Question

What determines the length of an element's atomic radius? I. The number of valence electrons II. The number of electron shells III. The number of neutrons in the nucleus A. I only B. II only C. I and II only D. I, II, and III

Short answer

B. II only

Step-by-step solution

Understanding Atomic Radius

Atomic radius is the distance from the nucleus of an atom to the outer boundary of its electron cloud. It is influenced by the electron configuration of the element.

Factor I: Number of Valence Electrons

Valence electrons are the outermost electrons of an atom. While they do affect the chemical reactivity of an element, they do not significantly determine the size of the atomic radius. The correct way this factor might indirectly affect the radius is through effective nuclear charge, but it isn't a direct determinant of atomic radius size.

Factor II: Number of Electron Shells

The number of electron shells (energy levels) directly affects the atomic radius. The more electron shells an atom has, the larger its atomic radius, because electron shells are layers that add bulk to the atom.

Factor III: Number of Neutrons in the Nucleus

The number of neutrons in the nucleus affects the mass of an atom but not the atomic radius. Neutrons contribute to the atomic mass but don't change the volume occupied by the electron cloud.

Determining the Correct Answer

Based on the analysis above, the number of electron shells is directly correlated with the length of an element’s atomic radius. Hence, the correct answer is the factor that includes the number of electron shells.

Key concepts

Valence Electrons

Valence electrons are the outermost electrons in an atom. They play a key role in chemical reactions and bonding.
These electrons, due to their position in the outermost shell, determine how an element interacts with others.

However, when we talk about the length of the atomic radius, valence electrons have a more indirect role. They influence the effective nuclear charge, which can slightly affect the atomic radius.
But on their own, valence electrons do not significantly determine the size of the atom.

Instead, the atomic radius is mainly influenced by factors like the number of electron shells and effective nuclear charge.

Electron Shells

Electron shells, also known as energy levels, have a direct impact on the atomic radius.
Each shell can be thought of as a layer around the nucleus where electrons reside.

The more electron shells an atom has, the larger its radius becomes.
This is because each additional shell adds a level of spacing around the nucleus, increasing the overall size.
A good way to understand this is to imagine layers of an onion; each new layer increases the onion's size.

As you move down a group in the periodic table, atoms gain more electron shells, resulting in a larger atomic radius.

Effective Nuclear Charge

The effective nuclear charge (often abbreviated as Zeff) is the net positive charge experienced by an electron in a multi-electron atom.
It is a crucial concept for understanding atomic structure.

While protons in the nucleus create a strong positive charge, this charge is partially offset by the shielding effect of inner-shell electrons.
Effective nuclear charge can be calculated using the formula: Zeff = Z - S
Where Z is the atomic number (total number of protons), and S is the shielding constant.

This means effective nuclear charge increases across a period on the periodic table as more protons are added without adding additional shielding electrons.
When Zeff increases, it pulls the outer electrons closer to the nucleus, slightly decreasing the atomic radius.

Therefore, effective nuclear charge indirectly influences the atomic radius, but it is not the most direct determinant.