Unit 1 Atom Structure PDF

Unit 1 : Structure of atom Atom : The smallest indivisible particle of an element is called as atom. ATOMIC NUMBER : The atomic number is simply the number of protons/ electrons present in an atom. it is denoted by the capital letter Z. ATOMIC MASS NUMBER : The atomic mass number or nucleon...

Unit 1 : Structure of atom Atom : The smallest indivisible particle of an element is called as atom. ATOMIC NUMBER : The atomic number is simply the number of protons/ electrons present in an atom. it is denoted by the capital letter Z. ATOMIC MASS NUMBER : The atomic mass number or nucleon number, is the total number of protons and neutrons (together known as nucleons) in an atomic nucleus. OR The sum of protons and neutrons present in the nucleus of an atom.  ISOTOPES Isotopes are the atoms of the same element having same atomic number but different atomic mass number.  ISOBARS Isobars are the atoms of the different element having same atomic mass number but different atomic number. ISOTONES : Isotones are the atoms of different elements having different atomic number and atomic mass number but same number of neutrons present in their nucleus.  ATOM STRUCTURE BOHR’S ATOMIC MODEL Dr. Kashmiri Khamkar MIT WPU, PUNE 6 In the year 1913, Niels Bohr proposed an atomic structure model, describing an atom as a small, positively charged nucleus surrounded by electrons that travel in circular orbits around the positively charged nucleus like planets around the sun in our solar system, with attraction provided by electrostatic forces, popularly known as Bohr’s atomic model. It was basically an improved version of Rutherford’s atomic model overcoming its limitations. On most of the points, he is in agreement with him, like concepts of nucleus and electrons orbiting it. Salient features of Niels Bohr atomic model are: Dr. Kashmiri Khamkar MIT WPU, PUNE 7 ▪ Each atom is made up of centrally situated positively charged nucleus and negatively charged electrons are revolving around the nucleus. ▪ Electrons revolve around the nucleus in stable orbits without emission of radiant energy. Each orbit has a definite energy and is called an energy shell or energy level. ▪ An orbit or energy level is designated as K, L, M, N shells. When the electron is in the lowest energy level, it is said to be in the ground state. ▪ Each energy level is made up of sub-energy levels, which are designated by s, p, d and f. Dr. Kashmiri Khamkar MIT WPU, PUNE 8 ▪ There is an electrostatic force of attraction between positively charged protons situated in the nucleus and negatively charged electrons revolving around the nucleus. This electrostatic force is balanced by centrifugal force. Therefore though there is electrostatic force between electrons and protons, electrons do not fall in the nucleus. ▪ An electron emits or absorbs energy when it jumps from one orbit or energy level to another. When it jumps from a higher energy level to lower energy level it emits energy while it absorbs energy when it jumps from a lower energy level to a higher energy level. ▪ The energy absorbed or emitted is equal to the difference between the energies of the two energy levels (E1, E2) and is determined by Plank’s equation.  LIMITATIONS OF BOHR’S ATOMIC MODEL ▪ Bohr's model of an atom failed to explain the Zeeman Effect (effect of magnetic field on the spectra of atoms). ▪ It also failed to explain the Stark effect (effect of electric field on the spectra of atoms). ▪ It violates the Heisenberg Uncertainty Principle. It could not explain the spectra obtained from larger atoms.  QUANTUM NUMBERS : An electron in an atom has some location relative to the nucleus and is associated with some energy. These two properties of an electron are specified by use of numbers called quantum numbers. Each electron is characterised by four quantum numbers. 1. The principal quantum numbers , n 2. The azimuthal quantum number , l 3. The magnetic quantum number , m 4. The spin quantum number , s These quantum numbers are like labels used to describe completely the electron in an atom. Dr. Kashmiri Khamkar MIT WPU, PUNE 1 1  PRINCIPAL QUANTUM NUMBER The quantum number is related to the energy of the electron and its average distance from the nucleus. Hence it gives the idea of the effective volume of the orbital in which the electron is present. It can have any positive integral value except 0. Thus n=1,2,3,4 etc. These values represent the main energy levels. As the value of n, increases the electrons get further away from the nucleus and its energy also increases. Dr. Kashmiri Khamkar MIT WPU, PUNE 1 2  AZIMUTHAL QUAANTUM NUMBER This quantum number also called angular momentum quantum number. It indicates the sublevel in which the electron is present. It also gives an idea of the shape of the orbital in which the electron is located. It may have all possible whole no. values from 0 to (n-l) where n is the principal quantum number of the electron For example : When n=1 , (K) l=0 ( only one sub level) : s n=2 , (L) l=0,1 (2 sub levels/ orbitals) : s, p n=3, (M) l=0,1,2 (3 sub levels/ orbitals) : s, p,d n=4 , (N) l=0,1,2,3 (4 sub levels/ orbitals) : s,p,d,f Dr. Kashmiri Khamkar MIT WPU, PUNE 1 3 Dr. Kashmiri Khamkar MIT WPU, PUNE 1 4  The Magnetic quantum number , m It provides information about the orbital that an electron resides. It is used to represent the orientation of an atomic orbital in space. The magnetic quantum number / orbital quantum number. specifies the shape of an atomic orbital in space. Dr. Kashmiri Khamkar MIT WPU, PUNE 1 5  SPIN QUANTUM NUMBER Electrons which are in the same orbital have identical first three quantum number. For example, the 2 electrons in the shell of a helium atom are in the K shell with n = 1, l = 0, and ml = 0. Their spin moments cannot be identical, so one is ms = -1/2 and the other is ms = +1/2. Visually, we draw this as a subshell with 1 "up" electron and 1 "down" electron. Dr. Kashmiri Khamkar MIT WPU, PUNE 20 Hund's rule of maximum multiplicity Dr. Kashmiri Khamkar MIT WPU, PUNE 21 HUNDS RULE OF MAXIMUM MULTIPLICITY HUND’S RULE : Pairing of electrons in the orbitals belonging to the same subshell does not occur unless each orbital belonging to that sub-shell has got one electron each. Dr. Kashmiri Khamkar MIT WPU, PUNE 14 Dr. Kashmiri Khamkar MIT WPU, PUNE 23  What is the Aufbau Principle? Aufbau is a german word meaning building up. The building up of orbital means filling up of orbitals with electrons in the ground state of an atom. The Aufbaus principle is based on 1. Increasing order of energies of orbitals 2. Paulis exclusion principle 3. Hunds rule of maximum multiplicity. The Aufbau principle dictates the manner in which electrons are filled in the atomic orbitals of an atom in its ground state. It states that electrons are filled into atomic orbitals in the increasing order of orbital energy level. According to the Aufbau principle, the available atomic orbitals with the lowest energy levels are occupied before those with higher energy levels. Dr. Kashmiri Khamkar MIT WPU, PUNE 24  ENERGIES OF ORBITALS Orbitals are filled in order of increasing energy. According to the Aufbau principle, electrons first occupy those orbitals whose energy is the lowest. This implies that the electrons enter the orbitals having higher energies only when orbitals with lower energies have been completely filled. The order in which the energy of orbitals increases can be determined with the help of the (n+l) rule, where the sum of the principal and azimuthal quantum numbers determines the energy level of the orbital. The usual sequence in which the electrons occupy various orbitals is as follows : 1s