Among all elements, helium (He) has the smallest atomic radius (approximately 31 pm). Among non-noble gas elements, fluorine (F) has the smallest atomic radius (approximately 64 pm). Atomic size decreases across a period (left to right) due to increasing nuclear charge pulling electrons closer, and increases down a group as new electron shells are added. This makes fluorine the smallest non-noble atom and helium the smallest atom overall.
Helium (He) has the smallest atomic radius of all elements (~31 pm).
Fluorine (F) has the smallest atomic radius among non-noble gas elements (~64 pm).
Atomic size decreases across a period (left to right) due to increasing nuclear charge.
Atomic size increases down a group due to addition of new electron shells.
Smallest atoms are in the top-right of the periodic table; largest in the bottom-left.
Fluorine is also the most electronegative element (Pauling value: 3.98).
Hydrogen atomic radius (~53 pm) is larger than helium despite being the lightest element.
Caesium (Cs) is among the largest atoms in the periodic table.
Overall smallest atom: Helium (He) ā¢ Atomic radius: ~31 pm (picometres) ā¢ 2 electrons, 2 protons ā fully filled K shell ā¢ Very high nuclear charge relative to electron count
Smallest non-noble gas atom: Fluorine (F) ā¢ Atomic radius: ~64 pm ā¢ Electron configuration: 2, 7 ā¢ Period 2, Group 17 ā rightmost non-noble element in Period 2
Why fluorine is small: ā¢ High nuclear charge (9 protons) pulls 9 electrons very close ā¢ Only two electron shells (n=1 and n=2) ā small overall size ā¢ Highest electronegativity of all elements (3.98 on Pauling scale)
Note: Hydrogen has the simplest atom (1 proton, 1 electron) but its atomic radius (~53 pm) is larger than helium and fluorine.
Across a period (left ā right): ā¢ Atomic size DECREASES ā¢ Reason: nuclear charge increases (more protons) but electrons are added to the same shell ā electrons are pulled closer to the nucleus ā¢ Example (Period 2): Li > Be > B > C > N > O > F > Ne
Down a group (top ā bottom): ā¢ Atomic size INCREASES ā¢ Reason: new electron shells are added, increasing the distance from nucleus to outermost electrons ā¢ Example (Group 1): Li < Na < K < Rb < Cs
Summary: ā¢ Smallest atoms: top-right corner of periodic table ā¢ Largest atoms: bottom-left corner of periodic table ā¢ He, F have smallest radii; Cs, Fr have largest radii
Selected atomic radii for reference:
ā¢ He: 31 pm (smallest overall) ā¢ F: 64 pm (smallest non-noble gas) ā¢ O: 66 pm ā¢ N: 71 pm ā¢ H: 53 pm ā¢ C: 77 pm ā¢ Cl: 99 pm ā¢ Na: 186 pm ā¢ K: 227 pm ā¢ Cs: 265 pm (among largest)
Trend reason summary:
For exam purposes: ā¢ Smallest atom (overall): He ā¢ Smallest non-noble gas / most electronegative: F ā¢ Atomic size increases down a group, decreases across a period
Helium (He) has the smallest atomic radius of all elements (~31 pm). Among non-noble gas elements, fluorine (F) has the smallest atomic radius (~64 pm). Atomic size decreases across a period due to increasing nuclear charge.
Across a period (left to right), the number of protons increases while electrons are added to the same energy level (shell). The increased nuclear charge pulls electrons closer to the nucleus, reducing the atomic radius.
Down a group, each successive element has one more electron shell (principal quantum level). The outermost electrons are farther from the nucleus, and there is more electron shielding. These factors increase the atomic radius.
Fluorine has 9 protons, giving it a very high nuclear charge. It has only two electron shells (n=1 and n=2). The strong nuclear attraction pulls the 9 electrons very close to the nucleus, making it the smallest among non-noble gas elements.
No. Hydrogen has 1 proton and 1 electron with atomic radius ~53 pm. Helium (31 pm) and fluorine (64 pm) are smaller despite having more electrons, because their higher nuclear charge pulls electrons much closer.
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