2. HISTORY OF THE ATOM 460 BC Democritus develops the idea of atoms He got the idea that everything was made of small bits by looking at sand on the beach. He pounded up materials in his pestle and mortar until he had reduced them to smaller and smaller particles which he called ATOMA (greek for indivisible)

3. HISTORY OF THE ATOM 340 BC Aristotle thought that everything was made of four “elements”: Earth, Air, Fire and Water Aristotle was wrong, but his idea was accepted for the next 2000 years. Aristotle’s competing idea

4. HISTORY OF THE ATOM 1808 John Dalton Revived Democritus’ idea Dalton said that all matter was made up of tiny spheres that were somehow different from each other, resulting in different elements. He called these tiny spheres ATOMS

5. HISTORY OF THE ATOM 1898 Joseph John Thomson Discovered the electron While working with cathode rays, Thomson discovered that they were made of tiny, identical and negatively charged particles which were later called ELECTRONS

6. HISTORY OF THE ATOM Thomson developed the idea that atoms were made up of electrons scattered unevenly in a soup of positive charge to balance the electron's negative charge. 1904 Thomson creates a new model of the atom. like plums surrounded by pudding. PLUM PUDDING MODEL

7. HISTORY OF THE ATOM 1910 Ernest Rutherford discovers the nucleus His team fired alpha particles at a piece of gold foil which was only a few atoms thick. They found that although most of the particles passed straight through, About 1 in 10,000 hit something. It was as if you fired a cannon at a sheet of tissue paper and the cannonball bounced back.

8. HISTORY OF THE ATOM gold foil helium nuclei They found that while most of the particles passed through the foil, a small number were deflected and some even bounced straight back. helium nuclei

9. HISTORY OF THE ATOM Rutherford’s new evidence led him to propose a new model with a central nucleus. He suggested that the positive charge was all in the central nucleus, instead of spread out like in Thomson’s model. It was this small, massive, positively charged nucleus that deflected the particles in the experiment.

10. HISTORY OF THE ATOM 1913 Niels Bohr creates a new model of the atom. Bohr studied under Rutherford at the Victoria University in Manchester. Bohr refined Rutherford's model by adding that the electrons were in orbits around the nucleus, like planets going around the sun. Each orbit could only hold a set number of electrons.

11. Bohr’s Atom electrons in orbits nucleus

12. ATOMIC STRUCTURE Particle proton neutron electron Charge + 1 No charge 1 1 1/1800 Mass

13. HELIUM ATOM + N N + - - proton electron neutron Shell Remember that models like this are not drawn to scale!

14. ATOMIC NOTATION the number of protons AND neutrons in the nucleus the number of protons in the nucleus He 4 2 Atomic number Mass number To find the number of neutrons, you have to subtract the atomic number from the mass number.

15. More About the Atomic Number The atomic number is also the ID number of the element. Only helium has atomic number 2. If a nucleus has 26 protons, it must belong to an iron atom. The Periodic Table is arranged by atomic number

16. What about the electrons? The electrons weigh almost nothing. They are held by the nucleus by electrical attraction. Atoms are electrically neutral, which means that the number of electrons in a neutral atom must be the same as the atomic number: number of electrons = number of protons

17. ATOMIC STRUCTURE Electrons are arranged in Energy Levels or Shells around the nucleus of an atom. first levela maximum of 2 electrons second levela maximum of 8 electrons third levela maximum of 8 electrons

18. ATOMIC STRUCTURE There are two ways to represent the atomic structure of an element or compound; 1.Electronic Configuration 2.Bohr Diagrams

19. ELECTRONIC CONFIGURATION With electronic configuration elements are represented numerically by the number of electrons in their shells and number of shells. For example; N Nitrogen 7 14 2 in 1 st shell 5 in 2 nd shell configuration = 2, 5 2 + 5 = 7

20. ELECTRONIC CONFIGURATION Write the electronic configuration for the following elements; Ca O ClSi Na 20 40 11 23 8 17 16 35 14 28 B 11 5 a)b)c) d)e)f) 2,8,8,22,8,1 2,8,72,8,42,3 2,6

21. BOHR DIAGRAMS With Bohr diagrams, elements and compounds are represented by Dots or Crosses to show electrons, and circles to show the energy levels. For example: Nitrogen N XX X X XX X N 7 14

22. BOHR DIAGRAMS Draw the Bohr diagrams for the following elements; OCl 817 16 35 a)b) O X X X X X X X X Cl X X X XX X X X X X X X X X X X X X

23. SUMMARY 1. The Atomic Number of an atom = number of protons in the nucleus. 2. The Atomic Mass of an atom = number of Protons + Neutrons in the nucleus. 3. The number of Protons = Number of Electrons. 4. Electrons orbit the nucleus in shells. 5. Each shell can only carry a set number of electrons.