What is Elements
The electronic configuration of atoms provides a theoretical foundation for the periodic classification. Elements of a group exhibit similar chemical properties because they have same electronic configuration of their outermost shell. Depending upon the type of orbital receiving the valence electron, the elements can be classified into following four blocks
Types of Elements
S-Block Elements
- In these elements, valence electron(s) enters in s-orbital.
- Group-1 and 2 (IA and IIA) of the periodic table belong to this block.
- Group-1 ( IIA) of the periodic table is collectively called as alkali metals.
- Ground-2 ( IIA) of the periodic table is collectively called as alkaline earth metals.
- The general electronic configuration of s-block elements is (noble gas) ns? for alkali metals and (noble gas) ns1 for alkaline earth metals.
- These elements are soft metals, electropositive and form basic oxides.
P-Block Elements
- In p-block elements, valence electron(s) enters in p-orbital.
- Valence shell electronic configuration is ns2 np1-6
- Group-13 to 18 (IIIA to VIIIA) are p-block elements in the periodic table.
- It is the only block which contains metals, non-metals and metalloids.
- Heavier members of this block show inert pair effect, i.e., their lower valency becomes more stable. e.g., +2 valency is more stable for Pb as compared to its +4 valency. Similarly, +1 valency is more stable for Tl as compared to its +3 valency.
d-Block Elements
- These elements are called transition elements (except zinc, cadmium and mercury).
- In d-block elements, valence electron(s) enters in d-orbital.
- Valence shell electronic configuration is (n-1)d1-10 ns1-2
- This block contains the elements of group-3 to 12 of the periodic table.
- The three series of transition metals are known 3d series (Sc to Zn), 4d series (Y to Cd) and 5d series (La to Hg, excluding Ce to Lu).
f-Block Elements
- The f-block consists of two series lanthanoids (fourteen elements following lanthanum) and actinoids (fourteen elements following actinium) of the periodic table.
- Electronic configuration of lanthanoids is 6s25d0-24f1-14
- Electronic configuration of actinoids are irregular.
- Elements of this block are also called inner-transition elements.
- Elements beyond uranium (at. no. 92) are man-made elements i.e., synthesised by man artificially. So, these are called transuranic or synthetic elements. All these elements are radioactive in nature. e.g., neptunium (Np), plutonium (Pu), americium (Am), curium (Cm), berkelium (Bk), californium (Cf), einstenium (Es), mendelevium (Md), etc.
Trends in Modern Periodic Table (Periodic Properties)
The properties which are repeated at regular intervals are known as periodic properties. These properties show a regular order along a group and period.
1. Valency
It generally increases from 1 to 7 in a period with respect to hydrogen but with respect to oxygen, it first increases from 1 to 4 and then decreases to o.
For alkali metals (i.e., sodium, potassium, etc) it is 1, for alkaline earth metals (i.e., magnesium, calcium, etc) it is 2, for aluminium it is 3 and for nitrogen it varies from -3 to +5.
2. Atomic Size
It refers to the radius of an atom. It generally increases on moving down the group because new shells are being added as we go down the group. It decreases along a period from left to right. This is due to an increase in nuclear charge which tends to pull the electrons closer to nucleus and reduces the size of the atom. Thus, size of alkali metals is largest and that of halogens is smallest in a period. Size of noble gases is larger as compared to corresponding halogens.
3.Ionisation Energy
It is the energy required to remove an electron from an isolated gaseous atom in its ground state. It generally increases along a period from left to right due to increase in effective nuclear charge but ionisation energy of group-2 elements (Be, Mg, Ca, Sr) is larger than the ionisation energy of group-3 (B, Al, Ga, In) elements.
Similarly, ionisation energy of group-15 elements (N, P, As) is larger than ionisation energy of group-16 (0, S, Se) elements because of the stable configuration of group-2 elements (ns) and group-15 elements ( ns2,np3) than that of group-3 elements (ns2,np1) and group-16 elements (ns2,np4) respectively. generally decreases along a group on moving downwards due to increase in atomic size.
4.Electron Gain Enthalpy
It is defined as the enthalpy change accompanying the process when an extra electron is added to neutral gaseous atom to convert it into an anion. The energy released during the process is called electron affinity (EA).
EA increases across a period from left to right but EA of group-2, group-15 and group-O is zero or positive. It decreases on moving down the group. It is highest for chlorine.
5.Electronegativity
It is the tendency of an atom in a molecule to attract the shared pair of electrons towards itself. It increases regularly along a period from left to right and decreases on moving down a group. It is highest for fluorine.
6.Metallic Character
It is the tendency of an element to form cation by the loss of electrons. It decreases along a period from left to right and increases in a group on moving downwards. Thus, metallic elements occupy the left hand columns of the periodic table.
7.Non-Metallic Character
It is the tendency of an element to form anion by the gain of electrons. It increases along a period from left to right and decreases in a group on moving downwards.
8.Density
It is defined as mass per unit volume and increases on moving down a group as well as along a period from left to right but density of gold is higher than that of mercury. The order of density of steel, mercury and gold is steel < mercury – gold.
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