ATOMIC STRUCTURE HISTORY OF THE ATOM 460 BC

ATOMIC STRUCTURE

HISTORY OF THE ATOM 460 BC Democritus develops the idea of atoms ATOMA (greek for indivisible) he pounded up materials in his pestle and mortar until he had reduced them to smaller and smaller particles which he called

Democritus’ Theory 1. All matter is composed of atoms, which are bits of matter too small to be seen. These cannot be further split into smaller portions. 2. There is a void, which is empty space between atoms. 3. Atoms are completely solid. 4. Atoms are homogenous, with no internal structure. 5. Atoms are different in their: sizes, shapes and weight.

HISTORY OF THE ATOM 1808 John Dalton ATOMS suggested that all matter was made up of tiny spheres that were able to bounce around with perfect elasticity and called them

16 X + 8 Y 8 X 2 Y Law of Conservation of Mass 2. 1

HISTORY OF THE ATOM 1898 Joseph John Thompson ELECTRON found that atoms could sometimes eject a far smaller negative particle which he called an

HISTORY OF THE ATOM 1904 Thompson develops the idea that an atom was made up of electrons scattered unevenly within an elastic sphere surrounded by a soup of positive charge to balance the electron's negative charge like plums surrounded by pudding. PLUM PUDDING MODEL

2. 2

J. J. Thomson, measured mass/charge of e(1906 Nobel Prize in Physics) 2. 2

Measured mass of e(1923 Nobel Prize in Physics) e- charge = -1. 60 x 10 -19 C Thomson’s charge/mass of e- = -1. 76 x 108 C/g e- mass = 9. 10 x 10 -28 g 2. 2

HISTORY OF THE ATOM 1910 Ernest Rutherford oversaw Geiger and Marsden carrying out his famous experiment. they fired Helium nuclei at a piece of gold foil which was only a few atoms thick. they found that although most of them passed through. About 1 in 10, 000 hit the detector plate.

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

(1908 Nobel Prize in Chemistry) α particle velocity ~ 1. 4 x 107 m/s (~5% speed of light) 1. atoms positive charge is concentrated in the nucleus 2. proton (p) has opposite (+) charge of electron 3. mass of p is 1840 x mass of e- (1. 67 x 10 -24 g) 2. 2

HISTORY OF THE ATOM Rutherford’s new evidence allowed him to propose a more detailed model with a central nucleus. He suggested that the positive charge was all in a central nucleus. With this holding the electrons in place by electrical attraction However, this was not the end of the story.

Because there was a problem…. It was known that H contains only 1 proton, He contains 2. So, the ratio of mass must be H: He=1: 2. However, the ratio is 1: 4!! Rutherford said there must be another subatomic particle…. But, unfortunately he couldn’t find what it was…

HISTORY OF THE ATOM 1913 Niels Bohr studied under Rutherford at the Victoria University in Manchester. Bohr refined Rutherford's idea by adding that the electrons were in orbits. Rather like planets orbiting the sun. With each orbit only able to contain a set number of electrons.

Bohr’s Atom nucleus electrons in orbits

ATOMIC STRUCTURE Electrons are arranged in Energy Levels or Shells around the nucleus of an atom. • first shell a maximum of 2 electrons • second shell a maximum of 8 electron • third shell a maximum of 18 electrons • fourth shell a maximum of 32 electrons

ATOMIC STRUCTURE There are two ways to represent the atomic structure of an element or compound; 1. Electron Configuration 2. Electron Dot Structure

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

ELECTRON CONFIGURATION Write the electronic configuration for the following elements; a) Ca 20 b) Na 11 c) 23 40 2, 8, 8, 2 2, 8, 1 17 14 d) Cl 35 2, 8, 7 O e) Si 28 2, 8, 4 8 16 2, 6 f) B 5 11 2, 3

Electron Dot Structure With Dot Structure diagrams elements and compounds are represented by Dots or Crosses to show electrons in the valence, outermost, shell. For example: X Nitrogen X N XX X N 7 14

Electron Dot Structure Draw the Electron Dot Structure diagrams for the following elements: XX 8 17 a) O b) 16 Cl 35 XX X X O X X X Cl X XX X X X

HISTORY OF THE ATOM 1932 James Chadwick He found the presence of neutron by bombarding a thin sheet of a metal with radiation. A high-energy radiation was emitted by the metal. However, the rays are not distracted by electrical charge. That’s why it’s named neutron because it is neutral. It’s mass is slightly different from the proton.

ATOMIC STRUCTURE Particle Charge Mass (g) proton + ve charge 1. 6 x 10 -24 neutron No charge 1. 6 x 10 -24 electron -ve charge 9. 1 x 10 -28

HELIUM ATOM Shell proton + - N N + electron What do these particles consist of? - neutron

ATOMIC STRUCTURE He 2 4 Atomic number the number of protons in an atom Atomic mass the number of protons and neutrons in an atom number of electrons = number of protons

ATOMIC STRUCTURE

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