In our 28th post , we have already introduced the concept of electronic configurations and the various rules that determine the sequence of filling of the shells and subshells, in an atom.In this post, let us examine the same topic, in relation to position of the elements in the periodic table.
The periodic table shows us the sequential filling of the electrons .The energy of the orbitals determines the sequence of filling- Lower energy orbitals are always preferred over high energy ones.The table is thus divided into 4 blocks namely – s,p,d, f blocks, depending on the occupation of the respective orbitals by the valence electrons of an element.
s-block elements :
- The valence electrons in the these elements are only in the s-orbital.
e.g. Li (Z=3) 1s2 2s1.
Na(Z=11) 1s2 2s2 2p6 3s1.
K (Z=19) 1s2 2s2 2p6 3s2 3p6 4s1.
- There can be maximum 2 electrons in the valence shell as s-orbital can only accommodate 2 electrons in it.
- Alkali(Group 1) and Alkaline earth metals (Group2) and Helium are all s-block elements.
- The General electronic configuration of this block is –
Group 1 elements – [Noble gas configuration] ns1 , where n = 2 to 7. Group 2 elements and helium – [Noble gas configuration] ns2, ,where n = 2 to 7.
p-block elements :
- This block contains groups 3A to 8A(columns 13, 14, 15, 16, 17, and 18)with the exception of Helium (He is a s block element).
- The valence electrons occupy p orbitals i.e the last electron enters the p-orbital.
e.g. Carbon 1s22s22p2.
Silicon 1s2 2s2 2p6 3s2 3p2.
Bismuth 1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 4d10 5s2 5p6 4f14 5d10 6s26p3.
- This block has metals(e.g Lead,Aluminium,tin,bismuth), non-metals(e.g halogens,phosphorous,sulphur,carbon) and metalloids(e.g. silicon,arsenic,germanium,antimony)
- General electronic configuration of this block is –
Group 3A (Boron family or ICOSAGENS) – ns 2 np 1
Group 4A (Carbon family or TETRAGENS) – ns 2 np 2
Group 5A (Nitrogen family or PNICTOGENS)- ns 2 np 3
Group 6A (Oxygen family or CHALCOGENS) – ns 2 np 4
Group 7A (Halogen family) – ns 2 np 5
Group 8A (Noble gases)- ns 2 np 6 (except Helium)
- There can be maximum 6 electrons in the p orbital and 2 electrons in the s orbital.So according to the electronic configurations given above, there can be 3 to 8 electrons in the outer shell of these elements.
(Note – The outer shell/orbit of these elements consists of s and p orbitals).
d – block elements :
1)These elements are found in groups 3 to 12 in the periodic table.
2)In these elements , the penultimate(n-1) d-orbital is filled with electrons. As we have seen in post 28 , the sequence of filling the orbitals is such that the energy of the ns shell is always less than (n-1) d shell .So the electrons fill the ns shell first and later they enter the penultimate (n-1)d orbital.
e.g. energy of 4s orbital < energy of 3d orbital. (refer to the energy level diagram of orbitals in post 28).So, electrons enter 4s orbital first and 3d orbital later.
[The order of filling of electrons in orbitals is 1s 2s 2p 3s 3p 4s 3d 4p 5s 4d 5p 6s 4f 5d 6p 7s 5f 6d .]
3)d- and s- orbitals can hold maximum of 10 and 2 electrons respectively.Thus , the number of valence electrons in these elements ranges from 3- 12.Generally the group number tells us the number of valence electrons in that group, but with transition metals this cannot be said with exactness.
Group 3: 3 valence electrons
Group 4: 2 to 4 valence electrons
Group 5: 2 to 5 valence electrons
Group 6: 2 to 6 valence electrons
Group 7: 2 to 7 valence electrons
Group 8: 2 or 3 valence electrons
Group 9: 2 or 3 valence electrons
Group 10: 2 or 3 valence electrons
Group 11: 1 or 2 valence electrons
Group 12: 2 valence electrons
These elements have thus have variable oxidation states.
e.g. Iron ion can exist in two states, Fe+2 and Fe3+.
4)The general outer electronic configuration of transition elements is (n-1)d1-10ns1-2.
5)These metals are placed in between the metals on the extreme left of the periodic table and nonmetals on the extreme right of the periodic table.Thus, they are called ‘Transition Elements’.All these elements are metals so they are also referred to as ‘Transition Metals’.
6)Transition metals have very strong metallic bonds and thus have high melting and boiling points(except Zn,Cd and Hg – on account of completely filled orbitals).
7)There are 4 sets oaf transition series –
First transition Series – 3d orbital gets filled – Scandium (Z=21) to Copper (Z=29).
Second transition series – 4d orbital gets filled – Yttrium (Z=) to Silver(Z= )
Third transition series – 5d orbital gets filled – Lanthanum (Z=) to Hafnium (Z= ) except Cerium to Lutetium.
Fourth transition series – 6d orbital gets filled – Actinum (Z=) to (Z=) – This is an incomplete period.
f-block elements –
- These are called the inner transition elements and they are placed separately at the bottom of the main periodic table.
- The two inner transition series are –
Lanthanide series – 4f orbital gets filled – Cerium (Z=58 ) to Lutetium(Z=71)
Actinide series – 5f orbital gets filled – Thorium (Z= 90) to Lawrencium(Z=103)- After Uranium(Z=92), the other elements in this group are synthesized synthetically.
- All actinides are radioactive.
- The general outer electronic configuration of Actinides is 5f1-14 6d0-1 7s2
This post is an overview of the various types of elements in the periodic table.We shall learn about these various set of elements in greater detail later.Till then ,
Be a perpetual student of life and keep learning..
References and further reading :