Problem 23 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following is a favourable factor for cation formation? (a) high electron affinity (b) high electronegativity (c) small atomic size (d) low ionization potential

Step-by-Step Solution

Answer

(d) Low ionization potential is the favorable factor for cation formation.

Step 1: Understanding Cation Formation

Cations are positively charged ions formed when an atom loses one or more electrons. To determine what facilitates cation formation, we should consider factors that make electron removal easier.

Step 2: Evaluating Option (a) High Electron Affinity

Electron affinity is the energy change when an atom gains an electron. High electron affinity favors gaining electrons, not losing them, so it is not favorable for cation formation.

Step 3: Evaluating Option (b) High Electronegativity

Electronegativity is the tendency of an atom to attract electrons. High electronegativity does not favor losing electrons, making this option unfavorable for forming cations.

Step 4: Evaluating Option (c) Small Atomic Size

A smaller atomic size generally leads to higher ionization energy because the electrons are closer to the nucleus and more tightly held. This does not favor cation formation as it resists electron loss.

Step 5: Evaluating Option (d) Low Ionization Potential

Ionization potential, or ionization energy, is the energy required to remove an electron from an atom. A low ionization potential means less energy is needed to remove an electron, making it favorable for cation formation.

Key Concepts

Electron Affinity

The concept of electron affinity refers to the amount of energy released or consumed when an electron is added to a neutral atom in the gaseous state. This is important because it tells us how much an atom 'wants' to gain an electron.

While cations are formed by losing electrons, electron affinity deals with how well an atom can gain one. High electron affinity means the atom can easily acquire an electron, which is not helpful for cation formation.

Instead, elements with low electron affinity are usually better suited to form cations as they do not have a strong pull towards retaining additional electrons, aiding the process of electron loss.

Electronegativity

Electronegativity is a measure of an atom's ability to attract and hold onto electrons.

An atom with high electronegativity has a strong tendency to attract electrons towards itself and form negative ions, which in turn counteracts the formation of cations. This is because cations require an atom to lose electrons and form a positive charge, which is the opposite of what high electronegativity promotes.

Thus, elements lower on the electronegativity scale are typically more inclined to lose electrons and form cations.

• High electronegativity -> Attracts more electrons.
• Low electronegativity -> Can lose electrons more easily.

Atomic Size

Atomic size or atomic radius is the distance from an atom's nucleus to the outermost electrons.

A smaller atomic size leads to a greater attraction between the nucleus and the electrons, making it more difficult for the atom to lose an electron and form a cation.

On the other hand, larger atomic size usually results in a smaller pull on the outer electrons, which can more easily be lost, facilitating cation formation.

• Small atomic size: Stronger pull on electrons -> Harder to lose electrons.
• Large atomic size: Weaker pull on electrons -> Easier to lose electrons.

Ionization Potential

Ionization potential, also known as ionization energy, refers to the energy required to remove an electron from an atom or ion.

Low ionization potential is a key factor in the formation of cations because it indicates that the atom can lose an electron more easily without needing much energy input.

Conversely, a high ionization potential means that it requires more energy to remove an electron, making it less favorable for forming positive ions. Therefore, atoms with low ionization energies are more likely to form cations.

• Low ionization potential -> Easier electron loss -> Favorable for cation formation.
• High ionization potential -> Difficult electron loss -> Not favorable for cation formation.