Which of the following elements has the largest atomic radius calcium sodium potassium magnesium

NaMgKCa

Answer : C

Solution : Na and K are in the same group and K is below than Na, so K will have higher atomic radius i.e., `K gt Na`. In a period on moving from left to right, atomic radius decreases. Since, K and Ca are in the same period and K is in 1 group and Ca is in 2 group, so atomic radius of K will be more than Ca, i.e., `K gt Ca` <br> Also Na and Mg are in the same period, but Na belongs to 1 group and Mg belongs to 2 group, so atomic radius of Na is more than Mg, i.e., `Na gt Mg`. Thus, if we take all these together we get `K gt Na gt Mg and K gt Ca gt Mg`. Hence, we can say the atomic radius of K is largest.

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Solution

(c) K. Potassium (K) with an atomic number of 19 has the largest atomic radius. This is because sodium and potassium are elements of group 1. On moving from top to bottom in a group, the atomic radius increases. Magnesium and calcium are elements of group 2. The size of the atomic radius decreases on moving from left to right in a period of the periodic table

Periodic Classification of Elements

Variation of Atomic Size

Standard X Chemistry

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Atomic radii reported in units of picometers (pm).

Data taken from John Emsley, The Elements, 3rd edition.  Oxford:  Clarendon Press, 1998.

The atomic radius is the distance from the nucleus of an atom to the outermost electrons.  Since the orbitals around an atom are defined in terms of a probability distribution in quantum mechanics, and do not have fixed boundaries, determining where an atom "stops" is not very straightforward.  By comparing the bond lengths of a number of representative compounds of an element, an average size for most atoms can be determined.

The atomic radius can also be defined in other ways.  The van der Waals radius (also known as the nonbonding atomic radius) is the radius of an atom which is not bonded to other atoms; this is determined by measuring the distance between atomic nuclei which are in direct but nonbonding contact with each other in a crystal lattice.  The covalent atomic radius (also known as the bonding atomic radius) is determined for metals by taking one-half of the distance between two adjacent atoms in a metallic crystal, or one-half the distance between like bonded atoms for nonmetals.

Unfortunately, it is not possible to determine the radius for every element on the periodic table in the same way, and consequently, it is sometimes difficult to make comparisons between different sets of data.  In the table above, most of the atomic radii listed are average atomic radii, while for the halogens (Group 7A) and the noble gases (Group 8A) the covalent radius is used.

Atomic radii vary in a predictable way across the periodic table.  As can be seen in the figures below, the atomic radius increases from top to bottom in a group, and decreases from left to right across a period.  Thus, helium is the smallest element, and francium is the largest.

• From top to bottom in a group, orbitals corresponding to higher values of the principal quantum number (n) are being added, which are on average further away from the nucleus, thus causing the size of the atom to increase.
• From left to right across a period, more protons are being added to the nucleus, but the electrons which are being added are being added to the valence shell, not to the lower energy levels.  As more protons are added to the nucleus, the electrons in the valence shell feel a higher effective nuclear charge — the sum of the charges on the protons in the nucleus and the charges on the inner, core electrons.  (See figure below.)  The valence electrons are therefore held more tightly, and the size of the atom contracts across a period.

The following charts illustrate the general trends in the radii of atoms:

Atomic size decreases across a Period, and increases down a Group. Between Na, Mg, and Be, sodium should have greatest atomic size.

Given that atomic size decreases across a Period, the sodium atom is larger than that of magnesium. Both third row metals are larger than second row beryllium, inasmuch as the third Period metals build on the atomic radius (the electron shell) already possessed by the second Period.

You have to know that for a given Period, atomic size decreases across from left to right. And you will be given a Periodic Table in any Chemistry exam.

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