Chemistry: The Atom

Page 1

Atomic theory

John Dalton developed the first atomic theory

atoms are

1. Very small

2. Have indivisible parts cannot be broken

3. Atoms of the same element have the same mass and chemical properties

Page 2

William Crooks 1870

discovered that the cathode rays travelled in a straight line.

How? Passed electricity through a a gas vacuum tube. Electricity traveled from a cathode to anode (negative electrode to positive electrode)

Crooks called them cathode rays

Evidence of the cathode rays

1. Fluorescent glow

2. Paddle moved

3. Shadow cast by cross ✝️

Page 3

J.J Thomps 1887

Discovered that cathode rays were negatively charged. Measured the charge to mass ratio (e/m) for electrons

How 1: Added a (-) and a (+) plate to Crookes vacuum and found the cathode rays were attracted to the positive plates which meant cathode rays were negatively charged

How 2: Used a magnetic field from electromagnets to deflect the electrons

Page 4

Johnstone Stoney 1880

Named the cathode rays electrons

Page 5

Robert Millikan

used his ‘oil drop’ experiment to measure the size of the charge of the electrons

Page 6

J.J Thompson plum pounding model

large positive sphere and negatively charged electrons throughout the atom were his development of a model of an atom

Page 7

Rutherford gold foil test

Proved the plum pudding model was incorrect by

using a gold foil test

Fired positive radioactive particles alpha particles at extremely thin foil

Finding

large number alpha particles (proton) passed straight through

Many deflected at small angles

Few particles deflected backwards as were repelled strongly

Observation

Positive charge was concentrated n very dense small nucleus

large amount of empty space in an atom which is occupied by very small electrons

positive proton particles

Had equal size to electrons

Page 8

James Chadwick

Discovered the neutron

Page 9

Bohr Model

central nucleus

Electrons orbiting around nucleus

Suggested electrons orbited with increasing distance away from nucleus

Each orbit had different energy level

Page 10

Atomic Number and Nucleus

atomic number: is the number of protons in an element Z is the symbol (determines element)

Atomic mass: is the number of protons and neutrons in an element A is the symbol

To calculate number of neutrons : atomic mass - atomic number (A-Z)

Page 11

Mass spectrometer: Works by separating positive ions by mass when moving in a magnetic field

Vaporization

Ionization

Accerlation

separation

Determination