ATOMIC STRUCTURE R.SANGEETHA VSMHS SCHOOL, CHENNAI

ATOMS AND SUBATOMIC PARTICLES An atom is very small. Its mass is between 10-21 and 10-23g. A row of 107 atoms (10,000,000 atoms) extends only 1.0 mm. The subatomic particles are: 1.proton(+ve charge) 2.electron(neutral) 3.electron(-ve charge)

Properties of the proton, neutron, and electron PARTICLE ACTUAL MASS (g) RELATIVE MASS (amu) RELATIVE CHARGE PROTON 1.6726 X 10-24 1.007 +1 NEUTRON 1.6749 X 10-24 1.008 ELECTRON 9.108 X 10-28 5.45 X 10-4 -1

A. Atomic Number Equals Electrons or Protons Atomic number= number of protons per atom= number of electrons per neutral atom B. Mass Number Equals Protons plus Neutrons Mass number = number of protons + number of neutrons

Thomson’s Model Of Atom Thomson in 1904 put forward a model of atom according to which, an atom consists of a sphere of positive charge in which the negatively charged electrons are embedded like the seeds in water melon. the model was rejected as it could not explain Rutherford’s scattering experiment Embedded electrons Diffused positive charge

Rutherford's Experiment In 1904 Rutherford investigated the radioactive rays with the help of strong electric and magnetic fields and classified them into three kinds Alpha(α),Beta(β) and gamma(γ). Alpha particles were identified with helium atoms that have a mass of 4 and the charge on them as 2+. Rutherford performed many experiments in radioactivity. Scattering of alpha particles is one such experiment led to a further understanding of atomic structure.

Results of Rutherford’s Experiment 1.There is a lot of hollow space in the atom. 2.The central part of the atom called nucleus, is positively charged. 3.The deflection of positively charged α particles is due to repulsion by the positively charged nucleus. The space occupied by the central part i.e. nucleus of the atom is much less than the much larger space in which the electrons revolve around the nucleus in closed orbits. 4.The nucleus is heavy and dense. 5.Since the nucleus is very small only a few α particles get deflected. these α particles get scattered through large angles by the nucleus because of its large mass and positive charge.

Bohr’s atom model To overcome the objections raised against Rutherford’s model of an atom, Neils Bohr in 1913 put forward a new theory of atomic structure known as Bohr’s theory. The important postulates are: 1.Atom consists of a small but massive nucleus situated at the centre with the electrons revolving around it in closed circular orbits. 2. The electrons in an atom revolve around the nucleus in definite circular paths known as energy levels. the energy of the electron remains constant as it moves in the same orbit. This implies that each orbit is associated with a certain definite amount of energy.

3.The different energy levels or shells are also called as stationary states designated by K,L,M,N etc. starting from the nucleus as shown in figure or with integers 1,2,3,4 etc. The energies of different energy shells follow the sequence 1<2<3<4 etc. or K<L<M<N etc. 4.Energy is emitted or absorbed only when an electron moves from one energy level to another. The amount of energy thus emitted or emitted is equal to the difference of energies of the two energy levels concerned. Since the various energy levels have certain definite or discrete quantities called quanta or photons.

Rules for filling up of electrons in the atom 1.Pauli’s exclusion principle “No two electrons in an atom can have all the four quantum numbers same.” 2.Hund’s rule of maximum multiplicity This rule states that “electron pairing will take place only when all the available sub shells have at least one electron each.” 3.Aufbau principle “This principle states that electrons enter in to orbital in the increasing order of energy.”