The Development of the Model of the Atom

What did John Dalton believe?

• Atoms were thought to be tiny spheres that could not be divided.

What was the first subatomic particle to be discovered?

First subatomic particle to be discovered was the electron which is negatively charged. Led to the plum pudding model of the atom. 

What was the Plum Pudding Model?

• It is a ball of positive charge with electrons embedded in the ball of positive charge.

• No empty space in the atom

• Mass spread throughout 

What experiment did Rutherford (with assistants: Geiger and Marsden) carry?

Alpha particles scattering experiment

What did the Alpha scattering experiment do?

•  firing alpha particles at very thin sheets of gold foil. 

What were the results of the alpha scattering experiment?

1. Result 1: Most alpha particles passed straight through the gold foil

   Conclusion: The mass of the atom is concentrated in the nucleus

   Conclusion: Most of the atom is empty space

2. Result 2: Some alpha particles were deflected

   Conclusion: The atom has a small positively charged nucleus

   
   

Why was the Plum Pudding model replaced with the Nuclear model?

The plum pudding model does not explain the results of the alpha particle scattering experiment as it shows the whole atom as a ball of positive charge with no empty space. Therefore, the experiment led to the plum pudding model of the atom being replaced with the nuclear model of the atom.

Who created the Nuclear model?

Rutherford

What was the nuclear model?

• Positive charge concentrated at the centre

• Electrons outside the nucleus

• Most of the atom is empty space

• Mass is concentrated at the centre 

What did Niels Bohr suggest?

• Electrons orbit the nucleus

• Electrons are at specific distances from the nucleus 

What did later experiments discover?

• Positive charge of any nucleus could be subdivided into a whole number of smaller particles, each particles having the same amount of positive charge (protons)

• The second particle to be discovered is proton. 

What did James Chadwick discover?

• Provided the evidence to show the existence of neutrons within the nucleus.

What are the discovery dates of each subatomic particle?

Date of discovery	Subatomic particle
1897	Electron
1920	Proton
1932	Neutron

What is the arrangement of the Modern periodic table?

• The periodic table is arranged in order of atomic number (proton number).

• Elements are arranged in groups and periods.

• Groups are vertical columns.

• Periods are horizontal rows.

• Transition metals are located between group 2 and 3.

• The group number indicates the number of electrons atoms have in their outer shell. 

• The period number indicates the number of electron shells an atom has. 

• Elements in a group have similar properties because they all contain the same number of electrons in the outer shell. Electrons are responsible for chemical reactions.

• The table is called a periodic table because similar properties occur at regular intervals.

How was the early periodic table arranged?

John Newlands and his periodic table

• Ordered his table in order of atomic weights 

• Realised similar properties occurred every eighth element – law of octaves

• Had more than one element in a box

• Noble gases are missing as they were not discovered

What were the disadvantages of John Newlands periodic table?

• Did not leave gaps

• Had many dissimilar elements in a column. E.g. Iron and oxygen are in the same column.

How did Dmitri Mendeleev arrange his table?

• Ordered his table in order of atomic weights, but not always strictly – i.e. in some places he changed the order based on atomic weights. E.g. He changed Te and I around.  

• Left gaps for elements that he thought had not been discovered yet. 

• Noble gases are missing as they were not discovered

Why was Dmitri Mendeleev’s table accepted?

• Predicted properties of undiscovered elements

• Elements with properties predicted by Mendeleev were discovered 

• Elements were discovered which fitted into the gaps so elements with similar properties could be placed together.

What does the atomic number (bottom number) show?

Number of protons

What does the mass number show?

Total number of protons and neutrons

How is the modern periodic table arranged?

1. In increasing atomic number

2. elements in the same group have similar chemical properties and similar reactivity

What do periods on the periodic table show?

The number of electron shells

What do groups on the periodic table show?

The no. of electrons in outer shell