Question

Explain why cations are smaller than their uncharged atoms and anions are larger than their uncharged atoms.

Short answer

Cations are smaller than their uncharged atoms because the loss of electrons reduces electron-electron repulsion and increases the effective nuclear charge, pulling electrons closer to the nucleus. Anions are larger because the gain of electrons increases electron-electron repulsion and enhances the electron shielding effect, causing electrons to spread out.

Step-by-step solution

Understanding Cations

Cations are positively charged ions that are formed when an atom loses one or more electrons. The loss of electrons reduces the electron-electron repulsion in the atom, which allows the electrons to be pulled closer to the nucleus. This results in a decrease in the atomic radius, making cations smaller than their neutral atoms.

Understanding Anions

Anions are negatively charged ions formed when an atom gains one or more electrons. Adding electrons increases the electron-electron repulsion, which makes it harder for the nucleus to hold the electrons tightly. This leads to an increase in the atomic radius, making anions larger than their neutral atoms.

Electron Shielding and Effective Nuclear Charge

The size of an ion is also affected by the electron shielding and effective nuclear charge. For cations, the loss of electrons can lead to a reduced shielding effect and a greater effective nuclear charge, further pulling the remaining electrons closer to the nucleus. In anions, the added electrons increase the shielding effect, reducing the effective nuclear charge and allowing the electrons to spread out more.

Key concepts

Cations and Anions

When atoms lose or gain electrons, they form ions. Two types of ions are cations and anions. Cations are positively charged ions that result from an atom shedding one or more electrons. This loss typically occurs in metals which start with loosely held outer electrons. Losing these electrons makes the atom more streamlined, so to speak, resulting in a positive ion that is smaller than the original atom due to less electron-electron repulsion.

On the flip side, anions are negatively charged ions created when an atom, usually a non-metal, gains extra electrons. These new electrons increase the 'family' of electrons surrounding the nucleus and repel each other due to their like charges. The increased repulsion pushes these electrons apart, making the anion larger than the atom before it gained extra electrons.

Electron-Electron Repulsion

It's like trying to hold a group of magnets that all have the same pole facing each other: they push away. That's how electron-electron repulsion functions within the atom. Electrons are negatively charged, and they all repel each other because of their like charges. This fundamental force in atomic structure affects the atomic radius. In cations, when the outer electrons are removed, the remaining electrons have less repulsion between them and can be pulled closer to the nucleus by its positive charge. However, in anions, the added electrons cause more repulsion, which pushes the electrons further apart, resulting in a larger atomic radius compared to neutral atoms.

Electron Shielding

Guess what happens when there's a lot of people between you and something you're trying to see? Your view gets blocked – this is similar to the concept of electron shielding. In an atom, the inner shells of electrons can act as a 'shield', preventing the outer electrons from feeling the full pull of the nucleus. As you could imagine, the more layers of electrons (or people), the less effective that pull (or sight) is.

For cations, electron shielding effect is reduced after the atom loses electrons, meaning the remaining electrons can feel a stronger pull from the nucleus. But with anions, additional electrons increase the shielding, making the pull from the nucleus less effective and giving electrons more space to spread out, hence enlarging the ion's radius.

Effective Nuclear Charge

Effective nuclear charge (ENC) is essentially the net positive charge experienced by electrons in the valence shell. This concept helps us understand why electrons remain in orbit around the nucleus and why their distance from the nucleus changes with the formation of ions.

When an atom becomes a cation, its electron shielding is reduced, and the ENC on the remaining electrons increases. As if the gravity around the nucleus has gotten stronger, the electrons are pulled in more tightly, reducing the radius of the ion. Conversely, for anions, the ENC is reduced due to increased distance from the nucleus and increased shielding from the extra electrons. This makes the nucleus's pull weaker on each electron, allowing the anion to have a larger radius than its neutral counterpart.