Trends in atomic and physical properties of alkali metals

A) The electronic configuration(EC)

  • The EC of alkali metal is ns1.
  • They have single electron in their outer most shell.
  • Therefore, their valency is also one.
  • They have a great tendency of losing electron from their outermost orbit.
  • Due to the above reason, they are considered to be the most electropositive elements.
  • For example: Na (11) =1s22s22p63s1
  • As the last electron is filled in s subshell, they are called s-block elements.

B) Density

  • It is quite low in alkali metals.
  • It increases down the group from Li to Cs due to increase in their atomic mass which compensates more than its bigger atomic size.
  • However, K is lighter than Na which is due to abnormal increase in its atomic size.
  • Thus, density follows the order as:

          Li< K< Na< Rb< Cs< Fr

C) Melting point (m.p) and boiling point (b.p)

  • The m.p and b.p of these metals are very low.
  • They decrease further with increase in atomic number due to weakening of metallic bond.
  • Fr is liquid at room temperature.

D) Softness

  • They are silvery white and soft wax and can be cut with knife.

E) Ionization energy (IE)

  • IE are very low due to presence of only one electron in outermost orbit.
  • They more often prefer to lose ns1 electron to change to M+
  • IE decreases down the group from Li to Fr due to increase in atomic radius and decrease in nuclear attraction.

Element                   Li      Na       K       Rb       Cs       Fr

IE1 (KJ mol-1)        520     496   419     403      376      400

F) Flame colouration

  • Alkali metals and their salts impart characteristics colour to their flame.
  • It is due to the release of excitation energy in the visible region.
  • The energy is absorbed by them which is released when heated in a bunsen flame.

                                           Li                   Na                             K                  Rb                   Cs

                     Crimson red     golden yellow           violet            violet             violet

G) Electropositive character

  • They are strongly electro positive.
  • It is due to their low ionization energies.
  • They form uni-positive ions.

H) Atomic radii

  • Atomic radii are largest in their respective periods.
  • They increase down the group due to increase in principal shells.

I) Ionic radii

  • Ionic radii of these metals are much smaller than their corresponding atomic radii.
  • It is due to lesser number of shells and contractive effect of increased nuclear charge.

J) Photoelectric effect

  • K and Na especially show photoelectric effect.
  • It is due to low ionization energies.

(Photoelectric effect – eject electrons when exposed to light).

K) Electrical conductivity

  • They are good conductors of heat and electricity.
  • Due to low ionization energies, their valence s-electrons are loosely held by the metal ions in solid state.
  • Electrical conductivity increases down the group.
  • It is due to the decrease in the ionization energy in the order as

          Li+< K+< Na+ < Rb+< Cs+

Alkali metals - General characteristics, Electronic configuration,  Electronegativity, Distinctive behavior, Chemical properties, Uses

Image source: braincart

L) Reducing character

  • All alkali metals are good reducing agents.
  • It is due to their low ionization energies.
  • There reducing character follows the order:

          Na <K< Rb< Cs< Li

  • The reducing character of metals depends upon a) heat of vaporization b) ionization energy and c) heat of hydration.
  • Since, Li+ ion has the smallest size, its heat of hydration is highest.
  • Therefore among alkali metals Li has the highest negative electrode potential and is the strongest reducing agent.

M) Crystal structure

  • All alkali metals possess body centered cubic structures with coordination number 8.




Trends in atomic and physical properties of alkali metals