Periodic Trends in the Properties of Elements: Atomic Mass, Variations, Reactivity

Periodic Trends in the Properties of Elements: The long form of the periodic table or the modern periodic table can also be called Bohr’s table since it adheres to Bohr’s scheme of arrangement of electrons around the nucleus. The modern periodic table is based upon the modern periodic law, which states that:

“The physical and chemical properties of the elements are periodic functions of their atomic numbers”.

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Atomic Number and Atomic Mass

The modem periodic law, therefore, focuses primarily on the atomic number of elements. It is, however, important to understand what are atomic mass and the difference between atomic mass and atomic number. 

The atomic mass of an element can be defined as:

“The average mass of an atom when compared to the \(\frac{{{\rm{KZ}}{{\rm{e}}^{\rm{2}}}}}{{{{\rm{r}}^{\rm{2}}}}}{\rm{ = }}\frac{{{\rm{m}}{{\rm{v}}^{\rm{2}}}}}{{\rm{r}}}\) of the mass of one carbon-\(12\) atom”.

On the other hand, atomic number is defined as:

“The number of protons present in the nucleus of an atom”.

Since the elements in the modern periodic table are arranged in the increasing order of atomic number, it has more importance in deciding the properties of an element.

Periodic System of Elements and Trends in Periodic Properties

The periodic properties of elements can be divided into two different categories:

1. Based on the properties of individual atoms: The properties, such as valency, atomic radii, ionic radii, electron-gain enthalpy, ionization enthalpy, and electronegativity, are unique properties of each element or atom. These properties are related to their electronic configuration.

2. Based on the properties of the group of atoms: Properties, such as heat of fusion and vaporization, melting point, boiling point, the energy of atomization, density, atomic volume, and so on, are properties of the group or a group of atoms and are indirectly associated with their electronic configuration.

‘The properties which are directly or indirectly associated with the atomic structure or the electronic configuration of the elements are called as atomic properties.’

Like electronic configurations, which are periodic functions of the atomic numbers of elements, the atomic properties are also periodic functions of their atomic numbers. Hence, the atomic properties are also called periodic properties. 

Thus, with the increase in the atomic number of the elements in a particular period or group, one can see a regular increase or decrease in a particular property of the element. 

Periodic properties, therefore, can be defined as:

‘The properties that are indirectly or directly related to the electronic configuration and which shows regular gradation when moving from top to bottom or left to right in a group or period respectively are known as periodic properties.’

The individual periodic properties of the elements in a periodic table are as given below.

Periodic Properties and their Variations in Groups and Periods

Some physical properties such as heat of fusion and vaporization, the energy of atomization, melting and boiling points, and so on show periodic variations. The significant physical properties (or periodic properties) of elements in the periodic table include:

1. Atomic Radius

The size of an atom or the atomic size is a very significant property of an atom because it determines several other physical and chemical properties of an atom. For the sake of defining atomic radius, an atom is considered a sphere. The radius of such a sphere is called its atomic radius. The atomic radius of an atom is defined as:

‘The distance between the centre of the nucleus and the outermost shell containing electrons’

Or

‘The distance between the centre of the nucleus and the point till which the probability of finding an electron is maximum.’

The estimate of the atomic size of an atom is usually made when they are in the combined state. 

There are three types of atomic radius, depending upon whether it is a metal or a non-metal. For non-metals, there are two types of atomic radius depending upon the bond formed:

a. Covalent Radius: Covalent radius is measured as one half of the distance between two nuclei when two similar elements (homonuclear, diatomic) are covalently bonded to form molecules. 

It is represented as \({{\text{r}}_{{\text{covalent}}}} = \frac{1}{2}\) [Internuclear distance between the two atoms]

Internuclear distance between the two atoms can also be called bond length. Hence, it is given as:

\({{\text{R}}_{{\text{covalent}}}} = \frac{1}{2}\)[Bond Length]

b. Metallic Radius: It is defined as the one half of the internuclear distance that separates two metal ions adjacent to each other in a metallic lattice. Metallic radius is expressed as angstrom units or picometers.

Between metallic radius and covalent radius, the metallic radius is longer because in a metallic lattice, the valence electrons are not stable but mobile. Therefore, they are weakly attracted to the kernels. 

2. Ionization Enthalpy

When energy is supplied to an atom, the electrons in lower energy levels get transported to the higher energy levels. When a sufficient amount of energy is supplied, an electron can be completely removed from the atom, therefore forming a positively charged ion. 

“The minimum amount of energy that needs to be supplied to remove a loosely bound electron from an isolated gaseous atom to form a gaseous cation is called as ionization enthalpy.”

Ionization enthalpy is represented by \({\Delta _{\text{i}}}{\text{H}}\). Ionization enthalpy or ionization energy is also known as ionization potential. When more than one electron is removed, the ionization enthalpy is called the successive ionization enthalpies. It is defined as the ionization enthalpy required to remove the first, second, third, etc., electrons from an isolated gaseous atom is called the successive ionization enthalpies.

3. Electron Gain Enthalpy

While ionization enthalpy is the energy required to remove an electron from an isolated gaseous atom to form a positive ion, energy is released when the electron is added to an atom to form a negative ion. This energy released is called the electron gain enthalpy.

Electron gain enthalpy is defined as:

“Energy released when an electron is added to a neutral, isolated gaseous atom to form a negative ion or anion.”

It can be represented as:

\({\text{X}}\left( {\text{g}} \right) + {{\text{e}}^{\text{-}}} \to {{\text{X}}^{\text{-}}}\left( {\text{g}} \right)\,\left( {{\text{Anion}}} \right)\Delta {\text{H}} = {\Delta _{{\text{eg}}}}{\text{H}}\)

So, when the amount of energy released is higher, the electron gain enthalpy of the element is also higher. Therefore, electron gain enthalpy is a measure of the strength with which the extra electron is attached.

4. Electronegativity

Electronegativity of an element is defined as the tendency of an atom to attract to itself the shared pair of electrons when in a covalent bond. While electron gain enthalpy and ionization energies are associated with atoms in an isolated state, electronegativity is a property of a bonded atom.

Periodic Table Properties and Trends

The table below gives the variations of periodic properties across a period and down the group of the periodic table.

Periodic Trends and Chemical Reactivity

The chemical properties of elements are associated closely with the periodic properties of elements. Hence, it is essential to understand the relationship between periodic trends in properties and the chemical reactivity of the elements based upon them.

1. Since the atomic radii and ionic radii decrease as we move from left to right in a period, the ionization enthalpies increases and the electron gain enthalpies become more negative. So, the I.E of the elements in the extreme left is the lowest, and the electron gain enthalpy is the highest or most negative. 
2. The chemical reactivity based upon the I.E, and electron gain enthalpy values, show that the two extremes of the table (left and right) have the highest reactivity.
3. While the left side of the periodic table with alkali metals is reactive due to their ability to lose electrons readily to form a cation, the right side with halogens is reactive due to their ability to gain electrons to form an anion. 
4. Hence, alkali metals are good reducing agents, while halogens are good oxidizing agents.
5. A reaction of the elements on the left side (alkali metals and alkaline earth metals) with oxygen gives basic oxides such as \({\text{N}}{{\text{a}}_{\text{2}}}{\text{O,}}\,{{\text{K}}_{\text{2}}}{\text{O}}\), etc_.
6. A reaction of elements on the right side with oxygen (Eg: halogens) gives acidic oxides (\({\text{S}}{{\text{O}}_{\text{3}}}{\text{,}}\,{\text{C}}{{\text{l}}_{\text{2}}}{{\text{O}}_{\text{7}}}\), and so on.)

On the other hand, the oxides formed by the elements in the centre of the periodic tables are amphoteric (\({\text{A}}{{\text{l}}_{\text{2}}}{{\text{O}}_{\text{3}}}{\text{,}}\,{\text{A}}{{\text{s}}_{\text{2}}}{{\text{O}}_{\text{3}}}\) and so on).

Summary

The elements in the modern periodic table are arranged according to the modern periodic law. Such an arrangement of elements in the periodic table, in increasing order of their atomic number, has resulted in some of their physical properties being repeated at regular intervals. These properties, called periodic properties, are significant in understanding the chemical reactivity of the elements. The significant periodic properties include atomic and ionic radii, ionization enthalpy, electron gain enthalpy and electronegativity. The periodic properties of elements show a standard variation depending upon their arrangement in groups and periods. 

FAQs on Periodic Trends in the Properties of Elements

Q.1. How do properties show periodic trends?
Ans: Some physical properties such as atomic radii, ionization enthalpy, electron gain enthalpy and electronegativity follow standard variations in elements from left to right of the periodic table and when moving from top to bottom in a group. 

Q.2. How do the periodic trends help in determining the possible properties of elements?
Ans: Since there is a standard variation in properties and particular trends shown in periodic properties by the elements in the periodic table, one can determine the possible properties of elements by looking at their position in the periodic table. For example, ionization energy increases from left to right in a period.

Q.3. What are 4 patterns in the periodic table?
Ans: Atomic radii decrease from left to right in the periodic table (periods) and increase from top to bottom in a group. Similarly, ionization enthalpy increase when we move along left to right from the periodic table and decreases as we move down from top to bottom in a group.

Q.4. What is the periodic variation of properties?
Ans: The properties that are indirectly or directly related to the electronic configuration and which shows regular gradation when moving from top to bottom or left to right in a group or period respectively are known as periodic properties.

Q.5. What properties are not periodic?
Ans: The individual properties of elements such as mass number, the density of elements, etc., are properties that are unique to a particular element, and are not periodic properties.

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