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# Atomic structure

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1. Atomic structure

2. Outcomes: You must be able to describe what an atom is made of and how atoms of an element are represented You should be able to describe the properties of all subatomic particles You could be able to explain what an isotope is

3. Everything is made up of atoms… … but what are atoms made up of? Atoms consist of much smaller particles called subatomic particles. (sub = under, below, beneath) These subatomic particles are: the proton the neutron and the electron

4. Atomic structure The protons and neutrons exist in a dense core at the centre of the atom. This is called the nucleus. The electrons are spread out around the edge of the atom. They orbit the nucleus in layers called shells.

5. Properties of the subatomic particles Protons are positively charged and have mass. Neutrons have no charge (they are neutral) and have mass Electrons are negatively charged and have no mass This can be summed up in the following table:

6. Using the Periodic Table We can use the Periodic Table to find out information about the atomic structure of each element. The bigger number is the mass number. It tells you how many protons and neutrons the atom has. The smaller number is the atomic or proton number. It tells you how many protons the atom has.

7. Important key points! An element always has the same number of protons – it’s like an ID card. E.g. a carbon atom has always 6 protons, nitrogen has 7, hydrogen has 1 etc. An atom is overall neutral – it does not have a charge! So the number of protons has to be equal to the number of electrons. Only protons and neutrons have mass – both of them make up the mass number. To find the number of neutrons, simply take away the number of protons from the mass number.

8. Practice How many protons, neutrons and electrons are in an atom of: H Mg F Na Al N

9. Answer: H p = 1, n = 0, e = 1 Mg p = 12, n = 12, e = 12 F p = 9, n = 10, e = 9 Na p = 11, n = 12, e = 11 Al p = 13, n = 14, e = 13 N p = 7, n = 7, e = 7

10. What is an isotope? Isotopes are atoms of the same element that have the same number of protons but different numbers of neutrons. E.g. p = 6 n = 6 p = 6 n = 7 p = 6 n = 8 • Isotopes are atoms with the same atomic number but different mass numbers.

11. Electronic structure

12. Outcomes: You must be able to define what energy levels or shells are You should be able to describe how electrons fill up these shells and draw the electronic structure of the first 20 elements You could be able to describe how it influences the properties of the element

13. Electron arrangement in atoms What information can you get out of the Periodic Table? Atomic number (smaller number)  number of protons  also: number of electrons, as an atom is neutral Mass number (bigger number)  number of protons AND neutrons  number of neutrons = mass number minus atomic number

14. Recap – electron arrangement in atoms What information can you get out of the Periodic Table? Period (rows)  number of electron shells Group(columns)  number of electrons on the outermost shell

15. Electron arrangement in atoms • Maximum number of electrons on each shell: • 1st shell: 2 • 2nd shell: 8 • 3rd shell: 8 • After that it gets more complicated.

16. 8p 8n What is the electron arrangement in the following atoms? • oxygen • protons? • neutrons? • electrons? • Number of shells? • Number of e- on last shell? 2, 6 8 8 8 2 6

17. 11p 12n What is the electron arrangement in the following atoms? • sodium • protons? • neutrons? • electrons? • Number of shells? • Number of e- on last shell? 2, 8, 1 11 12 11 3 1

18. Which element is it? beryllium

19. Which element is it? phosphorus

20. Which element is it? krypton

21. Which element is this? Helium Carbon Aluminium Chlorine Potassium 2 2, 4 2, 8, 3 2, 8, 7 2, 8, 8, 1

22. Let’s take a look atthe elements in one group: • What do elements in one group have in common? • They have the same number of electrons on the outer shell. • This means that elements in the same group have similar properties and react in a similar way. • What is different between them? • The further down the group you go, the more shells there are and the bigger the atom is.

23. Why are the noble gases so unreactive? • The way an element reacts is determined by the number of electrons on its outermost shell (highest energy level). • Only the outer shell electrons are used in bonding. • Through bonding atoms achieve a full outer shell and thus a more stable electron arrangement • The noble gases are very unreactive because they have a very stable electron arrangement – they all have a full outer shell ( = 8 electrons; He has 2)

24. What happens when atoms bond? • When atoms bond, only the electrons on their last shells are involved. • There are three types of bonding: • ionic bonding • covalent bonding • metallic bonding

25. 1. Ionic bonding • between metals and non-metals • involves the gain or loss of electrons • The ‘octet’ rule: an atom is more stable if it has a full outer shell (This is the reason why the noble gases are so unreactive!)

26. More detail… • When a metal reacts with a non-metal, it donates the electrons on the outermost shell. This means it has more protons than electrons, so it becomes a positively charged ion. • The non-metalaccepts the electrons from the metal. This means it has more electrons than protons, so it becomes a negatively charged ion.

27. More detail… • The bond is created through the strong electrostatic forces between the positively and negatively charged ions.

28. In an equation: • In the reaction between sodium and chlorine: • 2Na + Cl2 2NaCl • 2Na  2Na+ + 2e- • Cl2 + 2e-  2Cl- • We use dot&cross diagrams to represent ionic bonding.

29. + - x + Cl Na Cl Na x Dot&cross diagrams • Rules for drawing dot&cross diagrams: • only draw the outermost shell • use crosses for one element, dots for the other • the ions are drawn in brackets, with the charge at the top right-hand corner.

30. Dot&cross diagrams • Draw out dot&cross diagrams for the following ionic compounds: • Potassium chloride, KCl • Magnesium oxide, MgO • Calcium chloride, CaCl2 • Sodium sulfide, Na2S • Aluminium oxide, Al2O3

31. Cl Cl x x x x x x x Dot&cross diagrams - + K x 2- 2+ Mg O - 2+ Ca 2 + 2- 2 Na S 3+ 2- 2 3 Al O

32. Resources