Chemistry · Periodic Classification Of Properties · NEET
As you move left to right in a period, electrons are added to the SAME outer shell, but the number of protons in the nucleus also increases. More protons means a stronger pull (higher effective nuclear charge) on the outer electrons. The shell does not change, so the whole electron cloud is pulled inward and the atom gets smaller. Example: in period 2, Li is bigger than Be, which is bigger than B, and so on down to F.
Going down a group, each element has one MORE electron shell than the one above it. Adding a new shell puts the outer electrons farther from the nucleus. Even though the nuclear charge increases too, the extra shell and the shielding by inner electrons win, so the atom gets bigger. That is why Cs (bottom of group 1) is larger than Rb and much larger than Li.
You cannot measure a single atom because its electron cloud fades out with no clear edge. So chemists measure the distance between the nuclei of two bonded atoms and take HALF of it. For a covalent bond this gives the covalent radius. Example: in a Cl-Cl molecule the two nuclei are 198 pm apart, so the covalent radius of chlorine is 99 pm.
Covalent radius is half the distance between two atoms joined by a covalent bond, so the atoms are held close together. Van der Waals radius is half the distance between two atoms in NEIGHBOURING molecules that are only touching, not bonded, so this distance is larger. This is why noble gas radii look very big: they are monoatomic and cannot form covalent bonds, so we compare their van der Waals radii, which are naturally larger than covalent radii of other elements. Do not compare a noble gas radius with covalent radii directly.
Normally atomic radius increases down a group, so you expect Ga > Al. But between Al and Ga, ten 3d electrons are filled (the first d-block appears). These 3d electrons shield the nucleus poorly, so the outer electrons of Ga feel a stronger effective nuclear charge and get pulled in. Result: Ga (135 pm) is actually a little smaller than Al (143 pm). This is a favourite NEET trap in the group 13 radius order: B < Ga < Al < In < Tl.
It depends on the direction. Down a group, atomic radius INCREASES with atomic number because new shells are added. Across a period, atomic radius DECREASES even though atomic number increases, because electrons fill the same shell while nuclear pull grows. So there is no single answer for the whole table; always state period or group.
The correct order of atomic radii in group 13 elements is
Which of the following statements are true? A. Unlike Ga, that has a very high melting point, Cs has a very low melting point. B. On the Pauling scale, the electronegativity values of N and Cl are not the same. C. Ar, K+, Cl-, Ca2+ and S2- are all isoelectronic species. D. The order of first ionization enthalpies of Na, Mg, Al, Si is Si > Al > Mg > Na. E. The atomic radius of Cs is greater than that of Li and Rb.
Identify the incorrect statement: A. The largest and smallest species among Mg, Mg2+, Al and Al3+ are Al and Mg2+ respectively. B. IUPAC name of element Z=107 is Unnilseptium. C. Similarity of Li with Mg is the diagonal relationship. D. Oxidation state and covalency of Al in [AlCl(H2O)5]2+ are 3 and 6.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Atomic radius is the size of an atom, taken as half the distance between the nuclei of two identical atoms joined by a bond. We use half the bonded distance because a single atom has no sharp edge to measure.
Atomic radius decreases from left to right across a period (nuclear pull increases on the same shell) and increases from top to bottom down a group (a new shell is added each time).
Among common elements, caesium (Cs) has one of the largest atomic radii because it sits at the bottom-left region of the periodic table, where many shells and low nuclear pull make atoms big. In general, the bottom-left corner has the largest atoms.
Noble gases are monoatomic and do not form normal covalent bonds, so their size is given as the van der Waals radius (distance between touching, non-bonded atoms). Van der Waals radii are naturally larger than covalent radii, so noble gas values look big and should not be compared directly with covalent radii.
Yes. Atomic radius is the base concept for ionization enthalpy, electronegativity, ionic size and reactivity trends. NEET regularly asks size-order questions, including tricky anomalies like Ga < Al, so mastering the trend saves you many marks.
