Trends in atomic and physical properties of alkali metals

A) The electronic configuration(EC)

• The EC of alkali metal is ns^1.
• 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⁺

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.

References: 

i) https://study.com/academy/lesson/alkali-metal-elements-properties-characteristics-reactions.html

ii) https://www.thoughtco.com/alkali-metals-606645

Trends in atomic and physical properties of alkali metals