Question

for each of the following pairs, predict which atom or lon is smaller.

1. ______ li⁺, li
2. ______ o²⁻, o
3. ______ cl⁻, cl
4. ______ mg²⁺, al³⁺

Explanation:

Step1: Analyze the size of \(Li^+\) and \(Li\)

When an atom loses an electron to form a cation (\(Li^+\)), the number of electrons decreases while the nuclear charge remains the same. The remaining electrons are more strongly attracted to the nucleus. So, \(Li^+\) is smaller than \(Li\).

Step2: Analyze the size of \(O^{2 -}\) and \(O\)

When an atom gains electrons to form an anion (\(O^{2-}\)), the electron - electron repulsion increases. The added electrons expand the electron cloud. So, \(O\) is smaller than \(O^{2-}\).

Step3: Analyze the size of \(Cl^-\) and \(Cl\)

When an atom gains an electron to form an anion (\(Cl^-\)), the electron - electron repulsion increases. The added electron expands the electron cloud. So, \(Cl\) is smaller than \(Cl^-\).

Step4: Analyze the size of \(Mg^{2 +}\) and \(Al^{3+}\)

Both \(Mg^{2+}\) and \(Al^{3+}\) are isoelectronic (have the same number of electrons). The nuclear charge of \(Al^{3+}(Z = 13)\) is greater than that of \(Mg^{2+}(Z = 12)\). The greater nuclear charge in \(Al^{3+}\) pulls the electrons more strongly towards the nucleus. So, \(Al^{3+}\) is smaller than \(Mg^{2+}\).

Answer:

1. \(Li^+\)
2. \(O\)
3. \(Cl\)
4. \(Al^{3+}\)