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In a neutrally charged atom, the number of electrons is equal to the number of protons.
Therefore, the negative charge of each electron is cancelled out by the positive charge on each proton.
If the number of electrons and protons are not equal in an atom (or molecule),
a species known as an ion is formed.
Each atom of lithium has three protons.
This means that lithium also has 3 electrons because atoms are neutral.
If an electron is lost from the atom, there will be one more proton than electrons.
The charge of two electrons cancels out the charge of two protons and there is one proton left over, with a 1+ charge.
So, if you lose one electron from a lithium atom, a Li+ ion is formed.
A negative ion can be formed by an atom gaining electrons.
In a neutral atom of fluorine, there are nine protons and nine electrons.
If fluorine gains one electron, it will form a F- ion.
Ions can be formed by reactions between atoms
or through atoms being struck by high energy electrons.
Different atoms tend to form different ions.
The nature of the ion generated depends on the electronic structure of the atom.
I'll explain that in a future video.