Periodic Trends in Properties of Elements Atomic

Understanding the difference between Period and Group

In the periodic table, when you move from left to right, it’s called moving across a period. When you move from top to bottom, it’s called moving within a Group. So, a row(horizontal) is known as a Period while a column(vertical) is termed as a Group.

Periodic trends in properties of atomic elements

Elements that appear in the periodic table display a certain trend in their properties and chemical behaviour that remain consistent in a period or a group. These periodic trends can be seen in:

• Electronegativity

• Ionic radius

• Atomic radius

• Electron affinity

• Ionisation energy

• Chemical reactions of elements 

• Metallic character

• Shielding effect

Trends in ionisation energy

The ionisation energy is the minimum energy required to remove an electron from the outermost orbit of an isolated atom or molecule in its gaseous state. The ionisation energy increases as you move from left to right in a periodic table. On the contrary, it decreases as we move down a group. Ionisation energy is measured in kilojoules per mole (kJ/M) or electron volts (eV), which is also the unit of energy.

Some of the factors ionisation energy depends upon are-

• Atomic radius

A larger atomic radius means that the valence shell is farther away from the nucleus, and thus the valence electrons are loosely packed. It, thus, becomes easier to remove an electron from the outermost shell, implying that the ionisation energy decreases with the increase in atomic radius.

• Shielding effect

The outermost shell does not experience the complete positive charge of the nucleus as much as the inner shells do. This is known as the shielding effect. The higher the shielding effect, the easier it will be to remove an electron from the valence shell.

• Nuclear charge

The force of attraction of the protons present in the nucleus also determines the ionisation energy. The stronger the nuclear charge, the more difficult it will be to expel an electron from the outermost shell and vice versa.

Trends in atomic radius

When two identical atoms are bonded together, and the distance between their nuclei is halved, this measurement is called the atomic radius. It can also be understood as the distance between the nucleus and the outermost electron. The atomic radius of the periodic table tends to decrease across a period, with the only exception being oxygen. But when we move down vertically within a group, the atomic radius increases. This makes helium the smallest atomic radii and francium the largest atomic radii. 

Trends in ionic radius

You get an ionic radius when you halve the distance between the atomic ions in a crystal lattice. The ionic radius follows an increasing trend as you move top to bottom as each new row adds a new electron shell. The size of a cation is generally less than that of an anion.

When you move across a period, the ionic radius will decrease. This happens because more protons are being added to the nucleus, but the valence electrons remain consistent. Thus the size will decrease with the increase in the force of attraction. Non-metallic elements are seen as an exception here as their ionic radius increases because the number of electrons exceeds that of protons.

Trends in electronegativity

How eager an atom is to gain an electron is known as its electronegativity. It depends upon the electronic configuration of the atom. It increases across a period and decreases as you move down a group. Fluorine is the most electronegative element, while francium has the least electron gaining tendency. Going by the definition and concept, electronegativity is the opposite of ionisation energy.

Trends in metallic character

The metallic character tells you about how reactive an element is. Metallic properties include lusture, sonorosity, ductility, and conduction of heat and electricity. Metallic properties will increase down a group and decrease horizontally in a periodic table. As you move top to bottom in a group, the attraction between the nuclei and the valence electrons decreases, and thus the ability to conduct heat and electricity increases. Conversely, non-metallic character decreases down a group and increases across a period.

Trends in shielding effect

The electrons in the outermost shell will experience less nuclear charge as compared to those present in shells closer to the nucleus. This is known as the shielding effect and is caused by the electrons and their orbits that are present between the nucleus and the valence shell. As the size of an atom increases as we go down a group, the shielding effect will increase too. Since the size of the atom decreases as you move across a period, the shielding effect will decrease too.  

Trends in electron affinity

Electron affinity is the amount of energy released by an atom or a molecule when an electron is added to a neutral atom to form a negatively charged ion or an anion. Electron affinity increases as you move from left to right in a period . However, it may sometimes decrease as you move vertically downwards in a group. These trends do not always hold good and are not universally true.

Trends in valency

Valency is defined as the number of electrons the valence shell falls short of or exceeds to achieve stability. 

As you move left to the right in a periodic table, valency will initially increase and then start to decrease. It, however, shows no change as you move top to bottom in the periodic table. 

Conclusion

The Periodic Law forms the basis of the periodic trends in elements. It was discovered in 1871 by the Russian chemist Dmitri Mendeleev. Major periodic trends are electronegativity, ionisation energy, metallic properties, atomic radius, and ionic radius. The arrangement of elements gives rise to periodic trends is an important tool to predict an element’s properties quickly.