2.1.1 : The changing atom

Question 1

Describe Dalton’s atomic theory.

Answer 1

In the early 1800s, Dalton developed his atomic theory which stated that:
- atoms are tiny particles that make up elements
- atoms cannot be divided
- all atoms of a given element are the same
- atoms of one element are different from those of every other element
He said that atoms were tiny spheres and he developed the first table of atomic masses. Many of his predictions are still true and used in chemistry today.

Question 2

Describe JJ Thomson’s discovery of electrons.

Answer 2

-Thomson discovered that cathode rays were a stream of particles that:
- had a negative charge
- could be deflected by both a
magnet and an electric field
- had very very small mass
- These cathode rays were electrons and Thomson concluded that they must have come from within the atoms of the electrodes themselves. This disproved the idea proposed by the ancient Greeks and Dalton that an atom could not be split any further.

Question 3

What did Thomson suggest about the structure of an atom based off of his discovery of electrons?

Answer 3

Thomson suggested that atoms were made up of negative electrons moving around in a sea of positive charge. This formed the ‘plum pudding model’ in which the overall negative charge was equal to the overall positive charge, meaning the atom was neutral with no overall charge.

Question 4

Describe Rutherford’s gold leaf experiment.

Answer 4

• positively charged alpha particles were directed towards a sheet of very thin gold foil
• any change in direction (deflection) of the particles was measured

Question 5

Describe the results from Rutherford’s gold leaf experiment and what they show.

Answer 5

Results:
- most particles were not deflected at all
- however a small percentage of them were deflected through large angles
- very few were deflected back towards the source

This showed that:
- the positive charge of an atom and most of it’s mass is concentrated at the centre, nucleus
- negative electrons orbit the nucleus
- most of an atom’s volume would be the space between the nucleus and the orbiting electrons
- the overall positive and negative charges must balance each other out

Question 6

How did Niel Bohr change Rutherford’s model of an atom and how was this way better?

Answer 6

Bohr changed Rutherford’s model to show how electrons follow certain paths - this was the planetary atom, in which electrons orbited the nucleus in shells.

Bohr’s model helped to explain some periodic properties, such as:
- the spectral lines seen in emission spectra
- the energy of electrons at different distances from the nucleus.

Question 7

Describe how James Chadwick discovered the neutron.

Answer 7

Chadwick observed a new type of radiation emitted from some elements. He showed that this type of radiation was made up of uncharged particles with approximately the same mass as a proton. These unknown particles became known as neutrons as they had no charge.

Question 8

Evaluate the structure of an atom used in chemistry now?

Answer 8

Strengths:
- it’s easy to draw
- it allows us to predict how the atom will bond
- it clearly shows the structure in terms of electrons and a nucleus

Limitations:
- it’s 2D
- it becomes very complicated for elements heavier than calcium
- it fails to explain trends in ionisation energy

Question 9

Describe the current model of an atom.

Answer 9

• protons and neutrons found in the nucleus, which is at the centre of the atom
• electrons orbit the nucleus in shells
• the nucleus is tiny compared with the total volume of an atom
• the nucleus is extremely dense and accounts for most of the atom’s mass
• most of an atom is made up of empty space