STRUCTURE OF ATOM AND PERIODIC TABLE

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