Why do the negative ions have larger radii than the positive ions?
Because positive ions have stronger pulls on their electrons.
Electrons are negatively charged while protons are positively charged and they both exert pulls on each other. Negatively charged ions have higher negative charges within them, thus causing the electrons to spread out and move away from the nucleus much like how like magnets repel. Likewise, Positively charged ions have stronger positive charges within them and pull their electrons closer to them much like how magnets attract. Therefore, when an atom becomes a positive ion is pulls its electrons closer, decreasing is radius moreover, when it becomes a negative ion, it pulls its electrons closer and decreases the radius.
Addition of electrons forms negative ions which get larger.
Subtraction of electrons forms positive ions which get smaller
When an atoms attracts extra electrons it becomes a negative ion.
The negative ion is larger than the original atom. The positive nucleus remains the same, with the same attractive force. However this attractive force is now pulling on more electrons and therefore has less effect. This means with more electrons but the same number of positive protons, the size of the ionic radius will increase.
When an atom has its electrons attracted to another atom it becomes a positive ion. The positive ion is smaller than the original atom. The positive nucleus remains the same with the same attractive force. However this attractive force is now pulling on fewer electrons and therefore has a greater effect. This means that with fewer electrons but the same number of positive protons, the size of the ionic radius will decrease.
As the formation of a negative ion increases the ionic radius and the formation of a positive ion decreases the ionic radius the negative ion will be larger than the positive ion