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ATOMIC STRUCTURE

- An atom is made up of 3 particles: protons (+ve), electrons (-ve) and neutrons

- Atoms are neutral because they have the same number of protons and electrons

- If an atom loses or gains electrons it becomes an ion (a charged atom)

- If the atom loses electrons it becomes a +ve ion

- If the atom gains electrons it becomes a –ve ion

Name of particle

Relative mass

Relative charge

Proton

1

+1

Neutron

1

0

Electron

1/1840

-1

Mass number

A

Important definitions:

Atomic number = The number of protons in an atom

(* All the elements in the periodic table are arranged in order of their atomic number)

Mass number = The sum of the number of protons and neutrons in an atom

Number of neutrons = Mass number – Atomic number

Isotopes = Atoms of the same element, with the same number of protons but a different number of neutrons

Electron Arrangement:

- Electrons in an atom are arranged in shells (also known as energy levels)

- The atomic number tells you the total number of electrons in an atom

- Group number = Number of outer shell electrons (or number of electrons in the highest energy level)

- The electronic structure of an atom will determine its chemical properties (i.e. how it reacts)

- The 1st shell can hold 2 electrons; the 2nd and 3rd shell can hold 8 electrons

Sodium atom (Na) = 2,8,1 Sodium ion (Na+) = 2,8

* You must be able to write and draw the electronic structure of the first twenty elements (see example above)