It's a sub-atomic particle with negligible mass and unit negative charge.
It's a sub-atomic particle with mass nearby equal to that of a proton and carrying no charge.
It's the smallest possible particle of an element that can take part in a chemical reaction. It's electrically neutral and may or may not be capable of independent existence.
It's a positive or negatively charged particle with it's outermost shell complete and capable of independent existence is solution.
