Why anions are usually larger:

* Gain of electrons: Anions are formed when an atom gains one or more electrons. These extra electrons increase the electron-electron repulsion, making the electron cloud larger.

* Reduced effective nuclear charge: The added electrons also slightly reduce the effective nuclear charge experienced by each electron. This means the electrons are less strongly attracted to the nucleus, contributing to a larger atomic radius.

Exceptions and Considerations:

* Periodic trends: While anions are usually larger than their parent atoms, this trend is more pronounced for elements on the left side of the periodic table (especially Group 1 and 2). As you move across a period, the increase in nuclear charge becomes more dominant, and the size difference between the anion and neutral atom may become smaller.

* Specific elements: Some elements, like oxygen and fluorine, form anions that are actually smaller than their neutral atoms. This is due to the strong electronegativity of these elements, which leads to a significant contraction of the electron cloud.

* Ionic vs. covalent: The statement primarily applies to ionic compounds, where a complete transfer of electrons occurs. In covalent compounds, the concept of "anion size" is less well-defined, as the electrons are shared rather than transferred.

In summary:

While the general trend is for anions to be larger than the parent atom, there are exceptions and variations depending on the specific element and its position in the periodic table.