Final answer:
The size of atoms increases when they gain electrons and decreases when they lose electrons due to changes in the electrical charge and the attraction between electrons and protons.
Step-by-step explanation:
When atoms gain electrons, the size increases, and when electrons are lost, the size decreases. This is due to the fact that electrons and protons are strongly attracted to each other. The number of these particles and the strength of attraction between them greatly influences the size of an atom or an ion. In the case of an atom gaining electrons, it becomes a negative ion or anion, which leads to a reduced electrical charge and an increased size. Conversely, when an atom loses electrons, it becomes a positive ion or cation, resulting in a smaller size due to the reduced electron cloud creating less repulsion between remaining electrons and a stronger pull from the nucleus.
For example, if a fluorine atom gains an electron, it becomes a fluoride ion with an increased size and a charge of -1. On the other hand, metal atoms like lithium tend to lose electrons to achieve a more stable state, thus decreasing their size. In summary, the gain of electrons leads to a size increase, while the loss of electrons results in a size decrease, both as atoms transform into ions.