Alkali Metals - Practice Questions & MCQ

S-Block Elements - The s-block elements of the Periodic Table are those in which the last electron enters the outermost s-orbital. As the s-orbital can accommodate only two electrons, two groups (1 & 2) belong to the s-block of the Periodic Table.

Alkali Metals : Group 1 of the Periodic Table consists of the elements: lithium, sodium, potassium, rubidium, caesium and francium. They are collectively known as the alkali metals. These are so called because they form hydroxides on reaction with water which are strongly alkaline in nature. lithium , sodium , potassium , rubidium, caesium and francium except hydrogen are all alkali metals.

Alkaline Earth Metals : The elements of Group 2 include beryllium, magnesium, calcium, strontium, barium and radium. These elements with the exception of beryllium are commonly known as the alkaline earth metals. These are so called because their oxides and hydroxides are alkaline in nature and these metal oxides are found in the earth’s crust.

Physical Properties of Alkali Metals :

•  All the alkali metals are silvery white, soft and light metals.
• Because of the large size, these elements have low density which increases down the group from Li to Cs. However, potassium is lighter than sodium.
• The melting and boiling points of the alkali metals are low indicating weak metallic bonding due to the presence of only a single valence electron in them.
• The alkali metals and their salts impart characteristic colour to an oxidizing flame. This is because the heat from the flame excites the outermost orbital electron to a higher energy level. When the excited electron comes back to the ground state, there is emission of radiation in the visible region of the spectrum as given below:
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• The other physical properties of Alkali metals are:
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Physical properties of Alkali Metals- 2 :

Melting and Boiling Point

• The melting and boiling points of these metals are quite low.
• Melting point and Boiling point decreases down the group.
• This is because the metallic bonding becomes weak as we move down the group.
• The weak metallic bonding can be attributed to involvement of a single electron in the bonding.

Oxidation state

• All the alkali metals show an oxidation state of +1
• This is because upon loss of one electron, these attain stable noble gas configuration.
• All the alkali metal ions are diamagnetic in nature with no unpaired electons
• Alkali metals, however are paramagnetic in nature due to the presence of an unpaired electron.

Chemical Properties of Alkali Metals : The alkali metals are highly reactive due to their large size and low ionization enthalpy. The reactivity of these metals increases down the group.

• Reactivity towards air: The alkali metals tarnish in dry air due to the formation of their oxides which in turn react with
  moisture to form hydroxides. They burn vigorously in oxygen forming oxides.
• Reactivity towards water: The alkali metals react with water to form hydroxide and dihydrogen.
  
• Reactivity towards dihydrogen: The alkali metals react with dihydrogen at about 673K (lithium at 1073K) to form hydrides. All the alkali metal hydrides are ionic solids with high melting points.
• Reactivity towards halogens : The alkali metals readily react vigorously with halogens to form ionic halides, M⁺X^- .
  However, lithium halides are somewhat covalent.
• Reducing nature: The alkali metals are strong reducing agents, lithium being the most and sodium the least powerful
• Solutions in liquid ammonia: The alkali metals dissolve in liquid ammonia giving deep blue solutions which are conducting in nature.The blue colour of the solution is due to the ammoniated electron which absorbs energy in the visible region of light and thus imparts blue colour to the solution.

Chemical Properties of Alkaline Earth Metals :

• Reactivity towards Air :  Powdered beryllium burns brilliantly on ignition in air to give BeO and Be₃N₂ Magnesium is more electropositive and burns with dazzling brilliance in air to give MgO and Mg₃N₂ . Calcium, strontium and barium are readily attacked by air to form the oxide and nitride.
• Reactivity towards Water : Ca, Sr and Ba react with water with increasing vigour even in cold to form hydroxides. While Be and Mg are kinetically inert to water because of the formation of a layer.
• Reactivity towards the halogens: All the alkaline earth metals combine with halogen at elevated temperatures forming their halides.
• Reactivity towards hydrogen: All the elements except beryllium combine with hydrogen upon heating to form their hydrides, MH₂. BeH₂ , however, can be prepared by the reaction of BeCl₂ with LiAlH₄ .
• Reactivity towards acids: The alkaline earth metals readily react with acids liberating dihydrogen.
  M + 2HCl → MCl₂ + H₂
• Reducing nature: Like alkali metals, the alkaline earth metals are strong reducing agents. This is indicated by large negative values of their reduction potentials. However their reducing power is less than those of their corresponding alkali metals.
• Solutions in liquid ammonia: Like alkali metals, the alkaline earth metals dissolve in liquid ammonia to give deep blue black solutions forming ammoniated ions.