An anomalous behaviour differs from the norm or original order. In terms of properties, it is distinct from the others in its group. Anomalous behaviour means anomalous elements show unique properties and produce unique compounds.

Three factors cause the periodic table elements to behave anomalously.

• Compared to other atoms, their ions and atoms are small.
• The first element shows the highest electronegativity.
• No d–orbitals available. Only higher period members have d–orbitals and may use them for bond formation.

An example of an element showing anomalous behaviour is Beryllium with an atomic number 44  and having the symbol ‘Be.’ It is the first alkaline earth metal with anomalous behaviour. It is a steel-grey metal that is both strong and light; however, it is brittle. It is a divalent element present in minerals.

Anomalous Behaviour of Beryllium

Beryllium, the first element in group 2, has various properties that set it apart from the rest of the group.

Why does Beryllium have anomalous behaviour?

• With the highest ionisation energy, it is the smallest alkaline earth metal atom.
• Compared to other elements, Beryllium has a higher electronegativity.
• Beryllium compounds are mostly covalent due to increased electronegativity. The difference in electronegativity among Beryllium and other elements is usually low.
• The valence shell has no d orbitals.

Difference between Beryllium and other Alkaline Earth Metals 

• Alkaline earth metals are harder than Beryllium.
• Its melting and boiling points are greater than other elements in the group.
• It does not react with acids to release hydrogen like other elements in the group.
• Beryllium does not produce any colour in the flame test.
• Most Beryllium compounds are covalent, while most other elements’ compounds are ionic.
• It does not decompose water at high temperatures like the others.
• Its oxide, BeO, is amphoteric, while the other elements’ oxides are basic.

BeO+2HCl→BeCl2+H2OBeO+2HCl→BeCl2+H2O

BeO+2NaOH→Na2BeO2+H2OBeO+2NaOH→Na2BeO2+H2O

Beryllium’s complex compounds have a maximum coordination number of 44, whereas the other elements in the group may have a maximum coordination number of 66. The valence shell of Beryllium doesn’t have vacant d-orbitals, and other group members have vacant d-orbitals that they may use to get a coordination number of 6.

Chemical Properties of Beryllium

Alkaline earth metals create hydroxides with water. Because of forming a protective layer on its surface, Beryllium is the only element in Group 2 that does not react with water.

Metal oxides result from alkaline earth metals reacting with oxygen. Due to the protective layer created on the surface of these metals, only Beryllium in Group 2 reacts with air.

When heated, alkaline earth metals react with hydrogen to generate hydrides. All alkaline earth metal hydrides are metallic except Be, which forms a covalent hydride.

             X + H2 → 2XH2, where X is an alkaline earth metal

Fluorine, chlorine, bromine, and iodine react with alkaline earth metals to form ionic halides. However, Beryllium creates covalent halides.

M + X2 → MX2, where X is a halogen and M is an alkaline earth metal.

Uses of Beryllium

• Beryllium forms a variety of alloys.
• Beryllium is combined with copper to make strong springs for automobile shockers.
• Nuclear processes employ specific Beryllium isotopes.
• Beryllium is used in computers, missiles, and aircraft.

Anomalous Behaviour of Carbon

Tetravalency

There are four electrons on the surface of the carbon atom, but it needs four more to complete its octet. Carbon must first share electrons with other atoms in the presence of other particles to get all of them. CO2 has four covalent bonds because it shares electrons with other atoms. The tetravalency of carbon represents the number of carbon atoms in a given space, and carbon has four valances.

Catenation

Catenation involves joining carbon atoms to create covalent bonds, resulting in longer carbon chains and structures. Many organic substances exist on Earth because of this reason. Carbon is famous for its ability to catenate and is used in organic chemistry to examine various structures composed of catenated carbon atoms.

The small size of carbon

Because of the carbon atom’s small size, it is easier to form several bonds; thus, catenation is possible. Carbon has four electrons in its outermost shells, making it a half-filled element. The nucleus is stable because it may include both electrons bonded to one another and electrons not bonded to one another.

Electronegativity

Carbon may create pp–pp multiple bonds with other molecules and with itself. It might be due to its size and electronegativity. C = C, C° C, C = O, C = S, and C° N.

Anomalous Behaviour of Nitrogen

Nitrogen’s anomalous behaviour is due to its small size, strong electronegative nature, high ionisation energy, and non-availability of valence electrons in its d-orbital.

Reasons for anomalous behaviour of nitrogen

• Nitrogen’s electronegativity, ionisation enthalpy, small size, and the absence of d orbitals set it apart from the group.
• With equal size and electronegativity elements, nitrogen may form pπ -pπ multiple bonds.
• Heavy elements’ atomic orbitals are too large and diffuse to overlap to create pπ -pπ bonds.
• Nitrogen is a diatomic triple bond molecule (one s, two p), and its bond enthalpy (941.4 kJ mol–1) is high. Compared to bismuth, arsenic, phosphorus, and antimony, it forms P–P, As–As, and Sb–Sb single bonds. 
• However, the N–N bond is weaker than the P–P bond due to high inter electron repulsion. As a consequence, nitrogen has a weaker catenation tendency.

Conclusion

Even though scientists have seen trends throughout time, each element is different, and the second-period elements have particularly anomalous periodic properties.

Lithium, Beryllium, Boron, Carbon, Nitrogen, Oxygen, and Fluorine have slightly different periodic properties than the remainder of the Group 1, 2, and 13-17 elements. Lithium and Beryllium, for example, generate covalent compounds, whereas the rest of Groups 1 and 2 produce ionic compounds. In addition, unlike other Group 2 elements that create basic oxides, Beryllium’s oxide is formed when it combines with oxygen and is amphoteric in nature. Carbon, for example, may form multiple stable bonds, although Si=Si double bonds are uncommon.

So, the properties of the second period elements are different. In reality, they have periodic properties similar to the second element of the next group (for example, Lithium is similar to Magnesium, and Beryllium is similar to Aluminium), or they have a diagonal relationship.