Periodic Properties

The properties which are directly or indirectly related to their electronic configuration and show gradual change when we move from left to right in a period or from top to bottom in a group are called periodic properties. Such properties show a variation with change in atomic number.

Atomic volume, atomic size, melting size, melting point, boiling point, and density are important physical properties which show periodicity.

Some important chemical properties that exhibit periodicity are electronic configuration, ionization energy, electron affinity, electronegativity, metallic character, nature of oxides, oxidation state, and reducing character etc. (Specific heat, refractive index etc. are not periodic properties.)

• Atomic size

  It refers to the distance between the centre of the nucleus of an atom to its outermost shell of electrons.

Since the absolute value of atomic size cannot be determined, it is expressed in terms of the operational definitions such as ionic radius, covalent radius, Vanderwaal's radius, and metallic radius. The absolute value of atomic radius can not be determined because-

(i) It is not possible to locate the exact position of electrons in an atom as an orbital has no sharp boundaries.

(ii) It is not possible to isolate an individual atom for its size determination.

(iii) In a group of atoms, the probability distribution of electrons is influenced by the presence of neighbouring atoms. Thus, the size of an atom may change in going from one environment to other.

(a) Covalent radius:It is defined as half of the distance between two successive nuclei of two covalent bonded like an atom in a molecule. It is in the case of non-metal atoms.

If the bond length between two atoms is -d, then

covalent radius = 1/2 * [internuclear distance in two covalent bonded atoms]

$$\therefore Covalent\;radius = \frac{1}{2}d$$

(b) Vander Waals' radius:

It is defined as one-half of the distance between the nuclei of two non-bonded isolated atoms or two adjacent atoms belonging to two neighbouring molecules of an element in the solid state.

In general, Vanderwaals' radius is greater than the covalent radius of an atom.

(c) Metallic radius:It is defined as half the distance between two successive nuclei of two adjacent metal atoms in the metallic-closed packed crystal lattice.

(d) Ionic radius:It is the distance of the outermost shell of an anion or cation from its nucleus. It may also be defined as the effective distance from the nucleus of the ion up to the electrons in the outer shell to which it can exert influence in the ionic bond.

An atom can be changed into a cation (loss of an electron) or to an anion (by gaining of electrons). The size of the cation is always less than that of a parent atom (due to increased nuclear charge), whereas the size of the anion is always greater than the parent atom (due to decreased effective nuclear charge).

(e) Effective nuclear charge:It is defined as the net nuclear attraction towards the valence shell electron or in other words, the actual nuclear charge, where the electrostatic force of attraction is being experienced by the outer electron.
Greater the effective nuclear charge, more tightly is the hold with the nucleus. For example - In Na -

Generally, when an element (metal) loses an electron, its ionic radius decreases due to increased effective nuclear charge.

Atomic and ionic radii of some elements

In case of Chlorine atom-

Generally, when an element (non-metal) gains an electron, its ionic radius increases due to decreased effective nuclear charge.

Atomic and Ionic radii of some elements

Variation of Atomic Radii in Periodic Table

• Variation in a period

Generally, when we move from left to right in the same period, the atomic radii decreases due to increased effective nuclear charge and thus, the electron cloud is attached more strongly towards the nucleus which gives rise to a contraction in atomic radius in the period.

• Variation in a group

Generally, when we move from top to bottom in the same group, the atomic radii of the element increases because, as we go down from top to bottom of the group the nuclear charge increases, at the same time an extra shell is added in successive members. As a result, the size of the atom goes on increasing as we go down the group.

• Iso-electronic ions or species:

They are ions of the different elements which have the same number of electrons but the different magnitude of the nuclear charge. The size of isoelectronic ions decreases with the increase in the nuclear charge.

• Ionization energy (Ionization potential) (I.E or I.P)

It is the amount of energy required to remove the loosely bound electron from the isolated gaseous atom to change it into a gaseous cation.

$$A(g) + I.E \xrightarrow{} A^+(g) + .e^- $$

The amount of energies required to remove the subsequent electrons (2^nd,3^rd...) from the monovalent gaseous cation of the element one after the other is collectively called successive ionization energy.

Factors responsible for ionization energies:

(i) Atomic size or radius:Ionization energy is inversely proportional to the atomic size of the element i.e. higher the atomic size of the element, lower will be its ionization energy and vice versa. As the distance of the outer shell electron increases from the atomic nucleus, the attractive force decreases and the outer electrons are loosely held.

(ii) Shielding effect or number of intervening electrons or screening:The phenomenon in which the penultimate shell (n-1) electrons act as a screen or a shield between the nucleus and the valence cell electrons, thus reducing the effective nuclear charge is known as the shielding effect. The penultimate shell electrons repel the valence shell electrons to keep them loosely held with the nucleus. Evidently, greater the shielding effect, lesser is the effective nuclear charge and lesser is the ionization effect.

(iii) Nuclear charge: The greater the effective nuclear charge, the more strongly the electrons are held on by the nucleus. More energy is required to remove the electron and consequently higher is the ionization energy and vice versa.

(iv) Penetration effect:More penetrating (i:e closer) are the subshells of a shell to the nucleus, more tightly the electrons are held toward the nucleus and more is the ionization energy.

Ionization energy : s > p > d > f for a given shell.

Variation of Atomic Radii in Periodic Table

• Variation in a period

When we move from left to right, the ionization energy of the element increases because its nuclear charge increases and atomic radius decreases. As a result, the electron is more tightly held to the nucleus and it is more difficult to remove it.

• Variation in a group

When we go from top to bottom in the same group, I.E. of the elements decreases due to increase in the size of the atom. As a result, the electron becomes less and less firmly held to the nucleus and is able to be removed easily.

• Electron affinity (E.A)

It is the amount of energy released when a gaseous atom accepts the electron to form a gaseous anion.

$$X(g) + e^- \xrightarrow {} X^-(g) + E.A $$

Periodic relationship of electron affinity:As we go from left to right in the given period, the electron affinity of the element increases.

Similarly, when we go from top to bottom in the given group, the electron affinity of the element decreases.

• Electronegativity

It is the tendency of an atom in a molecule to attract the shared pair of electrons. It depends on a charge on the nucleus, the size of the atom, and shielding effect.

The most electronegative element in the periodic table is Flourine (4) and the least electronegative element is Caesium (0.7) and inert gases have zero electronegativity value.

Periodic relationship of electronegativity:As we go from left to right in a period, the electronegativity of the element increases due to increase in valence electrons and increased the effective nuclear charge. For example- In 2nd period-

As we go from top to bottom in the group, the electronegativity of the element decreases due to the increase in the size of the atom. For example-

• Atomic volume

It is the volume occupied by one mole of atoms of the elements at its melting point in the solid state.

$$Atomic\;volume= \frac{gram\;atomic\;mass\;of\;its\;element}{density}$$

Periodic relationship:Along the period, the atomic volume first decreases, reaches a minimum in the middle and then again increases because of the differences in the packing arrangements of their atoms in the solid state. Down the group, atomic volume goes on increasing gradually.

References:

Adhikari, Rameshwar; Khanal, Santosh; Subba , Bimala; Adhikari, Santosh; Khatiwada, Shankar Pd. Universal Chemistry XI. First. Vol. 1st. Kathmandu: Oasis Publication, 2069.

Chaudhary, Ganga Ram; Karna, Shila Kant Lal; Sharma, Kanchan; Singh, Sanjay; Gupta, Dipak Kumar. A Textbook of Higher Secondary Chemistry XI. Ed. 2nd. Kathmandu: Vidyarthi Pustak Bhandar, 2069 (2012).