AQA Science © Nelson Thornes Ltd C3 1.2 The modern periodic table How are the electrons arranged in an atom?

C3 1.2 The modern periodic table AQA Science © Nelson Thornes Ltd ATOMS All atoms of the same element have the same proton number. 1H1H 2 He 3 Li 5B5B 4 Be 6C6C 7N7N 9F9F 10 Ne 11 Na 12 Mg 16 S 17 Cl 18 Ar 19 K 20 Ca 8O8O 13 Al 14 Si 15 P Elements 1–20 Number of electrons = number of protons (unless the atom is charged).

C3 1.2 The modern periodic table AQA Science © Nelson Thornes Ltd The size of the nucleus compared with the size of the atom is like ‘the size of a man’s fist compared with the dome of St Paul’s Cathedral’ (Rutherford). INSIDE AN ATOM The nucleus contains almost the entire mass of the atom. Tiny electrons whizz around the nucleus. Almost all of the atom is empty space.

C3 1.2 The modern periodic table AQA Science © Nelson Thornes Ltd ELECTRONS Electrons can’t just go anywhere. They move in spaces called orbitals. nucleus electron Diagram not to scale. Orbitals have complicated shapes which are represented by the circles. The further away they are from the nucleus, the higher the energy of the electrons.

C3 1.2 The modern periodic table AQA Science © Nelson Thornes Ltd ENERGY LEVELS ON A HILLSIDE Part of a hill These stones have least energy because they are nearest the bottom. This stone has most energy because it has furthest to fall. Each different height from the ground is an ENERGY LEVEL. Stones must be at one level or another, not in between. Increasing energy

C3 1.2 The modern periodic table AQA Science © Nelson Thornes Ltd Increasing energy Level 1 Level 2 Level 3 Level 4 2 Nucleus The further away you move from the nucleus, the more orbitals available, but they are of higher energy. ENERGY LEVELS AROUND THE NUCLEUS OF AN ATOM Figures in red show maximum number of electrons in each energy level.

C3 1.2 The modern periodic table AQA Science © Nelson Thornes Ltd Energy Level 1 Level 2 Level 3 Level 4 2 Nucleus ARRANGEMENT OF ELECTRONS IN A HYDROGEN ATOM: 1 H Electronic configuration: 1. The electron occupies the lowest available energy level.

C3 1.2 The modern periodic table AQA Science © Nelson Thornes Ltd Energy Level 1 Level 2 Level 3 Level 4 2 Nucleus ARRANGEMENT OF ELECTRONS IN A HELIUM ATOM: 2 He Electronic configuration: 2. The electron occupies the lowest available energy level.

C3 1.2 The modern periodic table AQA Science © Nelson Thornes Ltd Energy Level 1 Level 2 Level 3 Level 4 2 Nucleus ARRANGEMENT OF ELECTRONS IN A LITHIUM ATOM: 3 Li Electronic configuration: 2, 1. The new electron occupies the lowest available energy level. filled orbital most recently filled orbital

C3 1.2 The modern periodic table AQA Science © Nelson Thornes Ltd Energy Level 1 Level 2 Level 3 Level 4 2 Nucleus ARRANGEMENT OF ELECTRONS IN A NEON ATOM: 10 Ne Electronic configuration: 2, 8. The new electron occupies the lowest available energy level. filled orbital most recently filled orbital

C3 1.2 The modern periodic table AQA Science © Nelson Thornes Ltd Energy Level 1 Level 2 Level 3 Level 4 2 Nucleus ARRANGEMENT OF ELECTRONS IN A POTASSIUM ATOM: 19 K Electronic configuration: 2, 8, 8, 1. The new electron occupies the lowest available energy level. filled orbital most recently filled orbital not available

C3 1.2 The modern periodic table AQA Science © Nelson Thornes Ltd Energy Level 1 Level 2 Level 3 Level 4 2 Nucleus ARRANGEMENT OF ELECTRONS IN A CALCIUM ATOM: 20 Ca Electronic configuration: 2, 8, 8, 2. The new electron occupies the lowest available energy level. filled orbital most recently filled orbital not available

C3 1.2 The modern periodic table AQA Science © Nelson Thornes Ltd Energy Level 1 Level 2 Level 3 Level 4 2 Nucleus ARRANGEMENT OF ELECTRONS IN A SCANDIUM ATOM: 21 Sc Electronic configuration: 2, 8, 9, 2. The new electron occupies the lowest available energy level. filled orbital most recently filled orbital

C3 1.2 The modern periodic table AQA Science © Nelson Thornes Ltd ELECTRON ARRANGEMENTS AND THE PERIODIC TABLE This periodic table shows all the elements in proton number order. The number of elements in each period is shown in red

C3 1.2 The modern periodic table AQA Science © Nelson Thornes Ltd FAMILIES OF METALS Group 1 3 Li 2, 1 11 Na 2, 8, 1 19 K 2, 8, 8, 1 37 Rb 2, 8, 18, 8, 1 55 Cs 2, 8, 18, 18, 8, 1 Group 2 4 Be 2, 2 12 Mg 2, 8, 2 20 Ca 2, 8, 8, 2 38 Sr 2, 8, 18, 8, 2 56 Ba 2, 8, 18, 18, 8, 2 Increasing reactivity

C3 1.2 The modern periodic table AQA Science © Nelson Thornes Ltd FAMILIES OF NON-METALS Group 6 8 O 2, 6 16 S 2, 8, 6 Group 7 9 F 2, 7 17 Cl 2, 8, 7 35 Br 2, 8, 18, 7 53 I 2, 8, 18, 18, 7 Increasing reactivity

C3 1.2 The modern periodic table AQA Science © Nelson Thornes Ltd THE NOBLE GASES Group 0 2 He 2 10 Ne 2, 8 18 Ar 2, 8, 8 36 Kr 2, 8, 18, 8 54 Xe 2, 8, 18, 18, 8 Almost completely unreactive

C3 1.2 The modern periodic table AQA Science © Nelson Thornes Ltd FORMING A SODIUM ION Sodium atom, Na, 2, 8, 1. Sodium ion, Na +, 2, 8. Na(g)  Na + (g) + e – The sodium ion is much smaller than the sodium atom.

C3 1.2 The modern periodic table AQA Science © Nelson Thornes Ltd Energy Level 1 Level 2 Level 3 Level 4 Nucleus FORMING POSITIVE IONS The more energy the electron already has, the less extra energy is needed to remove it. Complete orbitals shield the outer electrons from the pull of the nucleus. This means it takes less energy to remove an electron from an outer orbital than from an inner one. Li  Li + + e – requires 520 kJ/mol. Cs  Cs + + e – requires 376 kJ/mol. Given enough extra energy, an electron can escape from the pull of the nucleus. The higher its energy level, the less extra energy it will need. Na  Na + + e – requires 496 kJ/mol. Estimate the values for K and Rb.

C3 1.2 The modern periodic table AQA Science © Nelson Thornes Ltd FORMING A CHLORIDE ION Cl(g) + e –  Cl – (g) The chloride ion is larger than the chlorine atom. Chlorine atom, Cl, 2, 8, 7. Chloride ion, Cl –, 2, 8, 8.

C3 1.2 The modern periodic table AQA Science © Nelson Thornes Ltd Energy Level 1 Level 2 Level 3 Level 4 Nucleus FORMING NEGATIVE IONS The stronger the attraction of the nucleus for an extra electron, the more energy will be given out when a negative ion is formed. Complete orbitals shield the outer electrons from the pull of the nucleus. So the larger the atom, the less its attraction for an extra electron. Cl + e –  Cl – gives out 349 kJ/mol. Estimate the value for iodine. The nucleus can attract electrons into the outer shell of the atom. The smaller the atom, the stronger the pull of the nucleus. Br + e –  Br – gives out 325 kJ/mol.