Periodicity in Some Properties of Elements

Periodic table of elements

We have all heard the term ‘matter’. In simple words, matter is anything that occupies space and has mass. These are of several types. The basic unit of matter is atoms or elements. There were 31 known elements in 1800. By 1865, the number of known elements had more than doubled to 63. At present, there are 118 elements. Each of them has specific physical and chemical properties.

In this article, we will study the periodic table of elements, along with the periodicity of elements. In addition, we will learn about the modern periodic table, which is an extended version of the traditional periodic table.

Dmitri Mendeleev curated the periodic table in 1869. He arranged a group of elements having similar properties into the table. The similarities between the elements led him to arrange them into four blocks on the periodic table. The blocks depend on the type of atomic orbitals with electrons filling them.

The elements are:

• The s-Block Elements (Group 1-2)
• The p-Block Elements (Group 13-18)
• The d-Block Elements or Transition Elements (Group 3-12)
• The f-Block Elements or Inner-Transition Elements (Lanthanoids and Actinoids)

The elements mentioned above have a specific arrangement in the table. Mendeleev arranged these elements in horizontal rows and vertical columns based on their increasing atomic weights. He placed the elements that have similar properties in the same vertical column. Additionally, he used chemical properties to classify elements.

Periodicity of elements

Several trends or variations appear in the properties of elements. These variations change with an increase in atomic numbers and are called periodicity. Consistent and predictable shifts in the atomic structure are responsible for periodicity.

Mendeleev used the periodicity of elements to find the gaps in the periodic table. He used those gaps to insert extra elements. Moreover, it also helps scientists discover other elements. The periodicity of elements is also used to predict the behaviour of elements when they react with one another. In addition, it helps display periodic variations in physical properties.

Modern periodic table

The extended form of the traditional periodic table overcomes the shortcomings of Mendeleev’s periodic table. Henry Moseley devised the modern periodic table in 1913. The table was expanded upon later by Bohr and Bury, leading to the long form of the periodic table, also known as the modern periodic table

Out of all the types of periodic tables used to classify elements, long-form is most commonly used. It is also known as Bohr’s table. 

There are several advantages and disadvantages involved in the modern periodic table. The advantages are as follows:

• No reverse order for placing the atomic number
• Interpreting the properties of the electronic configuration
• There is no concept of subgroups, and the series is odd and even.
• Splitting of triad positions
• Positions of lanthanides and actinides

The disadvantages are as follows:

• Defects in hydrogen and helium positions
• The elements of the f-block are not fixed and can detach themselves from the main periodic table.
• Placing the isotopes of the lighter elements
• Lanthanides and actinides do not fall under the primary group of elements. The periodic table shows them separately at the bottom
• The electronic configuration is not the sole contributor to determining the place.

Periodicity in some properties of elements

There are specific patterns in the properties of chemical elements. These patterns appear in the periodic table as periodic trends. Their inclusion in the properties of elements defines periodicity. The periodic trends include:

Atomic Radius: It is the distance between the centre of the nucleus and the valence shell of an atom. It contributes to both the physical and chemical properties of an atom.

There are four types of atomic radius:

• Covalent radius: We define it as half of the inter-nuclear distance of two bonded atoms.

• Metallic radius: They are half of the inter-nuclear distance. This nuclear distance is between the two nearest metal atoms in a metallic crystal. The metallic radius is always greater than its corresponding covalent radius.
• Van der Waal’s radius: It is half of the inter-nuclear distance of two adjacent atoms. These atoms belong to two neighbouring molecules of an element in the solid state.
• Ionic radius: It is the radius of a monatomic ion in an ionic crystal structure.

Periodic Trends

The periodic table of elements reveals patterns in the characteristics of chemical elements, known as periodic trends. The following are the primary periodic trends: 

Metallic character: These chemical properties are exhibited by metallic elements. The metallic character decreases across the periodic table from left to right. It happens as the elements allow more electrons to form anions.

The metallic character also decreases from the bottom to the top of the table because of the shorter atomic radius. Moreover, metallic elements lose electrons to form cations. Due to their high oxidation numbers, they are oxidised and are powerful reducing agents.

Ionisation potential: It is a quantitative measure of energy. This process involves removing an electron from one mole of an isolated gaseous atom in its ground state. It is measured in electron volts.

Ionisation potential is proportionate to the effective nuclear charge on the nucleus of an atom. Also, it is proportional to the atomic radius and shielding effect. It also depends on the penetration effect and makes the orbitals stable.

Electron gain enthalpy: It is the quantitative measure of enthalpy change. The addition of an electron to a neutral gaseous atom accompanies it. A negative ion results from the conversion of this atom.

Electron affinity: It is the quantitative measure of the energy released. It takes place when the outermost shell of one mole of an isolated gaseous atom experiences addition of an electron in its ground state. The factors determining electron affinity are the same as its Ionisation Potential.

Electronegativity: It is a chemical property that refers to the property of an atom or a functional group to attract electrons. Electronegativity is directly proportional to the effective nuclear charge. Also, the oxidation state of an atom is proportional to the atomic radius. It also depends on the hybridisation state of an atom in a molecule.

Chemical Reactivity: Atomic radius typically decreases from left to right during a period. The same applies to an ionic radius. Because of the ionisation, enthalpy increases from left to right in a period. The electron gain enthalpy is negative. 

An element at the extreme left of the periodic table has the lowest ionisation enthalpy and vice versa. It results in high chemical reactivity at the two extremes of the periodic table. The reactivity decreases towards the centre of the table.

The metallic and non-metallic characteristics become relatable in this case. These are reducing and oxidising natures of elements.

These trends arise because of the changes in the structure of atoms of the elements within their groups and periods. But a few exceptions exist. For example, the ionisation energy of Group 3 and Group 6 are exceptions.

Conclusion

Element is the fundamental unit of all matters. Mendeleev places the arrangement of elements into horizontal rows and vertical columns depending on their increasing atomic weights. These types of elements are p-block, s-block, f-block, and d-block. These variations change with an increase in atomic numbers called periodicity. 

According to Mendeleev, periodic trends accompanied by an increase in atomic number are periodicity. This article outlines some of the key trends for your understanding.