Diagonal relationship between elements

The first element in every group of the periodic table is often the smallest and has the highest electronegativity compared to the rest of the group members. As a result, the first elements tend to have properties that differ from the rest of the group, but resemble those of the next lower elements diagonally. This relationship is called the diagonal relationship.

Diagonal relationship can also be explained in terms of the polarizing power of the diagonal elements. Polarizing power refers to the ability of a positive ion to polarize the negative ion. When positive and negative ions approach each other, their shapes may be distorted. The extent of this distortion is known as polarizability.

Polarization is the distortion or deformation of an electron cloud of an anion by a cation. If polarization is small, an ionic bond is formed. If it is large, the electrons in the anion are drawn toward the cation to the extent that a covalent bond is formed. The polarizing power and polarizability of an ion are affected by:

1. The size of the ion
2. The charge on the ion

The polarizing power is higher for smaller ions with high effective nuclear charge. Across a period from left to right, ionic radii decrease and effective nuclear charge increases. Down a group, ionic radii increase while the effective nuclear charge decreases. Thus, diagonal elements have similar polarizing power, which is why they exhibit similar properties.

Electronegativity comparison

Element	Electronegativity
Li	1.0
Be	1.5
B	2.0
C	2.5
N	3.0
O	3.5
F	4.0

Diagonal relationships are most significant between the following pairs:

1. Li and Mg
2. Be and Al
3. Be and Si

Examples of diagonal relationship

1. Lithium and Magnesium: Lithium (Li) shares similar properties with Magnesium (Mg) in terms of electronegativity and ionic properties.
2. Beryllium and Aluminium: Beryllium (Be) shows properties similar to Aluminium (Al), such as amphoteric oxides.

Anomalous behavior of the first element in a group

The first element in each group of the periodic table often exhibits some properties that are different from the other elements in the same group. This is called anomalous behavior. Several factors contribute to this, including:

1. The first element has the smallest atomic and ionic size.
2. The first element has the highest ionization energy.
3. The first element has the highest electronegativity.
4. The first element has higher electron affinity.

Anomalous behavior of lithium (Li)

Lithium, being the first element in the alkali metals group, shows several unusual properties compared to other alkali metals:

1. Formation of covalent compounds: Lithium forms covalent compounds, such as LiCl, while other alkali metals form ionic compounds like NaCl.
2. Reaction with nitrogen: Lithium reacts with nitrogen gas to form an ionic nitride, Li3N, while other alkali metals do not react with nitrogen.
3. Reaction with water: Lithium reacts slowly with cold water, unlike other alkali metals, which react vigorously with water.
4. Hydrated chloride: Lithium forms hydrated chloride (LiCl·2H2O), while other alkali metals form anhydrous chloride.
5. Burning in air: Lithium burns to form monoxide (Li2O) while other alkali metals form peroxides or superoxides.
6. Formation of acetylides: Lithium does not form acetylides with acetylene (C2H2), unlike other alkali metals.
7. Hydrolysis of salts: Lithium is the only alkali metal whose salts may undergo hydrolysis.

Chemical equations for lithium

Reaction with water:

2Li + 2H2O → 2LiOH + H2

Decomposition of lithium hydroxide:

2LiOH → Li2O + H2O

Anomalous behavior of beryllium (Be)

Beryllium exhibits different properties compared to other alkaline earth metals due to its small size and high electronegativity. Examples include:

1. Reaction with alkali: Beryllium reacts with concentrated alkali solutions to form hydroxo-complexes and hydrogen gas, which other alkaline earth metals do not.
2. Amphoteric oxide: Beryllium oxide (BeO) and hydroxide (Be(OH)2) are amphoteric, while other group metals form basic oxides.
3. Hydrolysis of chlorides: Beryllium chloride (BeCl2) hydrolyzes in water:

BeCl2 + H2O → Be(OH)2 + 2HCl

Chemical equations for beryllium

Reaction with alkali:

Be + 2NaOH + 2H2O → Na2Be(OH)4 + H2

Anomalous behavior of fluorine (F)

Fluorine exhibits distinct behaviors compared to other halogens due to its high electronegativity, absence of d-orbitals, and small atomic size. Key properties include:

1. Monovalency: Fluorine is monovalent (forms only one bond), while other halogens like chlorine and iodine show covalencies of 3, 5, or 7.
2. Highest oxidation state: Fluorine forms compounds with the highest oxidation states, such as SF6.
3. Hydrogen bonding: Fluorine forms hydrogen fluoride (HF), which is strongly hydrogen-bonded, unlike other halogen hydrides.

Chemical reactions involving fluorine

Reaction with water: Fluorine liberates oxygen from water:

2F2 + 2H2O → 4HF + O2

Reaction with oxygen: Fluorine evolves oxygen from hot alkalis:

2F2 + 2NaOH → 2NaF + O2