numbers of an element in the following way: Na Symbol of the element 11 Atomic number 23 Mass number

8. Calculating the number of protons, neutrons and electrons in an atom. Atomic Number = 35 Number of protons = 35 Number of electrons = 35 Mass number = 80 Number of neutrons = 80 – 35 = 45

9. Arrangement of electrons in atoms. The electrons in atoms are arranged in energy levels round the nucleus. The first energy level (lowest energy) can hold up to 2 electrons. Nucleus with 1 proton 1 electron in first energy level Nucleus with 2 proton + 2 neutrons 2 electrons in first energy level

10. The second electron energy level. The second electron energy level in an atom can hold up to 8 electrons. The diagram shows an atom of fluorine, Nucleus with 9 protons + 10 neutrons 2 electrons in first energy level 7 electrons in second energy level The electron arrangement in this atom would be written as 2,7

11. . The third electron energy level. The third electron energy level in an atom can also hold up to 8 electrons. Nucleus containing 15 protons and 16 neutrons. The diagram below shows an atom of 2 electrons in first energy level 8 electrons in second energy level 5 electrons in third energy level The electron arrangement is shown as 2,8,5

12. Isotopes. Isotopes are atoms having the same atomic number (number of protons) but different mass numbers (number of neutrons). Shown below are the three isotopes of hydrogen. Nucleus with 1 proton and no neutrons Nucleus with 1 proton and 1 neutron Nucleus with 1 proton and 2 neutrons