CHAPTER 2 STRUCTURE OF ATOM BY DR. VIDHI GROVER

CHEMISTRY THE SCIENCE IN WHICH WE STUDY ABOUT Matter is made up of small indivisible particles called atoms. Atom can neither be created nor be destroyed. Atoms always combine in a fixed ratio to form a compound. Atoms of the similar elements have similar shape ,properties,structure in all respects. Atoms of the different elements have different shape ,properties,structure in all respects.

Atoms of the same element are exactly alike. H H

Atoms of different elements are different. O H

Compounds are formed by the joining of atoms of two or more elements. H H O

BOHR’S ATOMIC MODEL i) An atom has a positively charged nucleus at its centre and most of the mass of the atom is in the nucleus. ii) The electrons revolve around the nucleus in special orbits called discrete orbits. iii) These orbits are called shells or energy levels and are represented by the letters K, L, M, N etc. or numbered as 1, 2, 3, 4, etc. iv) While revolving in the discrete orbits the electrons do not radiate energy.

ATOM It is the smallest particle which can not be further sub divided.

Parts of Atom Nuclear part Extra –nuclear part Proton neutron electron

ATOMIC STRUCTURE Particle Charge Mass proton + ve charge 1 neutron No charge 1 electron -ve charge nil

Subatomic Particles (SAP’s) Particle & Symbol Relative Electrical Charge Relative Mass Actual Mass (kg) Proton (p+, 11H) # determines elements identity +1 positive 1.007 1.673*10-27 1836 times greater than the mass of an e- Neutron (n0,01n) 0 uncharged 1.009 1.675*10-27 Electron (e-, -10e) -1 negative 0.0005 9.109*10-31

Discoveries Electron J.J Thomson Proton Goldstein Neutron Chadwick

RELATED TERMS ATOMIC NO. MASS NO. The atomic number of an element is the number of protons present in the nucleus of the atom of the element. Eg :- Hydrogen – Atomic number = 1 (1 proton) Helium - Atomic number = 2 (2 protons) The mass number of an element is the sum of the number of protons and neutrons (nucleons) present in the nucleus of an atom of the element. Eg :- Carbon – Mass number = 12 (6 protons + 6 neutrons) Mass = 12u

number of electrons = number of protons ATOMIC STRUCTURE He 4 Atomic mass the number of protons and neutrons in an atom 2 Atomic number the number of protons in an atom number of electrons = number of protons

Isotopes :- Atoms of the same element having the same atomic numbers but different mass numbers. Eg :- Hydrogen has three isotopes. They are Protium, Deuterium (D) and Tritium (T). H H H Protium Deuterium Tritium Carbon has two isotopes. They are :- C C Chlorine has two isotopes They are :- Cl Cl 3 1 2 1 1 1 12 14 6 6 35 37 17 17

Isobars :- Isobars are atoms of different elements having different atomic numbers but same mass numbers. These pairs of elements have the same number of nucleons. Eg :- Calcium (Ca) – atomic number - 20 and Argon (Ar) – atomic number 18 have different atomic numbers but have the same mass numbers – 40. Iron (Fe) and Nickel (Ni) have different atomic numbers but have the same atomic mass numbers – 58. 40 40 Ca Ar 20 18 58 Fe 58 Ni 26 27

ISOTONES Atoms of the different elements having same number of neutrons are called isotones. For example:- 14C 6 15N 8

ORBITAL Its a 3D region in space where the probability of finding the electron is maximum about 90-95%. Types – s p d f

s Orbital shape (Starts at n=1, l = 0) The s orbital has a spherical shape centered around the origin of the three axes in space. 1 arrangement (ml = 0)

p orbital shape (Starts at n=2, l = 1) +1 -1 There are three dumbbell-shaped p orbitals in each energy level above n = 1, each assigned to its own axis (x, y and z) in space. 3 arrangements (ml = -1 to +1)

Difference orbit orbital It is a circular path traced by e-. It has 2D representation Orbits are K L M N It is circular in shape It is non directional in nature The maximum no. of electron in orbit is 2n2 It is not accordance with hesienberg uncertanity principle It is a region in space where the probability of finding the e- is maximum. It has 3D representation Orbitals are s p d f They have different shapes It is directional in nature The maximum no. of electron in orbit is two It is accordance hesienberg uncertanity principle

1. Electronic Configuration ATOMIC STRUCTURE There are two ways to represent the atomic structure of an element or compound; 1. Electronic Configuration 2. Dot & Cross Diagrams

DISTRIBUTION OF ELECTRONS IN DIFFERENT SHELLS The distribution of electrons in the different shells was suggested by Bhor and Bury. The following are the rules for filling electrons in the different shells. i) The maximum number of electrons in a shell is given by the formula 2n2 where n is the number of the shell 1, 2, 3 etc. First shell or K shell can have = 2n2 = 2 x 12 = 2x1x1 = 2 electrons Second shell or L shell can have = 2n2 = 2 x 22 = 2x2x2 = 8 electrons Third shell or M shell can have = 2n2 = 2 x 32 = 2x3x3 = 18 electrons Fourth shell or N shell can have = 2n2 = 2 x 42 = 2x4x4 = 32 electrons and so on. ii) The maximum number of electrons that can be filled in the outermost shell is 8. iii) Electrons cannot be filled in a shell unless the inner shells are filled.

ELECTRONIC CONFIGURATION With electronic configuration elements are represented numerically by the number of electrons in their shells and number of shells. For example; Nitrogen configuration = 2 , 5 7 2 in 1st shell 5 in 2nd shell N 2 + 5 = 7 14

ELECTRONIC CONFIGURATION Write the electronic configuration for the following elements; 20 11 8 Na O Ca a) b) c) 16 23 40 2,8,8,2 2,8,1 2,6 17 14 5 Cl Si B d) e) f) 11 35 28 2,8,7 2,8,4 2,3

N DOT & CROSS DIAGRAMS Nitrogen With Dot & Cross diagrams elements and compounds are represented by Dots or Crosses to show electrons, and circles to show the shells. For example; X Nitrogen N 7 X X N X X 14 X X

2 ways to write electron configurations 1 s value of energy level sublevel no. of electrons spdf NOTATION for H, atomic number = 1 Orbital Box Notation Arrows show electron spin (+½ or -½) ORBITAL BOX NOTATION for He, atomic number = 2 1s 2  26

Aufbau’ principle According to this in the ground state of the atoms , the orbitals are filled according to their increasing energies. The order is : 1s, 2s , 2p , 3s , 3p , 4s , 3d , 4p , 5s , 5p , 6s

Aufbau Principle -- “Bottom Up Rule” 28

Hund’s rule of maximum multiplicity The pairing of electrons does not takes place until orbital belonging to same sub shell get singly occupied Eg : Nitrogen (N) =7

Pauli’s exclusion principle Its states that no two electrons in an atom can have the same set of all four quantum nos. Eg: for k-shell n=1 , l=o , m=0 , s= +1/2 , -1/2

Basic Principle: electrons occupy lowest energy levels available Rules for Filling Orbitals Bottom-up (Aufbau’s principle) Fill orbitals singly before doubling up (Hund’s Rule) Paired electrons have opposite spin (Pauli exclusion principle) 31

Orbital energy ladder Energy n = 4 n = 3 n = 2 n = 1 f d p d s p s p s 32

Shorthand notation practice [Noble Gas Core] + higher energy electrons Examples ● Aluminum: 1s22s22p63s23p1 [Ne]3s23p1 ● Calcium: 1s22s22p63s23p64s2 [Ar]4s2 ● Nickel: 1s22s22p63s23p64s23d8 [Ar]4s23d8 {or [Ar]3d84s2} ● Iodine: [Kr]5s24d105p5 {or [Kr]4d105s25p5} ● Astatine (At): [Xe]6s24f145d106p5 {or [Xe]4f145d106s26p5}

  Phosphorus Symbol: P Atomic Number: 15 Full Configuration: 1s22s22p63s23p3 Valence Configuration: 3s23p3 Shorthand Configuration: [Ne]3s23p3     1s 2s 2p 3s 3p Box Notation 34

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