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Alkali Metals


The alkali metals are the elements in Group I of the periodic table of the elements. These elements are arranged in increasing order of atomic number, they are Lithium (Li), Sodium (Na), Potassium (K), Rubidium (Rb), Caesium (Cs) and Francium (Fr).

The elements of Group I illustrate more clearly that any other group of elements, the effects of increasing size of atoms or ions on the physical and chemical properties. Metals are found on the left hand side and in the middle of the periodic table. Metals including the alkali metals have similar physical properties.

Alkali Metals Definition

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Define alkali metals

"Alkali metals are Group I elements and their hydroxides are vary strong alkalies". Alkali metal means any metal of the group Na, K, Li, Rb and Cs. In general the oxides of metallic elements are basic and the oxides of nonmetallic elements are acidic.

Elements in the first two groups of the periodic table are some of the most reactive elements known. In fact many of the elements are so reactive that water or in some cases air can cause them to explode or catch fire.

Alkali Metals in Water

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The alkali metals fizz and push around on the water surface like a hovercraft. This is due to the liberation of hydrogen gas as they react with water. Lithium, sodium and potassium react with water to produce a colorless gas and an alkaline solution that turns red litmus paper blue. Hence lithium, sodium and potassium exhibit similar chemical properties.

The observation also show that the reactivity of the alkali metals in their reactions with water increases from lithium $\rightarrow$ sodium $\rightarrow$ potassium.

Alkali metals with water to produce metal hydroxide solution and hydrogen gas.

2Li(s) + 2H2O(l) $\rightarrow$ 2LiOH(aq) + H2(g)
2Na(s) + 2H2O $\rightarrow$ 2NaOH(aq) + H2(g)
2K(s) + 2H2O $\rightarrow$ 2KOH(aq) + H2(g)

Is Hydrogen an Alkali Metal?

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Hydrogen is placed at the top of group 1 of periodic table. Its position however is not justified because of its resemblance with alkali metals as well as with halogens.

Hydrogen resembles with alkali metals in the following ways.
  1. Both hydrogen and alkali metals have one electron in their outermost shell.
  2. Both show oxidation state of +1 in their compounds.
  3. Both alkali metals as well as hydrogen form compounds with electronegative elements.
  4. Both alkali metals and hydrogen are very good reducing agents.

Alkali Metal Cations

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Alkali metal cations are more complex, hard, non polarizable. The properties of alkali metal cations are
  1. Little fixed preference for particular coordination geometries.
  2. Relatively high free energies of hydration.
  3. Affinity for highly charged non polarizable bases.

Alkali Metals List

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Alkali metals are flammable and highly reactive. When they are in contact with water they produce flammable hydrogen gas a strong caustic runoff and excessive heat. Alkali metals are quite soft compared to other metal goods. The list of alkali metals are given below.
  1. Lithium
  2. Sodium
  3. Potassium
  4. Rubidium
  5. Cesium
  6. Francium

Periodic Table Alkali Metals

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Lithium (Li), Sodium (Na), Potassium (K), Rubidium (Rb), Cesium (Cs) and Francium (Fr) are the elements of IA group of the periodic table. These elements are known as alkali metals because that form water soluble hydroxides which are strongly alkaline. Alkali metals periodic table belong to s-block elements.

The alkali metals on periodic table is shown below.

Periodic Table Alkali Metals

Reactivity of Alkali Metals

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Alkali metals are highly reactive metals and the reactivity gradually increases down the group. Alkali metals reactivity are very high due to their low ionization energy and relatively low heat of atomization.

Reaction with water - Alkali metals react with water liberating hydrogen. The reaction becomes increasingly violent on descending the group. Na and K always catch fire.

2M + H2O $\rightarrow$ 2MOH +H2

Reaction with air - All alkali metals tarnish in air due to the formation of oxide or hydroxide on the surface. When heated in excess of air they form oxides Li2O, Na2O2, KO2, RbO2 and CsO2 respectively.

Reaction with liquid ammonia - Alkali metals dissolve in liquid NH3, he solubility is as high as 5M. The formed solutions are colorless and metastable.

Alkali Metals Valence Electrons

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Alkali metals have only one valence electron. The force of attraction of the nucleus is very small on this valence electron and it can be removed by supplying a small amount of energy. Once they have lost the valence electron the alkali metal atoms becomes positively charged.

M $\rightarrow$ M+ + e-

Where M is an alkali metal. All the alkali metals tend to lose one electron in the outermost shell and become uni positive ions of the type M+.

Melting Point of Alkali Metals

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Alkali metals have low density because of their large atomic size. Due to weak intermolecular attraction these metals have low melting points. Thus the energy required for binding the atoms in the crystal lattice of the metal is low. The melting point of alkali metals also decreases from lithium to cesium.

Melting Point of Alkali Metals

Ionization Energy of Alkali Metals

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The atoms of alkali metals are the largest in their respective periods. Therefore the single electron in the outermost shell of these elements is loosely held by the nucleus and hence can be easily removed. Consequently the ionization energies of alkali metals are comparatively small.

The ionization energies of alkali metals decrease down the group because the effect of increasing size out weighs the increasing nuclear charge.

Ionization Energy of Alkali Metals

Uses of Alkali Metals

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  1. Lithium stearate is used in making automobile grease.
  2. Lithium is alloyed with Pb to make white metal which is used to make bearings for motor engines.
  3. Large amounts of Na2SO4 are used to make soaps, detergents, paper, textiles and glass.
  4. KOH is used to make potassium phosphates and also soft soaps both of which are used in liquid detergents. KBr find an extensive use in photography.
  5. Sodium vapor is used to fill street lamps which emits a characteristic yellowish orange glow.
  6. Caesium is used in vapor from in the atomic clock the vibration of its atoms acting as a regulator.

Characteristics of Alkali Metals

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Properties of alkali metals

Following are some of the physical properties of alkali metals given.
  1. They are relatively soft metals in purified forms.
  2. When freshly cut they are silvery colored but soon oxidize in air, turning darker in color.
  3. Their melting and boiling points becomes lower as their atomic weights and size increase.
  4. Their atomic volumes become larger as their atomic mass increases and thus their densities also increase.
  5. They produce colored flames when burned
  6. Alkali metals react with water and weak acids. Those with higher atomic weight reacts explosively.
  7. Their alkaline nature is above pH 7.