STRUCTURE OF ATOM AND PERIODIC TABLE
INRODUCTION
The elements are arranged in the periodic table in such a way that their chemical and physical properties can be predicated .We can find out that properties of an element if its position in the periodic table are known .The basis for the arrangement of the various elements in the periodic table is the similarities and differences in their atomic structure
Atoms are minute practice .If ten lakhs of atoms are arranged one after the other,it would come to a length of one millimeter only .Scientists have developed certain ideas about internal structure of the atom which is not visible even through the most powerful microscope .It is possible to explain the properties of atoms and elements on the basis of these ideas
You have learnt that the electrons of an atom are arranged in different shells around a nucleus. The basis of this arrangement is the energy level of the electrons .The attraction of the nucleus on their electrons decreases as the distance from the nucleus increases therefore, the energy of the electrons increase as the distance from the nucleus increases. that is the electrons in the first shell K will have the least energy .Likewise ,the energy of electrons progressively increases from L to M and toN. Hence each shell can be considered as an energy level You have learn to calculate the maximum number of electrons that each shell can accommodate using the formula 2n2
ELECTRON CONFIGURATION IN SHELL AND SUB SHELL
Bohr models of potassium calcium and scandium are given below
POTASSIUM
CALCIUM
SCANDIUM
Element | Shells | |||
K | L | M | N | |
19K | 2 | 8 | 8 | 1 |
20Ca | 2 | 8 | 8 | 2 |
21Sc | 2 | 8 | 8 | 3 |
Higher the potential energy of an electron greater will be it’s a distance from the Nucleus .there are some areas where the possibility of finding an electron is greatest .These areas are found around the nucleus ,one around the other
Imagine a glass marble faced with in a ball .This ball is faced with in another ball which is in intern contained with in bigger ball each ball is then equivalent to an area where electron can be found. These can be called as Shells .It is important to release that shells are not physical .These are merely shape of areas where the probability of finding electrons is the highest .The electron with lowest potential energy will be found in the shell closest to the nucleus .As the potential energy increases,the electrons will be distributed in shells further away from the nucleus
When there are more than one electron in a shell ,They are arranged according to their potential energy .Electron with least potential energy will be placed at lowest level in the shell .Electrons with higher potential energy will go to higher levels
In other words, it is similar to the arrangement of electrons around the nucleus .electrons are found in areas arranged one around the other ,energy levels with in the shells also follow the same arrangement
These energy levels with in the shells called Sub Shells these are named as s,p,d,f
Like shells, sub shells can accommodate only a limited number of electrons .The maximum number of electrons that can be accommodates in ‘S’ sub shell is 2
Shell Number | 1 | 2 | 3 | 4 | ||||||
Name of The Shell | K | L | M | N | ||||||
Maximum Number of Electrons | 2 | 8 | 18 | 32 | ||||||
Sub Shell | s | s | p | s | p | d | s | p | d | f |
Number of electrons in Sub Shell | 2 | 2 | 6 | 2 | 6 | 10 | 2 | 6 | 10 | 14 |
Sub shell electron configuration
When more electrons come to a particular shell in an atom, they are arranged in different sub energy levels. That is, electron filling takes place in sub shells. Let us see how electron filling takes place in sub shells.
Take the case of hydrogen atom, There is only one electron. It goes to the ‘s’ sub shell in the first shell. The sub shells electron configuration of Hydrogen atom can be illustrated as under
Sub shell electronic configuration of Lithium, aluminum, Potassium
Element | Electron configuration in Shell | Electron configuration in Sub Shell |
Lithium , Aluminum Potassium | 2,1 2,8,3 2,8,8,1 | 1s2 , 2s1 1s , 2s2,2p6,3p1 1s , 2s2,2p6,3s2,3p6,4s1 |
The sub shell Electronic Configuration of Potassium 1s2 , 2s2,2p6,3s2,3p6,4s1 ,In this case of Potassium the filling of electrons in the orbital based upon the increasing energy level .4s orbital is less than 3d so 4s is filled 1st before 3d
Electrons filling take place in the ascending order of the energy of the sub shells. This is known as Aufbau Principle
BLOCKS IN PERIODIC TABLE
In the modern Periodic table elements are classified in to 18 groups based on the electron configuration
s,p BLOCK ELEMENT
The subshell electron configuration of elements of the first and second group is given below.
Group – 1 | Group - 2 |
Li – [He] 2s1 Na – [Ne] 3s1 K – [Ar] 4s1 Rb – [Kr] 5s1 Cs – [Xe] 6s1 Fr – [Rn] 7s1 | Be – [He] 2s2 Mg – [Ne] 3s2 Ca – [Ar] 4s2 Sr – [Kr] 5s2 Ba – [Xe] 6s2 Ra – [Rn] 7s2 |
The above table shows that the last electrons entered into the ‘s’ subshell, so these elements are called ‘s’ block elements.
Consider the elements Al, Si, P, S, Cl, Ar and the electronic configurations are given below:
Al
1s2, 2s2, 2p6, 3s2, 3p1
Si
1s2, 2s2, 2p6, 3s2, 3p2
P
1s2, 2s2, 2p6, 3s2, 3p3
S
1s2, 2s2, 2p6, 3s2, 3p4
Cl
1s2, 2s2, 2p6, 3s2, 3p5
Ar
1s2, 2s2, 2p6, 3s2, 3p6
The above elements shows that last electron enter into the ‘p’ sub shell so these elements are called ‘p’ block elements.
s, p block elements shows tendency of periodicity and the elements are metals, non metals, metalloids, solids, liquids and gases. So s, p elements are called representative elements.
d block elements
The group of elements in between s and p blocks are called d block elements.
Elements | Atomic Number | Electronic Configuration |
Sc Ti V Mn Co Ni Zn | 21 22 23 25 27 28 30 | [Ar]3d1 4s2 [Ar]3d2 4s2 [Ar]3d3 4s2 [Ar]3d5 4s2 [Ar]3d7 4s2 [Ar]3d8 4s2 [Ar]3d10 4s2 |
The above table shows that last electron enter in to the d sub shell so these elements are called d block element ,and these elements are also called Transition elements
In the compounds FeCl2 and FeCl3 shows two oxidation state of iron
When the compound FeCl3 is formed one of the electrons in 3d subshell is also lost along with two electrons in the 4s sub shell .Thus ,iron loses three electrons .This happens so because the energy difference between 4s and 3d su shells is very little .So ,iron exhibits two oxidation states .All transitional elements ,excepts Zn,show different oxidation states
f-Block elements
You already know about lanthanones and actionones in the periodic table .In them ,the last electron goes to the f subshell of the ante penultimate shell .Now you know for sure which are the ‘f’block elements
IONIZATION ENERGY
The change that happens to the size of an atom along the periods and groups in the periodic table .As the size of the atom increases ,the distance from the nucleus to the outermost shell also increases .The positively charged nucleus of the atom attracts the electron .So if electrons are to be removed from the outermost shell of an atom ,they are to be brought out of the field of attraction of the nucleus .For this ,certain amount of energy is to be provided .This is called the ionization energy
