The correct order of atomic radius of Li, Na, Be and O is
- (a)Na > Li > Be > O
- (b)Na > Be > Li > O
- (c)Be > Li > Na > O
- (d)O > Be > Li > Na
Correct — A, Na > Li > Be > O. The four elements can be placed with two rules and no memorised numbers. Lithium, beryllium and oxygen all sit in the second period, where atomic size generally decreases from left to right: the added electrons go into the same valence shell while the nuclear charge keeps rising, so the outer electrons are pulled in harder. That fixes Li > Be > O. Sodium sits directly below lithium in Group 1, and down a group the radius increases regularly, because the principal quantum number rises and the filled inner shells screen the outer electron from the nucleus. So sodium is the largest of the four and the whole order is Na > Li > Be > O. The NCERT table gives the numbers and they line up exactly: sodium 186 picometres, lithium 152, beryllium 111 and oxygen 66.
- (b)Na > Be > Li > O — Puts beryllium ahead of lithium. Both are in period 2 and beryllium is to the right of lithium, so beryllium at 111 picometres is smaller than lithium at 152, not larger.
- (c)Be > Li > Na > O — Gets the period-2 pair the wrong way round and also puts sodium below lithium in size. Sodium is one period lower, so it is the largest atom here at 186 picometres.
- (d)O > Be > Li > Na — Reverses the trend completely. Oxygen is far to the right of period 2 and is the smallest of the four at 66 picometres, not the largest.
Atomic radius is measured indirectly. For a non-metal it is taken as the covalent radius — half the internuclear distance between two identical atoms joined by a single bond, so the 198-picometre bond in a chlorine molecule gives chlorine a radius of 99 picometres. For a metal it is the metallic radius, half the distance between adjacent atoms in the metallic crystal. Across a period the effective nuclear charge grows while the valence shell stays the same, so size falls; down a group the valence electrons occupy a higher shell and are shielded by the filled shells beneath, so size grows.
The safe method for any ordering question of this kind is to place the elements on a rough sketch of the periodic table rather than to recall figures. Lithium, beryllium and oxygen lie along period 2 at atomic numbers 3, 4 and 8; sodium is at 11, one row down from lithium. Read the sketch from bottom-left to top-right and the order falls out. Two cautions are worth attaching. The noble gases are left out of these comparisons altogether, because their published values are van der Waals radii and are much larger than the covalent radii of their neighbours. And atomic radius is not the same as ionic radius — fluorine's atom is 64 picometres but its fluoride ion is 136, while sodium's atom is 186 and its sodium ion only 95, so an anion is always bigger than its parent atom and a cation always smaller.
- NCERT atomic radii in picometres for period 2: Li 152, Be 111, B 88, C 77, N 74, O 66, F 64.
- For period 3: Na 186, Mg 160, Al 143, Si 117, P 110, S 104, Cl 99.
- Down Group 1 the radius rises steadily — Li 152, Na 186, K 231, Rb 244, Cs 262.
- Atomic size generally decreases across a period as effective nuclear charge increases.
- A cation is smaller than its parent atom and an anion larger; noble gases are excluded from these comparisons.
Bottom-left is biggest and top-right is smallest, which reads directly as Na > Li > Be > O.
- Ranking by atomic number and assuming a heavier atom must be a bigger one — oxygen is heavier than lithium but much smaller.
- Mixing atomic radius with ionic radius, where a cation shrinks and an anion swells.
- Trying to slot a noble gas into such an order; its listed radius is a van der Waals value and is not comparable.
Asked as an ordering item mixing one Group 1 pair with two period-2 neighbours, so both the across-period and the down-group trend are needed.
Consider the following statements with reference to the Periodic Table of chemical elements: I. Ionisation potential gradually decreases along a period. II. In a group of elements, electron affinity decreases as the atomic weight increases. III. In a given period, electronegativity decreases as the atomic number increases. Which of these statement(s) is/are correct?
- (a) I only
- (b) II only
- (c) I and III
- (d) II and III
Answer(b) II only
The same two directions, applied to the other periodic properties. Statement II is accepted there because atomic size grows down a group — precisely the rule that puts sodium ahead of lithium here — while I and III fail because both ionisation potential and electronegativity rise across a period as radius falls.
- practice — not a real PYQ
Which one of the following has the largest atomic radius?
- (a)Fluorine
- (b)Chlorine
- (c)Potassium
- (d)Magnesium
Answer(c) Potassium — at 231 picometres it is the furthest down and to the left of the four, while fluorine at 64 picometres is the smallest.
- practice — not a real PYQ
Among the isoelectronic species, the one with the smallest radius is
- (a)the oxide ion
- (b)the fluoride ion
- (c)the sodium ion
- (d)the magnesium ion
Answer(d) the magnesium ion — all four have ten electrons, so the one with the largest nuclear charge holds them most tightly and is the smallest.