Atomic Theory

What is an Atom?

� An ATOM is the smallest part of an

element that has all of the element’s

properties.

� Atoms of different elements are different

from each other.

Atomic Theory

� This is the study of the nature of atoms

and how they combine to form all types of

matter.

� It explains how atoms make up their

elements and how they make up all other

forms of matter, including compounds and

mixtures.

Ancient Atomic Theories

� The idea that matter is made up of combinations of simpler forms of matter is very ancient.

� Ancient Greeks – thought that matter was all made up of 4 elements – Fire, Water, Earth and Air

� Ancient China – thought there were 5 elements – Fire, Water, Wood, Metal and Earth

John Dalton

� In the early 1800’s a scientist and teacher,

John Dalton, suggested that matter is

made up of atoms.

� He believed that atoms could vary in size,

mass or colour.

Dalton’s Atomic Theory

� All matter is made up of small indivisible

particles called atoms

� All atoms of an element are identical to

each other in size and mass

� Atoms of different elements have different

properties

� Atoms of different elements combine in

specific ways to form new substances

Dalton’s Atoms

J.J. Thomson

� In the late 1800’s an English physicist researched atoms and decided that he thought atoms might be made of a combination of particles.

� He experimented with electric currents in glass tubes called cathode ray tubes.

� Using this tube he was able to cause non-radioactive atoms to produce streams of negatively charge particles that were later found to be electrons.

J.J. Thomson

� In 1897, Thomson proposed a

revolutionary new model for atoms, in

which each atom was composed of

smaller particles

� He discovered that atoms were composed

of both positive and negatively charged

particles.

Thomson’s Model of the Atom

This can be

compared to as a

blueberry muffin.

The blueberries

represent negative

charges, and the

batter of the muffin

represents positive

charges.

� New Zealand born scientist tested

Thompson’s model of the atom while

working in England

� Rutherford’s famous “Gold Foil”

experiment led him to include a nucleus in

his atomic model

� Rutherford shot positively charged

particles at a very thin foil of pure gold

� In the experiment most high speed

positive particles went right through the

gold foil, however some bounced back

WHY? :. Because of the gold’s NUCLEUS!

� Rutherford discovered the nucleus. The nucleus is the centre of the atom

� He also discovered that the nucleus contains most of the atom’s mass. The nucleus is far smaller than the rest of the atom.

� He calculated that comparing the size of the nucleus to other atomic sub-particles was like comparing a football field to a green pea.

The nucleus

� Based on the “Gold Foil” experiment

Rutherford revised the existing atomic

model to include a nucleus

� Similar to Thomson’s model except that

all of the atom’s positive charge and

mass were concentrated at a tiny point in

the centre of the atom and the electrons

surrounding this point took up an

enormous amount of space.

� Chadwick was Rutherford’s student and he refined the concept of the nucleus

� He discovered that the nucleus contains neutral (no charge) particles called neutronsand positive charge particles called protons

� Each neutron has almost the same mass as each proton in the atom

� Danish physicist that helped transform Rutherford’s model to a model that we use today

� Bohr suggested that electrons surround the nucleus in specific energy levels called energy shells

� He found that electrons jump between these energy shells by gaining or losing energy

� The max number of electrons in the first

three energy shells are; 2, 8, and 8

respectively

� Most advanced and

accurate model of the

atom

� Electrons exist in

specific energy levels

that surround the

nucleus in a form

resembling a cloud