2. Atoms All elements are made up of very small particles called atoms (look back to previous notes). All atoms in an element are the same. Atoms arent EMPTY SPACE. They are actually made up of even smaller particles called sub-atomic particles. Three sub-atomic particles make up an atom. They are called:- 1. PROTONS 2. NEUTRONS 3. ELECTRONS

3. Sub-Atomic Particles in an Atom (1) The structure of an atom is shown below. The centre of the atom is hard and is called the NUCLEUS. It contains PROTONS and NEUTRONS. The ELECTRONS are very light and are found MOVING around the outside of the nucleus. The simplistic shape of an atom is represented by a sphere (3-D), or a circle (2-D). However, the diagram above is more accurate. The outline shape of the atom is actually the flight path of the electrons.

4. Sub-Atomic Particles in an Atom (2) The 3 sub-atomic particles have different properties. Particle Mass (amu) Charge Location Symbol Proton 1 1+ Nucleus p + Neutron 1 0 Nucleus n 0 Electron 0 1- Moving e - outside nucleus ***amu = atomic mass units***

5. Sub-Atomic Particles in an Atom (3) A simplified diagram of an atom. + - - The charge on the nucleus is POSITIVE due to the PROTONS present. The ELECTRONS are NEGATIVELY charged. Atoms are said to be ELECTRICALLY NEUTRAL, because the POSITIVE charges of the PROTONS are EQUAL to the NEGATIVE charges of the ELECTRONS, and CANCEL OUT.

6. Important Numbers Atoms of different elements vary in size and mass due to them containing different number of sub-atomic particles. How do we know the numbers of sub-atomic particles different atoms contain?????? We use two very important numbers called the:- »1. ATOMIC NUMBER »2. MASS NUMBER

7. 1. The Atomic Number The ATOMIC NUMBER of an atom is equal to the number of PROTONS in the nucleus of the atom, which is the same as the POSITIVE charge on the nucleus. In a NEUTRAL ATOM, the ATOMIC NUMBER is also the same as the number of ELECTRONS. The ATOMIC NUMBER, is the number an atom has on the periodic table. 17 Cl The Atomic Number can be represented like this.

8. 2. The Mass Number The MASS NUMBER of an atom is the same as the TOTAL number of PROTONS and NEUTRONS in the NUCLEUS of an atom. 35 Cl 17 Mass number - Atomic number = Number of neutrons Mass number always goes here. PROTON + NEUTRONS Atomic number PROTONS = ELECTRONS

9. Using this information

10. 2. The Mass Number (cont) 7 LiThis atom has:- 3 protons (3+ charge on nucleus) 3 3 electrons 4 neutrons 39 K This atom has:- 19 protons (19+ charge on nucleus) 19 19 electrons 20 neutrons All atoms except 1 have both protons and neutrons in the nucleus. H 1

11. Isotopes ISOTOPES are atoms of the SAME element, with the SAME ATOMIC NUMBER but have a DIFFERENT MASS NUMBER ( This means a DIFFERENT NUMBER of NEUTRONS in the NUCLEUS). A machine called a MASS SPECTROMETER is used to show that most elements contain a mixture of isotopes. For example, CHLORINE is made up of 2 isotopes and HYDROGEN is made up of 3 isotopes. 35 37 1 2 3 Cl and Cl H, H and H 17 17 1 1 1

12. Electron Arrangement (how electrons are arranged in atoms) (1) Electrons move around (orbit) the outside of a nucleus. The electrons do NOT randomly do this. They orbit in neatly arranged in ENERGY LEVELS or SHELLS. + Third Energy Level has the highest energy Second Energy Level First energy level has the lowest energy Nucleus

13. Electron Arrangement (2) Imagine the energy levels/shells of an atom to be like the layers of an onion, with each layer only holding a MAXIMUM number of electrons. The diagram below is called a TARGET PICTURE. + FIRST SHELL - Can hold a maximum of 2 electrons SECOND SHELL - Can hold a maximum of 8 electrons THIRD SHELL - Can hold a maximum of 8 electrons

14. Electron Arrangement (3) Electrons dont fill energy levels in a random way. They fill the first energy level, then the second and so on, until all the electrons are inside a level. Electrons are put into a shell singly at first (at the 4 points of a clock face), then paired up. This is shown below. 7+ This atom is nitrogen, because its atomic number 7 (7 positive charges). The shorthand way of writing this electron arrangement is:- 2, 5

15. Electron Arrangement (4)

16. Electron Arrangement (5) A notation exists for writing electron arrangement, this is shown on page 1 of the data book. 14 + This is a SILICON atom, because the number of + charges is equal to the ATOMIC NUMBER. This means the number of - charges (electrons) are also 14. 14 electrons are arranged:- 2 in the first energy level, then 8 in the second, and lastly 4 in the third (adds up to 14.) Shorthand notation :- 2, 8, 4

17. Electron Arrangement (5) Chemists have shown that the electron arrangement of an atom is related to its CHEMICAL PROPERTIES (how it reacts). It has been found that elements in the same group of the periodic table have very similar properties. Looking at the electron arrangement of elements within a group it can be seen that they ALL have the SAME NUMBER OF OUTER SHELL ELECTRONS. It is these electrons which are involved in chemical reactions. The number of outer electrons is the SAME as the GROUP NUMBER of the ELEMENT.

18. Electron Arrangement (6) All react violently with water Very reactive, poisonous non-metals All unreactive gases

19. The Noble Gases All the noble gases have 8 outer electrons - a full outer shell (octet). For neon this is 2. Atoms with a full outer shell are unreactive and this explains the chemical properties of the noble gases. Atoms only react to get a full outer shell of electrons, thus becoming stable.