ELECTRONIC ARRANGEMENT IN ATOMS:
1. Maximum number of electrons that can be accommodated in a shell is given by 2n2 where n = shell number.
2. Outermost shell of an atom cannot accommodate more than 8 electrons, even if it has a capacity to accommodate more electrons. This is a very important rule and is also called the Octet rule. The presence of 8 electrons in the outermost shell makes the atom very stable.
In every energy level (shell or orbit) there are sub energy levels (sub shells or sub orbits).
Number of orbit = Number of sub orbits present in it.
The sub orbits are represented with the symbols ‘s’ ‘p’ ’d’ ‘f’ etc., sublevels in sublevels are called as orbitals. Number of orbitals in‘s’ sublevel=1, ‘p’ sublevel=3, ‘d’sublevel=5 and in ‘f’ sublevel=7.
The maximum number of electrons in sub orbit s=2, p=6, d=10, f=14. Completely filled sub orbit is stable than half filled and it is stable than incompletely filled sub orbit.
|
Number
of orbit |
Maximum number
of electrons that can be present in that orbit (2n2) |
Number of
sub orbits present in it |
Their symbols |
Maximum number of
electrons that can be present in them. |
| 1 | 2 | 1 | 1s | 2 |
| 2 | 8 | 2 | 2s,2p | 2,6 |
| 3 | 18 | 3 | 3s,3p,3d | 2,6,10 |
| 4 | 32 | 4 | 4s,4p,4d,4f | 2,6,10,14 |
| Element | Symbol |
Atomic
number |
Electronic configuration
(or Electron arrangement) K L M N |
Number of Unpaired electrons
(Valency) |
|---|---|---|---|---|
| Hydrogen | H | 1 | 1 = 1s1 | 1 |
| Helium | He | 2 | 2 = 1s2 It is inert gas. | 0 |
| Lithium | Li | 3 |
2, 1 = 1s2 2s1
This element prefers to lose one electron to get stable configuration of two electrons in the first orbit. That is why it prefers to form ionic compounds. |
1 |
| Beryllium | Be | 4 |
2,2 =1s2 2s22p02p02p0
There are no unpaired electrons in ground State. It prefers to lose ‘2’ electrons to get Stable electronic configuration. Then it forms ionic compound. If one of the 2s electron excites to 2p level It gets two unpaired electrons. 1s2 2s12p12p02p0 = valency = 2 |
0
2 |
| Boron | B | 5 |
2,3 = 1s2 2s22p12p02p0
In excited state the electronic configuration = 1s2 2s12p12p12p0 = 3 unpaired electrons = valency = 3 |
1
3 |
| Carbon | C | 6 |
2,4 = 1s2 2s22p12p12p0
In excited state the electronic configuration = 1s2 2s12p12p12p1 = 4 unpaired electrons = valency = 4 |
2
4 |
| Nitrogen | N | 7 |
2,5 = 1s2 2s22p12p12p1
This element cannot excite one electron from 2s level because there is no 2d sub level in second orbit. So it’s valency is always ‘3’ |
3 |
| Oxygen | O | 8 |
2,6 = 1s2 2s22p22p12p1
This element cannot excite one electron from 2s level because there is no 2d sub level in second orbit. So it’s valency is always ‘2’ |
2 |
| Fluorine | F | 9 |
2,7 = 1s2 2s22p22p22p1
This element cannot excite one electron from 2s level because there is no 2d sub level in second orbit. So it’s valency is always ‘1’ |
1 |
| Neon | Ne | 10 |
2,8 = 1s2 2s22p6
It is inert gas. |
0 |
| Sodium | Na | 11 |
2,8,1 = 1s2 2s22p6 3s1
This element prefers to lose one electron to get stable configuration of (2, 8) electrons in the 1st & 2nd orbits. That is why it prefers to form ionic compounds |
1 |
| Magnesium | Mg |
12 |
2,8,2 = 1s2 2s22p6 3s2
This element prefers to lose one electron to get stable configuration of (2, 8) electrons in the 1st & 2nd orbits. That is why it prefers to form ionic compounds |
2 |
| Aluminium | Al | 13 |
2,8,3 = 1s2 2s22p6 3s23p13p03p0
In excited state the electronic configuration = 1s2 2s22p6 3s13p13p13p0 = 3 unpaired electrons = valency = 3 |
1
3 |
| Silicon | Si | 14 |
2,8,4 = 1s2 2s22p6 3s23p13p13p0
In excited state the electronic configuration = 1s2 2s22p6 3s13p13p13p1 = 4 unpaired electrons = valency = 4 |
2
4 |
| Phosphorus | P | 15 |
2,8,5
= 1s2 2s22p6 3s23p13p13p13d03d03d03d03d0 In excited state the electronic configuration = 1s2 2s22p6 3s13p13p13p13d13d03d03d03d0 =5 unpaired electrons = valency = 5 |
3
5 |
|
Sulphur |
S |
16 |
2,8,6 = 1s2 2s22p6 3s23p23p13p13d03d03d03d03d0 In excited state the electronic configuration = 1s2 2s22p6 3s23p13p13p13d13d03d03d03d0 =4 unpaired electrons = valency = 4 In excited state the electronic configuration = 1s2 2s22p6 3s13p13p13p13d13d13d03d03d0 =6 unpaired electrons = valency = 6 |
2
4 6 |
|
Chlorine
Chlorine |
Cl
Cl |
17
17 |
2,8,7
= 1s2 2s22p6 3s23p23p23p13d03d03d03d03d0 In excited state the electronic configuration = 1s2 2s22p6 3s23p23p13p13d13d03d03d03d0 =3 unpaired electrons = valency = 3 In excited state the electronic configuration = 1s2 2s22p6 3s23p13p13p13d13d13d03d03d0 =5 unpaired electrons = valency = 5 In excited state the electronic configuration = 1s2 2s22p6 3s13p13p13p13d13d13d13d03d0 =7 unpaired electrons = valency = 7 |
1
3 5 7 |
|
Argon |
Ar |
18 |
2,8,8 = 1s2 2s22p6 3s23p6 It is inert gas. |
0 |
| Potassium | K | 19 |
2,8,8,1 = 1s2 2s22p6 3s23p64s1
This element prefers to lose ‘1’ electrons to get stable configuration of 2, 8, 8. That is why this element mainly forms ionic compounds. |
1 |
| Calcium | Ca | 20 |
2,8,8,2 = 1s2 2s22p6 3s23p64s2
This element prefers to lose ‘2’ electrons to get stable configuration of 2, 8, 8. That is why this element mainly forms ionic compounds. |
2 |
Once the ‘N’ level gets ‘2’ electrons the next 10 electrons go to ‘M’ level, to get ‘18’ electrons till ‘3d’ sublevel is filled. These ten elements are called Transition metals.
| Element | Symbol |
Atomic
number |
Electronic configuration
(or Electron arrangement) K L M N |
Number of
Unpaired electrons |
| Scandium | Sc | 21 | 2,8,9,2 = 1s2 2s22p6 3s23p6 3d14s2 |
1
Oxidation state = 2,3. |
| Vanadium | V | 22 | 2,8,10,2 = 1s2 2s22p6 3s23p6 3d24s2 |
2
Oxidation state = 2,3.4. |
| Titanium | Ti | 23 | 2,8,11,2 = 1s2 2s22p6 3s23p6 3d34s2 |
3
Oxidation state = 2,3.4,6. |
| Chromium | Cr | 24 |
2,8,13,1 = 1s2 2s22p6 3s23p6 3d54s1
(Half filled ’d’) |
6
Oxidation state = 1,2,3,4,5,6 |
| Manganese | Mn | 25 | 2,8,13,2 = 1s2 2s22p6 3s23p6 3d54s2 |
5
Oxidation state = 2,3.4, 5, 6, 7. |
| Iron | Fe | 26 | 2,8,14,2 = 1s2 2s22p6 3s23p6 3d64s2 |
4
Oxidation state = 2,3,4,6. |
| Cobalt | Co | 27 | 2,8,15,2 = 1s2 2s22p6 3s23p6 3d74s2 |
3
Oxidation state = 2,3.4,5 |
| Nickel | Ni | 28 | 2,8,16,2 = 1s2 2s22p6 3s23p6 3d84s2 |
2
Oxidation state = 2,3.4 |
| Copper | Cu | 29 |
2,8,18,1 = 1s2 2s22p6 3s23p6 3d104s1
(completely filled ’d’) |
1
Oxidation state = ,2,3. |
| Zinc | Zn | 30 | 2,8,18,2 = 1s2 2s22p6 3s23p6 3d104s2 |
2
Oxidation state = 2. |
When ‘M’ level gets ‘18’ electrons and is completely filled. Then ‘N’ level starts refilling.
| Element | Symbol |
Atomic
number |
Electronic configuration
(or Electron arrangement) K L M N |
Number of
Unpaired electrons |
| Gallium | Ga | 31 |
2,8,18,3
= 1s2 2s22p6 3s23p6 3d104s24p14p04p0 In excited state the electronic configuration == 1s2 2s22p6 3s23p6 3d104s14p14p14p0 =3unpaired electrons = valency =3 |
1
3 |
| Germanium | Ge | 32 |
2,8,18,4
= 1s2 2s22p6 3s23p6 3d104s24p14p14p0 In excited state the electronic configuration == 1s2 2s22p6 3s23p6 3d104s14p14p14p10 =4unpaired electrons = valency =4 |
2
4 |
| Arsenic | As | 33 |
2,8,18,5 =
1s2 2s22p6 3s23p6 3d104s24p14p14p14d04d0 In excited state the electronic configuration 1s2 2s22p6 3s23p6 3d104s14p14p14p14d14d0 =5unpaired electrons = valency =5 |
3
5 |
| Selenium | Se | 34 |
2,8,18,6 =
1s2 2s22p6 3s23p6 3d104s24p24p14p14d04d0 In excited state the electronic configuration 1s2 2s22p6 3s23p6 3d104s24p14p14p14d14d0 =4unpaired electrons = valency =4 In excited state the electronic configuration 1s2 2s22p6 3s23p6 3d104s14p14p14p14d14d1 =6unpaired electrons = valency =6 |
2
4 6 |
| Bromine | Br | 35 |
2,8,18,7 =
1s2 2s22p6 3s23p6 3d104s24p24p24p14d04d04d0 In excited state the electronic configuration 1s2 2s22p6 3s23p6 3d104s24p24p14p14d14d04d0 =3unpaired electrons = valency =3 In excited state the electronic configuration 1s2 2s22p6 3s23p6 3d104s24p14p14p14d14d14d0 =5unpaired electrons = valency =5 In excited state the electronic configuration 1s2 2s22p6 3s23p6 3d104s14p14p14p14d14d14d1 =7unpaired electrons = valency =7 |
1
3 5 7 |
| Krypton | Kr | 36 |
2,8,18,8 = 1s2 2s22p6 3s23p6 3d104s24p6
It is inert gas. |
0 |
In modern concept, the valence of an element is the number of unpaired electrons in the ground or excited state of the element.
An element goes to excited state only when it is combining with a more electronegative element. From an electron pair one electron excites to higher energy subshell, and then it gets two unpaired electrons to participate in bonding.
In excited state the unpaired electrons can participate only in covalent bonds.
How many unpaired electrons an element can have depends upon the electronegativity of the element with which it is combining.
Example: Chlorine can show valence ‘7’when it combines with oxygen. Chlorine can form ClF5 but nor ClF7. Iodine can form IF7.
However if the bonds is polar, we should not conclude the molecule is polar because the resultant of individual bond polarities is the polarity of molecule.
There is a fundamental difference between the oxidation state and valance of the element in a compound but for the sake of writing the formula we can consider them similar.