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Periodic trends refer to patterns in the properties of elements that emerge as you move across a period or down a group in the periodic table. These trends help predict and understand various elemental properties such as atomic size, ionization energy, and electronegativity.

When considering ionic size, it's important to note how these trends manifest. As elements gain electrons to form negative ions, we observe changes in size based on their position in the periodic table.

• Across a period, from left to right, the ionic size generally decreases in metals but increases in nonmetals. This is because nonmetals tend to gain electrons and can often form anions.
• Down a group, the ionic size generally increases as new electron shells are added, making the atoms larger in nature.

For elements in the same period, such as sulfur and chlorine, sulfur (\(\mathrm{S}^{2-}\)) is larger compared to chlorine (\(\mathrm{Cl}^{-}\)) when they both form ions. This is primarily because sulfur gains more electrons than chlorine does, resulting in a larger ionic radius.

Negative ions, also known as anions, form when an atom gains one or more electrons. This increase in electrons leads to a net negative charge. As a result, the ion typically becomes larger than its neutral atom counterpart.

Some key points to understand about negative ions:

• Gaining electrons means there are more electron-electron repulsions within the atom. This increases the size of the electron cloud.
• The more electrons an atom gains, the stronger the repulsion becomes, causing the ion to expand.
• In the case of \(\mathrm{S}^{2-}\) and \(\mathrm{Cl}^{-}\), even though both are gaining electrons, sulfur gains more electrons making it a \(2-\) ion compared to chlorine's \(1-\) ion. This additional electron in sulfur's case increases its size significantly more than that of chlorine.

Negative ions are crucial to understanding not only trends across the periodic table but also the underlying chemical behavior of elements.

Electron-electron repulsion refers to the forces between electrons that occur because electrons hold like charges and naturally repel one another. This concept plays a significant role in determining the overall structure of ions, especially negative ions.

Key aspects of electron-electron repulsion include:

• When electrons are added to an atom, they occupy the outer electron shells, increasing repulsion forces among electrons.
• This repulsion causes the electron cloud to expand, increasing the size of the ion.
• For ions with more electrons, such as \(\mathrm{S}^{2-}\), the repulsive forces are stronger compared to ions like \(\mathrm{Cl}^{-}\). This contributes to a larger ionic radius for sulfur.

Understanding electron-electron repulsion helps clarify why ions differ in size based on their electron configuration, which is crucial for making sense of periodic trends and ionic sizes.