Periodic Trends in the Periodic table

The periodic trends refer to the periodicity in properties of elements arranged in the periodic table. We can say that periodic trends occur in the periodic table due to the periodicity in the properties of elements. Periodicity refers to the repetition of similar properties of elements when we move down a group or across a row after a specific interval or gaps between their atomic numbers.

Some of the most common periodic trends in the modern periodic table are described below;

Atomic size:

It refers to the distance between the centre of the nucleus of an atom and its outermost shell. Whereas, the atomic radius refers to the shortest distance between the atom's nucleus and its outmost shell or the distance between atoms of the same element in a molecule. The atomic radius is represented in picometre (pm) as it is very small. Picometre is even smaller than a nanometre as 1 pm = 10^-12 metre, whereas, 1 nm = 10^-9 metre. Now, the atomic size decreases across a period due to an increase in the atomic number or number of protons. An increase in atomic number increases nuclear charge that increases pull on electrons, so, size of the atom tends to decrease. However, the atomic size increases while moving down the group due to an increase in the number of shells. An element with more number of shells will have more atomic size.

Metallic character or electropositivity or reactivity:

The metallic character refers to the tendency to lose electrons or electropositivity. More is the metallic character, more is the reactivity of an element. Now, as we move left to right in a period the metallic character as well as reactivity decreases. The reason for this trend is that due to a decrease in atomic size the pull on the electrons increases, so, the atoms tend to gain electrons instead of losing.

Whereas, while moving down the group the elements' electropositivity or tendency to lose electrons increases. The reason is that the atomic size increases as the number of shells increase and thus the pull on electrons decreases that increases the tendency to lose an electron or metallic character or reactivity of elements.

Non-metallic character or electronegativity:

It the tendency to gain or attract electrons. Electronegativity increases across a period from left to right as the atomic size decreases the pull on electrons increases which increases the element's tendency to gain electrons. Whereas, while moving down a group from top to bottom, the electronegativity decreases as the atomic size increases which reduce the nuclear pull on the electrons and thus the tendency of elements to gain electrons decreases.

However, there are few exceptions, such as in group 13, the electronegativity increases from Aluminium to Thallium and in the 14^th group tin is more electronegative than lead.

Valency:

The valency (the tendency to gain or lose electrons) first increases then decreases when we move from left to right in a period. On the other hand, the valency of elements remains the same while moving down the group.

Ionization energy:

It is the minimum amount of energy required to remove an electron from an atom in its neutral or gaseous state. It tends to increase while moving across a period as the nuclear charge increases across a period that increases pull on electrons and thus more energy is required to remove an electron from an atom's valence shell.

Now, ionization energy decreases down the group as the atomic size increases due to an increase in atomic size or energy shells. The increased atomic size tends to reduce pull or attraction for electrons, so less energy is needed to remove electrons from an atom's valence shell.

Electron affinity or electron gain enthalpy:

It is the amount of energy, which is released when a neutral atom gains an electron and becomes an ion. It increases across a period due to an increase in the nuclear charge or force of attraction on electrons. Due to this atoms becomes smaller and the added electrons come closer to the nucleus, which tends to increase the electron affinity. The electron affinity is negative when energy is released while the electron is added and positive when energy is supplied to an atom when an electron is added.

On the other hand, while moving down the group from top to bottom, the electron affinity decreases as the atomic size increases and thus pull on electrons decreases and the newly added electron tends to move away from the nucleus, so the force of attraction is not much strong which tends to decrease electron affinity.

Valence electrons:

The number of valence electrons (electrons in the valence shell) increases across a period from left to right. For example, the elements of third-period Sodium (Na), Magnesium (Mg) and Aluminium (Al) contain 1, 2, and 3 valence electrons respectively.

Whereas, while moving across a group from top to bottom, valence electrons remain constant. So, the number of valence electrons remains the same in all the elements of a group. For example, hydrogen, lithium, and sodium that belong to the first group have one valence electron.