The elements are arranged in such a manner in the periodic table that every element with similar chemical and physical properties will be placed together in columns known as groups. Based on this, several properties can be defined, like atomic radius, ionization enthalpy, stability, electron gain enthalpy, and so on. Even though the properties follow a proper order of either increase or decrease as one moves down the group, some elements that sit in the first place show anomalous properties. Their behaviours do not match the rest of the elements present in the group, which is why their study is so important in chemistry.

What Anomalous Behaviour of Elements?

Anomalous properties of any element are mainly due to the electronic configuration, the unique arrangement of the electrons in the shells, and the physical structures. 

Due to anomalous behavior in the periodic table, the modern-day representation is not considered perfect. It is a drawback that still needs to be rectified to explain why these elements with different physical and chemical properties have been grouped with other elements.

Why do the First Elements in the Periodic Table Groups Show Anomalies in their Behaviors?

Several reasons can generally explain why the first elements of the periodic table show anomalous properties. In this section below, some of these reasons are explained properly for giving a brief idea about such anomalies.

1. The first elements are always smaller in size than other elements present down the group. Due to smaller radii, the electrostatic force of attraction between the protons and the last shell electrons is maximum.
2. Also, the first element in the s-block groups have less ionization enthalpy, so they cannot easily form the ionic compounds like the other members in the same group.
3. Similarly, the electron enthalpy is also pretty high, so a lot of energy will be needed for an electron to enter the last unfulfilled subshell and complete the structure.
4. Because the s and p block elements do not have d orbitals, there won’t be any valence shell present in the element. Hence, electrons cannot jump to high energy orbitals when heat or any other energy is supplied from the outside.

Which First Elements of the Periodic Table Groups Show Anomaly?

Only the first elements of the s and p block show anomalous properties because these blocks follow the strict principles of electronic arrangements. They can form ionic bonds, and their last orbitals are highly unstable. Hence, these are also termed electropositive and electronegative groups, respectively.

The following first elements in the periodic table show the anomalous properties:

• Lithium from Group 1 or IA
• Beryllium from Group 2 or IIA
• Carbon from Group 14 or IVA
• Fluorine from Group 17 and VIIA

Apart from these elements, nitrogen, oxygen, and boron also show anomalous behaviors. However, the extent of anomaly is not great, so only the four elements mentioned above are considered most of the time.

Explanation of Anomalous Behavior of Lithium

Reason

Lithium is the first element of Group 1 or IA. As a result, it has an extremely small size and atomic radius. The last s-orbital has one electron that experiences a high electrostatic attraction from the three protons present in the nucleus. Due to this, the polarization power of the lithium ions is also pretty high. These two reasons can explain the anomalous behavior of lithium well.

Anomalous Behavior

• First of all, lithium has one electron in the s-shell, and therefore it should behave like all other alkali metals. However, this element forms a covalent bond due to the high polarization power and therefore, this is one of the anomalous properties of the first element.
• Also, lithium is quite hard compared to sodium and potassium of the same group, which are incredibly soft and can be cut down with a knife.
• Lithium chloride can crystallize in the presence of moisture which is one of the anomalous properties of the first element of Group 1 because no other alkali metal can form the hydrates.

Explanation of Anomalous Behavior of Fluorine

Reason

Fluorine belongs to the halogen family or Group 17. Being the first element with dense electron clouds in the last orbital, it shows a great anomaly in its behavior compared to other halogen series elements. The main reasons for such behavior are the absence of d orbitals in the valence shell, small structure, dense electron cloud, less affinity for electrons, and low bond dissociation enthalpy.

Anomalous Behavior

• Fluorine can exhibit a unique anomalous behavior in the periodic table by forming hydrogen bonds and giving compounds like HF.
• Fluorine cannot form ionic compounds easily, unlike other elements of the same group, because it has a low affinity for the incoming electrons.

Explanation of Anomalous Behavior of Beryllium

Reason

Beryllium is the first element of Group 2 or IIA, and it also shows anomalous properties. The main reason for such behavior is the small size compared to other elements of Group 2, high ionization enthalpy, polarization property, and increased electrostatic force of attraction between the nucleus and the 2s-orbital electrons.

Anomalous Behavior

• One of the significant anomalous properties of beryllium is its coordinate number 4, but all other alkali metals of Group 2 have a coordinate number 6.
• Beryllium has high melting and boiling points in comparison to other alkali metals. Therefore, it is said to be one of the hardest metals of the s-block in the periodic table.
• Also, beryllium has high ionization enthalpy, so it cannot form ionic compounds by losing the two electrons in the s-shell. Instead, it shares the electronic cloud to form the covalent bonds.

Summary:

With these elements showing a wide range of anomalous properties, it is very important to understand the differences between their physical properties and chemical behavior with respect to other elements belonging to a concerned group. Also, these elements are usually studied differently because of their properties since they can’t be aligned with the normal elements of the same group. Besides, their studies also determine the conditions required for making them chemically active and forming their compounds artificially. Apart from the s and p block elements, certain d-block elements also have anomalous properties but aren’t considered much because the d-block elements do not actually follow the group rules.