Using only the periodic table, predict the most stable ion for Na, Mg, Al, S, Cl, K, Ca, and Ga. Arrange these from largest to smallest radius, and explain why the radius varies as it does. Compare your predictions with Fig. \(8.8\).

Understanding the periodic table is fundamental in predicting ionic radii, as it is organized to showcase patterns in elements' properties. The table is arranged in rows called periods and columns known as groups. One crucial trend regarding ionic radius involves its variation across periods and down groups.

As we journey across a period from left to right, the ionic radius typically decreases. This shrinkage occurs because the atomic number rises, leading to a stronger positive charge in the nucleus that pulls electrons closer. However, as we travel down a group, ionic radii increase. This increment is because, with every step downwards, we introduce a new electron shell, further from the nucleus, offering more room and sometimes, shielded from the nucleus' pull by inner electrons.

Ion Size Comparisons

In a particular group, a cation will always be smaller than its parent atom, and an anion will be larger than its parent atom. The logic behind this is grounded in electron loss or gain — the former shrinks the electronic cloud surrounding the nucleus, while the latter increases electron-electron repulsion, leading to a more swollen cloud. These trends help explain why, for example, a potassium ion (K+) is larger than a magnesium ion (Mg2+), despite both belonging to the same period.

The formation of cations and anions is a response to an atom's urge to achieve a more stable electron configuration, often resembling the nearest noble gas. To reach this desired state, atoms will either lose or gain electrons.

In the process, ions with a positive charge, or cations, emerge when an atom surrenders electrons. Commonly, this occurs with metals like sodium (Na) and aluminum (Al), which will shed one or more electrons. This results in a smaller ionic radius due to the reduced repulsion between the remaining electrons, and the increased attraction from the positively charged nucleus.

Conversely, anions are born when an atom acquires extra electrons, becoming negatively charged. Nonmetals like sulfur (S) and chlorine (Cl) tend to follow this path, accepting electrons to fulfill their outer shells. Subsequently, the ionic radius enlarges since the added electrons increase repulsion among one another and expand the electron cloud.

The effect of an element's atomic number on ionic radius is a key concept in understanding ionic sizes. The atomic number, which is the count of protons in the nucleus, not only defines the element but also influences the size of an ion that the atom can form.

Every step up the atomic number scale represents an additional proton in the nucleus and typically an added electron orbiting the nucleus. This extra proton exerts a stronger pull on the surrounding electrons, a phenomenon known as an increased effective nuclear charge. As a result, the electrons are drawn in tighter, leading to a smaller atomic or ionic radius for ions across a period.

However, when examining the effect down a group, an increase in atomic number means the introduction of new electron shells, which outweighs the heightened nuclear pull. This causes the ionic radius to grow, as electrons are further removed from the nucleus, with each new shell. In this context, the atomic number serves as a guiding parameter in predicting ionic sizes and their trends within the periodic table.