Atomic StructureAtoms: Atoms are the smallest individual part of an element. They are tiny;0ne thousand millionth of a metre.Molecules:Two or more atoms of the same or different elements join together(bond) to form a molecules of a substance. eg. When atoms of elements of hydrogen and fluorine join together, they form a molecule of the compound Hydrogen Fluoride.So molecules are the smallest individual parts of a compound.Atomic structure.Atoms are not individible; they possess an internal structure. At the centre of the atom is a dense nucleus, consisting of protons which ahave a positive electric charge and mass equal to 1.67x10-24 gramms) and neutrons which are neutral and have a mass of 1.0087 amu.Around the nucleus are negatively charged electronshaving a charge of 1.602x10-19 coulumbs negative charge and mass of 9.10x10-18 gramms. It is the electrons that give a substance most of its physical and chemical properties. Different structures:Different structures were proposed for the atom. In 1903 J. J Thomson proposed a watermelon model of atomic structure. In 1911 Rutherford proposed a planetary model of atomic structure. in 1913 Neil's Bohr,a Danish scientist proposeda structure based on the quantum Theory of Radiation. Bohr proposed circular orbits to the electrons which go around the nucleus. It states that an electron remains constant as long as it revolves in the same orbit.later studies by Sommerfeld, Lande and others proved that electrons do not follow definite paths around the nucleus. Instead electrons are said to be found within certain regions called orbitals.
These are arranged around the nucleus in 'shells', each containing electron of a particular energy. For example, the first shell(1 )can holdup to two electrons, in a so called s-shell(1s), the second shell(2) can hold upto 8 electrons in the s-orbital (2s) and p-orbitals (2p).The third shell can hold 18 electrons93s, 3p, 3d(10 electrons), the fourth can have 32 (4s, 4p, 4d, 4f and so on).The systematic filling of electrons in an atom is called its electronic configuration. The filling up of electrons in various energy levels or orbitals depends upon certain principles and rules as follows.1. The maximum number of electrons in an orbit (1,2,3,4...)is 2n2 where n is the number of the orbit.2.An orbital can hold only a maximum of 2 electrons.3. Auf-Bau principle: The electrons always occupies the orbital having the least energy first.. The order of energies of orbitals can be found using a moeller diagram.4.Hund's Rule: "Electron pairing takes place only after all the available degenerate orbitals are occupied by one electron each."5. Pauli's Exclusion Principle; " No two electrons will have all the four quantum numbers same."The atomic orbitals have different shapes. The s-orbital is spherical in shape, p-orbitals(3)are dumbell in shape. The five d orbitals have distinct dumbell shapes.