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which of the following best helps to explain why the atomic radius of is greater than that of ?

The atomic radius of one element is greater than that of another mainly because of periodic trends : how atomic size changes across a period and down a group in the periodic table.

Since your question is written in a generic form (“atomic radius of ___ is greater than that of ___”) and the specific elements or options are missing, here is the general rule that such multiple‑choice questions almost always test:

Core ideas that explain a larger atomic radius

  1. Position on the periodic table (period and group)
    • Atomic radius increases down a group because atoms gain additional electron shells; the valence electrons are in orbitals farther from the nucleus, so the atom is larger.
 * Atomic radius **decreases across a period from left to right** because the number of protons increases while electrons are added to the same principal energy level; the stronger nuclear attraction pulls electrons closer, shrinking the atom.
  1. Effective nuclear charge(Zeff)(Z_{\text{eff}})(Zeff​)
    • ZeffZ_{\text{eff}}Zeff​ is the net positive charge felt by valence electrons after accounting for shielding by inner electrons.
 * A **higher** effective nuclear charge pulls electrons in more tightly, giving a **smaller** radius; a **lower** effective nuclear charge leads to a **larger** radius.
  1. Electron shielding and added shells
    • When you go down the table, inner electrons in additional shells shield the outer electrons more effectively from the nucleus, weakening the attraction and allowing the electron cloud to spread out.
 * This shielding effect outweighs the increase in nuclear charge down a group, so atoms get larger.
  1. Ion type (if the question involves ions)
    • A cation (positively charged ion) is smaller than its neutral atom because it has fewer electrons and reduced electron–electron repulsion; the remaining electrons are pulled closer to the nucleus.
 * An **anion** (negatively charged ion) is **larger** than its neutral atom because extra electrons increase electron–electron repulsion and expand the electron cloud.

What a typical correct option looks like

In a standard multiple‑choice version of your question (for example, “Why is the atomic radius of K greater than that of Br?”), the best explanation usually says something like:

“Potassium has its valence electrons in a higher principal energy level and/or experiences a lower effective nuclear charge than bromine, so its electrons are farther from the nucleus and the atomic radius is larger.”

In more generic wording, the correct choice is normally the one that refers to:

  • Electrons being in a higher principal energy level (more shells), and/or
  • A smaller effective nuclear charge on the outer electrons, often because of greater shielding by inner electrons.

How to use this to answer your actual question

If you can see the actual answer choices, pick the one closest in meaning to:

  • “The element with the larger radius has its valence electrons in an energy level farther from the nucleus (more shells).”
  • Or: “The element with the smaller radius experiences a larger effective nuclear charge on its valence electrons, pulling them closer to the nucleus.”

If you paste the specific options (A, B, C, D), I can point out exactly which one matches this reasoning.