Question

For each pair, identify the larger atom or ion. (a) \(\mathrm{Mg}\) or \(\mathrm{Mg}^{2+}\) (b) \(\mathrm{P}\) or \(\mathrm{P}^{3-}\)

Short answer

The larger atom/ion in each pair are: (a) Mg is larger than Mg2+; (b) P3- is larger than P.

Step-by-step solution

Identify Atomic Size

Determine the atomic size from the periodic table. The atomic size of Mg and P can be determined from their position in the periodic table. Metals (like Mg) can lose electrons and form cations while non-metals (like P) can gain electrons and form anions.

Predict Ionic Size for Mg or Mg2+

The atomic size of Mg is larger than Mg2+ because Mg2+ has lost two electrons resulting in a decrease in its atomic size. So, Mg is larger than Mg2+

Predict Ionic Size for P or P3-

The atomic size of P is smaller than P3- because P3- has gained three electrons resulting in an increase in its atomic size. So, P3- is larger than P

Key concepts

Periodic Table Trends

Understanding how atomic size changes across the periodic table is crucial for predicting the size of different atoms and ions. As we move from left to right across a period in the periodic table, the atomic size typically decreases. This happens because electrons are added to the same energy level, but the increasing nuclear charge pulls these electrons closer to the nucleus, reducing the size of the atom.

On the other hand, as we move down a group, atomic size increases. This is due to the addition of new energy levels, which are further away from the nucleus, resulting in a larger atomic radius. Taking these trends into account not only helps students in identifying the larger atom or ion in a pair but also reinforces conceptual understanding of the relationship between an element's position on the periodic table and its properties.

Cations and Anions

Cations and anions play a critical role in understanding atomic sizes. A cation is an atom that has lost one or more electrons, resulting in a positive charge. Because of this loss, the electrostatic repulsion among the remaining electrons decreases, and the electrons are drawn closer to the nucleus. This contraction results in a smaller atomic size compared to the neutral atom.

Conversely, an anion is formed when an atom gains electrons, becoming negatively charged. The addition of extra negatively charged electrons increases electron-electron repulsion, causing the atom to expand. Therefore, anions are larger in size compared to their neutral counterparts. Knowing whether an atom tends to form a cation or an anion, based on its position as a metal or non-metal, significantly aids in predicting changes in atomic size upon ion formation.

Ionic Size Prediction

The ionic size prediction is a logical extension of understanding periodic trends and the nature of cations and anions. To predict ionic size, one should consider the atomic size of the neutral atom and the charge of the ion. For example, the magnesium ion, \( \mathrm{Mg}^{2+} \), is smaller than its neutral atom counterpart because it has lost two electrons, decreasing electron-electron repulsion and allowing the remaining electrons to be pulled closer to the nucleus.

In contrast, the phosphide ion, \( \mathrm{P}^{3-} \), is larger than a neutral phosphorus atom because it has gained three electrons. This increases the repulsion between the electrons and the size of the electron cloud, resulting in a larger ionic radius. These predictions can be confirmed by referring to the periodic table and using the understanding of the charged particles' nature, benefiting students in their approach to comparing atomic and ionic sizes accurately.