Question

For parts of the free-response question that require calculations, clearly show the method used and the steps involved in arriving at your answers. You must show your work to receive credit for your answer. Examples and equations may be included in your answers where appropriate.

Answer the following questions related to K .

(a) In terms of atomic structure, explain why the atomic radius of K is larger than that of Na .

Answer 1

The atomic radius of potassium (K) is larger than that of sodium (Na) because K has an extra electron shell, increasing the electron-nucleus distance. The additional shell also causes increased shielding, which allows the outermost electrons to be held less tightly, and thus the atomic radius is larger.

Step-by-step explanation:

The question involves comparing the atomic radius of potassium (K) to that of sodium (Na) using principles of atomic structure. Atomic radius is generally understood to be the distance from the atomic nucleus to the outermost shell of electrons. To begin with, the atomic radius of K is larger than that of Na because K has more electron shells than Na. As you move down a group in the periodic table, each element has an additional electron shell compared to the one above it. This extra shell increases the distance between the nucleus and the outermost electrons, resulting in a larger atomic radius.

Furthermore, as we add protons and electrons, the increase in positive charge in the nucleus of K compared to Na attracts the electrons more strongly. However, because the additional electrons in potassium are to be found in a new, outer electron shell, the effect of the increased nuclear charge is somewhat mitigated by the shielding effect. This is where inner shells of electrons shield the outer electrons from the full charge of the nucleus. Because potassium's additional electron shell experiences more significant shielding, the outer electrons are held less tightly and the atomic radius expands.

The concept of shielding also explains differences in ionic radii, as shown in ions like Ca2+ compared to the neutral Ca atom. Upon losing electrons to form Ca2+, the radius decreases significantly due to reduced electron-electron repulsion and increased attraction from the nucleus. This trend in changing ionic and atomic radii across the periodic table, such as between different groups like K and Na, is underlined by the balance between nuclear charge, electron shielding, and the number of electron shells.