Question 1

What is the difference between van der Waals radius and covalent radius?

Accepted Answer

Van der Waals radius, or nonbonding atomic radius, is one-half the distance between adjacent nuclei in atomic solids. Covalent radius, or bonding atomic radius, differs for metals and nonmetals. In metals, it's one-half the distance between neighboring atom centers in crystal structures. In nonmetal diatomic molecules, it's one-half the distance between bonded atom centers.

Question 2

Why does atomic radius increase when moving down a group in the periodic table?

Accepted Answer

As you move down a group, the principal quantum number increases by one for each element, placing outer electrons farther from the nucleus. Although nuclear charge increases, the additional inner shell electrons shield outer electrons more effectively. This greater distance outweighs the increased nuclear charge, causing atomic radius to increase down the group.

Question 3

How does effective nuclear charge explain the decrease in atomic radius across a period?

Accepted Answer

Across a period, nuclear charge increases while the number of inner shell electrons remains constant. Outer electrons in the same valence shell do not shield each other effectively. As effective nuclear charge increases, the nucleus pulls outer electrons closer, decreasing atomic radius. This stronger pull creates the periodic trend of decreasing size from left to right.

Question 4

What role does electron shielding play in determining atomic size?

Accepted Answer

Inner shell electrons partially shield outer shell electrons from the nucleus's pull, reducing the effective nuclear charge felt by valence electrons. Core electrons shield efficiently, but outermost electrons do not shield one another effectively. The greater the effective nuclear charge after accounting for shielding, the stronger the nucleus holds outer electrons, resulting in a smaller atomic radius.

Question 5

Why do transition elements have relatively constant atomic radii across a row?

Accepted Answer

Transition elements maintain roughly constant atomic radii across each row because the number of electrons in the outermost principal energy level remains nearly constant. Although nuclear charge increases, the additional electrons enter inner d orbitals, not the valence shell. This results in a roughly constant effective nuclear charge experienced by outer electrons.

Question 6

How do alkali metals compare in size within their group?

Accepted Answer

Alkali metals show maximum atomic radius at the beginning of each period. Moving down group 1 from lithium to cesium, atomic radius increases significantly. This trend reflects the increasing principal quantum number for valence electrons, placing them progressively farther from the nucleus despite increasing nuclear charge.

Question 7

What determines whether an atom's size is defined by bonding or nonbonding radius?

Accepted Answer

Bonding radius applies when atoms form covalent bonds, measured as one-half the distance between bonded atom centers in molecules or crystal structures. Nonbonding radius applies to isolated atoms in solids, measured as one-half the distance between adjacent nuclei. The choice depends on whether atoms are chemically bonded or simply in contact within a solid structure.

原子半径和有效核电荷