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Class notes chemistry Chemistry Simplified NCERT Class 11
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Class notes chemistry Chemistry Simplified NCERT Class 11
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UNIT 10 THE S-BLOCK ELEMENTS In s-block elements the last electron enters the outermost s-orbital.
It includes elements of group 1 and 2 and He in group 18.
It consists of six elements Li, Na, K, Rb, Cs and Fr.
These are called alkalie metals because they react with water to form soluble hydroxides
which are strong bases or alkalies.
The group 2 includes Be, Mg, Ca, Sr, Ba and Ra.
These elements with the exception of Be are commonly known as alkaline earth metals.
They are so called because their oxides are found in the earth’s crust.
The general outer electronic configuration of s-block elements is ns1 or ns2.
DIAGONAL RELATIONSHIP
The similarity in properties shown by diagonally placed elements of second and third
periods in modern periodic table is called diagonal relationship.
Li shows similarities to Mg.
Be shows similarities Al.
CAUSE OF DIAGONAL RELATIONSHIP
As we move from left to right across a period, the electronegativity increases.
But it decreases when we move from top to bottom.
As a result of these two opposite changes, as we move diagonally, these two effects
tend to cancel each other and there is no marked change in electronegativity.
Because of the similar values of electronegativities, the diagonal elements have similar
chemical properties.
ANOMALOUS BEHAVIOUR OF THE FIRST MEMBER OF A GROUP
This anomalous behaviour of the first member in each group is due to
The very small size of the atoms
High Ionisation energies
High electronegativities
Absence of vacant d orbitals.
GROUP I ELEMENTS: ALKALI METALS
ATOMIC AND PHYSICAL PROPERTIES
1. Electronic Configuration
The general electronic configuration of alkali metals may be represented as
[Noble gas] ns1.
2. ATOMIC AND IONIC RADII
The alkali metals have the largest sizes in a particular period of the periodic table.
With the increase in atomic number, the atom becomes larger.
The monovalent ions (M+) are smaller than the parent atom.
PREPARED BY HAIZEL G. ROY, HSST (HG) CHEMISTRY, GOVT. H.S.S. KALAMASSERY, ERNAKULAM Page | 1
, The atomic and ionic radii of alkali metals increase on moving down the group.
3. IONISATION ENERGY
The alkali metals have low ionisation energies.
Their atomic sizes are quite large and the valence electrons are quite loosely held by
the nucleus.
On moving down the group, the ionisation energy decreases.
It is due to the increase in size of the atoms.
Iincrease in the magnitude of screening effect caused by the increase in the number of
intervening electrons.
4. Hydration Enthalpy
The hydration enthalpies of alkali metal ions decrease with increase in ionic sizes.
The smaller the ion, the more is the extent of hydration.
Due to this, in aqueous solution, the hydration enthalpy decreases in the order
Li +
> Na+ > K+ > Rb+ > Cs+
PHYSICAL PROPERTIES
1. Metallic Character
All the alkali metals are silvery white, soft and light metals.
The metallic character increases down the group.
2. Density
Because of the large size, alkali metals have low density.
The Density generally increases down the group.
3. Melting and Boiling Points
The melting and boiling points of alkali metals are low.
It indicates weak metallic bonding due to the presence of only a single valence electron
in them.
The melting and boiling points decreases down the group.
4. Flame Colouration
When alkali metals or their salts are heated in the flame of a Bunsen burner, they
impart characteristic colours to the flame.
Li Na K Rb Cs
Crimson red Golden Yellow Pale Violet Reddish Violet Sky Blue
Alkali metals have low Ionisation energy.
Therefore, the electrons present in their atoms get readily excited to higher energy
states by absorbing energy from the visible region of the light.
When these electrons jump back to their ground state, they emit energy in the form of
radiations.
PREPARED BY HAIZEL G. ROY, HSST (HG) CHEMISTRY, GOVT. H.S.S. KALAMASSERY, ERNAKULAM Page | 2
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