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Atomic radius

Definition of atomic radius

The atomic radius is the size of the atom, typically measured by the distance from the nucleus of the atom to the electron clouds around the nucleus. As there are no physical existence of orbital in atoms, it is difficult to measure the atomic radius. Thus sometime different methods are used to measure the radius while they are bonded in a molecule.

Ways to measure the atomic radius

According to the Heisenberg uncertainty principle, it is not possible to measure the momentum and the position of the electron simultaneously. So, as the position of electron is not certain rather they are explained as electron cloud around the nucleus, it is hard to measure an atomic radius accurately. There are different methods of calculating the radius by measuring the distance between two nucleus when an atom is bonded in a molecule.

Covalent radius

Covalent radius is the radius calculated when an atom is bonded covalently with another atom of same element. When an atom (A) is covalently bonded with another homonuclear atom (A), the atomic radius of that atom can be calculated by measuring the distance between the two nucleus of those two atoms. The diameter of an atom is equal to the distance between two nuclei and the atomic radius will be the half of it.

CK-12 Foundation [CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0)], via Wikimedia Commons

Metallic radius

The metallic radius is the radius calculated between two metal atoms bonded together in metal cluster. The metallic radius can also be calculated by measuring the distance between the two nuclei of the two atoms. The metallic diameter is equal to the distance between two nuclei and the radius is just half of it.

Ionic radius

Ionic radius is the radius calculated when an atom is bonded with another atom in a molecule by transferring electrons to make ionic bond. Here the radius of two ions are not equal and few other things need to be discussed before we calculate the ionic radius:

  • The atoms of different element, they must differ in electrons and protons, sometimes number of orbitals and so on.
  • Again the atomic radius is different because of their ionic status of the atom. The atom can be cation or anion by losing or gaining electron.
  • When an atom loses electron sometime it has less orbital than before. However the nucleus attract the rest of the electrons more tightly towards the nucleus. The ionic radius is thus reduced than atomic radius.
  • In the same way the anions can have lesser radius that the radius of its original neutral atom.

So to calculate the atomic radius, at first the distance between the nuclei is measured. Then depending on atomic size, the atomic radius of the two atoms are calculated. The bigger the atomic size, the larger the atomic radius.

Atomic radius in the periodic table

The atomic radius in the periodic table decreases across the period and increases down the group.

Cdang and Adrignola [GFDL (http://www.gnu.org/copyleft/fdl.html) or CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0)], via Wikimedia Commons

Atomic radius decreases across the period

Moving from left to right across a period, the number of protons and electrons increases while the number of energy shells stay same. Thus the increasing number of nucleus attracts the more electrons more tightly towards it and the atomic radius decreases.

Atomic radius increases down the group

Moving down a group, the number of energy shells also increases with the increase of protons and electrons. Thus the atomic radius also increases down the group in periodic table.

Exception

As in the transition metals the electrons are added in the inner shell, the nucleus attracts the increasing electrons towards it more and thus the radius decreased.

Summary

  • The atomic radius is the size of the atom, typically measured by the distance from the nucleus of the atom to the electron clouds around the nucleus.
  • Covalent radius can be calculated by measuring the distance between the two nucleus of two atoms in covalent compound.
  • The metallic radius can also be calculated by measuring the distance between the two nuclei of the two atoms in metal cluster.
  • Ionic radius are calculated by considering the atomic size of the two atoms.
  • The atomic radius in the periodic table decreases across the period and increases down the group. Exceptions are observed in transition metal elements.
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