Developing the Atomic Theory

Question 1

Empedocles (450 BC)

Answer 1

Empedocles proposed matter was composed of 4 elements
Earth, air, fire, and water
Experimentally determined that air is just not nothing because it takes up space; therefore must be a form of matter

Question 2

Democritus (400 BC)

Answer 2

Democritus suggested matter was made of tiny particles that can not be broken down further
Referred to the articles as atoms

Question 3

Alchemists (500-1600)

Answer 3

Alchemists attempted to turn cheap metals such as iron and lead into gold
Obviously unsuccessful
But, through their experiments they discovered many elements and compounds, as well as, lab techniques and equipment

Question 4

Boyle (1650)

Answer 4

Boyle defied an element as a pure substance that cannot be broken down further into simpler substances
Also disagreed with the 4 element model because he thought air to be more of a mixture

Question 5

Priestly & Lavoisier (Late 1700s)

Answer 5

Priestly experimentally isolated oxygen but it was Lavoisier who suggested it was an element and used it to conclude that air must be a mixture of at least 2 gases, 1 being oxygen

Question 6

Dalton (1808)

Answer 6

Dalton’s atomic model for matter:
All matter is made of atoms, which are particles too small to see
Each element has its own atom with its own mass
Compounds are created when atoms of different elements link to form molecules
Atoms can not be created or destroyed

Question 7

Faraday (Late 1800s)

Answer 7

Faraday found electric current could cause chemical changes in some compounds in solution
The atoms could gain electric charges and form charged atoms known as ions
This led to a modification in Daltons theory:
Matter must contain positive and negative charges
Opposite charges attract and like charges repel
Atoms form molecules because of electrical attraction between atoms

Question 8

Thomson (1904)

Answer 8

Thomson discovered electrons which changed Dalton’s theory again
Atoms contain negative particles called electrons
Electrons have a small mass and a negative charge
The rest of the atom is a sphere of positive charge
Electrons are embedded in the sphere which makes the atom neutral or uncharged

Question 9

Rutherford (1911)

Answer 9

Rutherford tested Thomson’s model in what is called ‘The Gold Foil Experiment’
He shot alpha particles (type of radiation) at a thin piece of gold foil, now the alpha particles should have all passed through the gold foil, according to Thomson, BUT some bounced back
This again led to changes in Dalton’s theory
An atom has a tiny, dense, positive core called the nucleus (which contains protons)
The nucleus is surrounded mostly by empty space containing rapidly moving negative electrons

Question 10

Bohr

Answer 10

Finally Bohr Suggested
Electrons move around the nucleus in nearly circular paths called orbits
Each electron in an orbit has a certain amount of energy
The farther the electron is from the nucleus the greater its energy
Electrons can not exist between orbits but can move up or down from 1 orbit to another with enough energy
Electrons are more stable at lower energy and when there closer to the nucleus
The order of filling of electrons in the 1st 4 orbital’s is 2, 8, 8, 18 (maximum values)