Atomic Structure

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Atomic Structure

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The structure of the atom

 

Fig 1 – Source: Pixabay

What is an atom made up of?

  • An atom is made up of a nucleus containing protons and neutrons.
  • Protons have a positive charge and neutrons have no charge.
  • Electrons are found moving around the nucleus in orbitals or shells. They have a negative charge.

Relative mass

Relative charge
Proton 1 +1
Neutron 1 0
Electron 0.0005 -1

 

Before an atom takes part in bonding it is neutral overall. This is because it has the same number of protons and electrons. The positive and negative charges cancel each other out.

The nucleus

  • The nucleus of an atom is positive overall.
  • Most of the mass of an atom is found in the nucleus.

Key

8 = Atomic number

15.999 = Mass number

Fig 2 – Source Pixabay

  • The atomic number of an atom is the same as the number of protons.
  • The mass number of an atom is the number of protons + neutrons.
  • The mass number is always the bigger number.
  • The number of neutrons = mass number – proton number.

Electronic Structure

 

Fig 3 – Electronic Structure

The number of electrons in an atom is the same as the atomic number.  

  • Electrons are found in shells orbiting the nucleus. Only a maximum of 2 electrons are found in the first shell, 8 in the second shell and 8 in the third shell. The next electrons will then fill the fourth shell.
  • Electrons will always fill the shell closest to the nucleus first.
  • It is the outer electrons only that take place in bonding.

E.g. Carbon has an electronic configuration of 2, 4. This means it has 2 electrons in the first shell and 4 in the second shell.

Electron patterns and groups

  • The number of electron shells will identify which period an atom is in. E.g. carbon has two electron shells and is found in period 2.
  • The number of electrons in the outer shell will tell you the group number e.g. Carbon has 4 electrons in its outer shell and is found in group 4.

How has the atomic model changed over time?

The table summarises the history of the atom.

 

Scientist

Date

Atomic model
 

John Dalton

 19th Century

Atoms were described as spheres or billiard balls.

 

J.J. Thomson

 1897

 

Discovered the electron. Plum pudding model – the atom is a ball of positive charge with electrons embedded within it.
 

Geiger, Marden and Rutherford

  

1909

 

Gold leaf and alpha particles experiment–mass is concentrated in a central nucleus that is charged. Most of the mass is in the nucleus. The rest of the atom is empty space.
 

 

Niels Bohr

  

 

1913

 

Electrons exist in shells orbiting the nucleus.

 
 

James Chadwick

 

 1932

Discovered neutrons in the nucleus.

Questions:

  1. What is the electronic configuration for Lithium?
  2. Explain why an atom is neutral overall.

 Answers:

  1. 2,1
  2. It has the same number of protons and electrons so it has an equal number of positive and negative charges.

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