Question

(a) Why are monatomic cations smaller than their corresponding neutral atoms? (b) Why are monatomic anions larger than their corresponding neutral atoms? (c) Why does the size of ions increase as one proceeds down a column in the periodic table?

Short answer

(a) Monatomic cations are smaller than neutral atoms because losing an electron results in a smaller electron cloud and increased effective nuclear charge, pulling remaining electrons closer to the nucleus. (b) Monatomic anions are larger than neutral atoms because gaining an electron increases electron-electron repulsion and reduces effective nuclear charge experienced by the added electron, resulting in a larger overall size. (c) Ion size increases down a column in the periodic table due to an increase in the principal quantum number (n), resulting in more electron shells and greater shielding, which reduces the effective nuclear charge experienced by outermost electrons, further increasing ion size.

Step-by-step solution

(a) Explanation for monatomic cations being smaller than neutral atoms

Monatomic cations are formed when a neutral atom loses one or more electrons. Losing an electron results in a smaller electron cloud around the atom's nucleus, while the number of protons in the nucleus remains the same. This increases the effective nuclear charge experienced by the remaining electrons, which pulls them closer to the nucleus. As a result, the overall size of the monatomic cation is smaller than its corresponding neutral atom.

(b) Explanation for monatomic anions being larger than neutral atoms

Monatomic anions are formed when a neutral atom gains one or more electrons. Gaining an electron increases the overall negative charge (electron cloud) around the nucleus, which increases electron-electron repulsion. The added electron also experiences shielding from the inner electrons, which reduces the effective nuclear charge experienced by that electron. Due to the increased electron-electron repulsion, these electrons are farther apart from each other, resulting in a larger overall size of the anion compared to its corresponding neutral atom.

(c) Explanation for the increasing ion size down a column in the periodic table

As you move down a column in the periodic table, the principal quantum number (n) increases. This means that electrons are occupying higher energy levels and are farther away from the nucleus. The increase in the number of electron shells in lower rows of the periodic table results in larger overall size of ions. Additionally, the increase in the number of electron shells leads to a greater degree of shielding, which reduces the effective nuclear charge experienced by the outermost electrons. This shielding effect further increases the size of the ion as one moves down a column in the periodic table.

Key concepts

Monatomic Cations

Monatomic cations are ions formed when a neutral atom loses one or more electrons. This loss of electrons results in a smaller electron cloud surrounding the nucleus. Interestingly, even though electrons are missing, the number of protons in the nucleus remains unchanged. This imbalance causes the protons to exert more pull on the remaining electrons.

• This increased attractive force is due to a relatively higher effective nuclear charge.
• The electrons are drawn closer to the nucleus, leading to a decrease in the size of the ion.
• This makes monatomic cations smaller than their original neutral atoms.

Understanding why cations are smaller is about observing this balance of forces. When there are fewer electrons to repel each other, and the protons' effect strengthens, the ionic radius shrinks.

Monatomic Anions

Monatomic anions arise when a neutral atom gains additional electrons. This results in more electrons orbiting the nucleus, creating a greater negatively charged electron cloud. Unlike with cations, this accumulation leads to increased electron-electron repulsion.

• With more electrons, the outermost layer experiences a push, making the ion expand.
• Additionally, new electrons experience shielding by inner electrons.
• This shielding effect lowers the effective nuclear charge experienced by the outer electrons.

Thus, monatomic anions are larger than the neutral atoms they come from. It's all about the expansion due to repulsion and the reduced pull from the nucleus.

Periodic Table Trends

Periodic table trends, specifically regarding ionic sizes, provide insight into how element characteristics change down the columns (groups). For ions, this change is predominantly about size increase as you move down a group.

• As you move down, the principal quantum number (\( n \)) increases, meaning electrons reside in higher energy levels further from the nucleus.
• This increase in energy levels results in more electron shells, or layers, around the nucleus.
• More shells mean increased size of ions.

Furthermore, with each added shell, electrons experience more "shielding," or blocking of nuclear charge by inner shells, reducing the nucleus's pull on outer electrons. This trend explains why ions get larger as one moves down a column in the periodic table.

Effective Nuclear Charge

Effective nuclear charge (ENC) is a crucial concept in understanding ionic radii and general atomic behavior. ENC refers to the net positive charge experienced by valence electrons in an atom or ion.

• The more protons in an atom, the greater the nuclear charge.
• The more electron shells between the nucleus and the valence electrons, the lower the effective nuclear charge due to shielding effects.
• For cations, ENC is stronger because there are fewer electrons to shield the outer electrons from the protons.

In contrast, when extra electrons are added to form anions, the ENC is reduced due to increased shielding and repulsion, causing the expanding of the electron cloud. Thus, understanding ENC aids in explaining why ions can be smaller or larger compared to their neutral atoms.