3.1.1 Atomic Structure - Protons, neutrons and electrons

Specification

Students should
  • understand the importance of these particles in the structure of the atom and appreciate that there are various models to illustrate atomic structure

Mass number

The atomic mass number is represented by the symbol (letter) 'A'. This is not to be confused with the relative atomic mass Ar.

The mass number gives the integral number of nucleons, protons and neutrons found in the nucleus of an atom.

The relative mass is a value that is not necessarily integral that compares a mass to the mass of a carbon isotope, assigned a value of exactly 12.0000 units.


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Atomic number

This is represented by the symbol (letter) 'Z'. It shows us the number of protons in an atom (and the number of electrons in a neutral atom.

Example: How many protons and electrons does an atom of iron contain?

The atomic number of iron is 26 therefore it contains 26 protons

The number of electrons = number of protons therefore there are 26 electrons


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AZE

Any isotope of any element can be defined by using the A value, the Z value and the element symbol.

Using the values of A and Z it is possible to calculate the number of sub-atomic particles within any specific isotope of an element.

Example: Determine the number and type of sub-atomic particles in the following atom:

The atomic number is 1 therefore there is 1 proton and 1 electron

The mass number is 3 therefore there are (3-1) = 2 neutrons

Example: Determine the number and type of sub-atomic particles in the following atom:

The atomic number is 5 therefore there are 5 protons and 5 electrons

The mass number is 11 therefore there are (11-5) = 6 neutrons


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Ions

The system can be extended to cover ions simply by adding the charge onto the element symbol. It is important to remember that a positive ion has LOST electrons.

Example: Determine the number of electrons in the following ion: Mg2+

The atomic number is 12 therefore in a neutral atom there would be 12 protons and 12 electrons.

However, the charge is 2+ therefore the atom has LOST 2 electrons

The remaining electrons then = 10 electrons


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quick check