Modern Periodic Table
* Atomic number is the number of protons in an atom.
* The number of electrons in an atom is equal to the number of protons.
* An atom is electrically neutral.
* Valence electrons: Electrons in the outermost shell of an atom.
* Electronic configuration: The arrangement of electrons in an atom around its nucleus.
* Bohr’s theory: Electrons are not located randomly around an atom’s nucleus, but occur in definite electrons shells.
* The physical and chemical properties of elements are a periodic function of their atomic numbers.
* Elements are arranged in the increasing order of atomic number.
The modern periodic table consists of:
* 7 horizontal periods
* 18 vertical groups
* The properties of the elements in the periodic table change gradually from left to right.
* Group I elements are called alkali metals.
* Group II elements are called alkali earth metals.
* Group VII elements are called halogens.
* Zero group elements are referred to as inert gases.
Periodic Classification of Elements – Periodic Properties
* The properties of elements, which recur at regular intervals, are called periodic properties.
* Atomic size increases in a group and decrease in period
* Lonization energy increase across a period and decrease down a group
* Electron affinity increase across a period and decrease down a group
* Electronegativity increase across a period and decrease down a group
* Metallic character decrease across a period and increase down a group
* Non Metallic character increase across a period and decrease down a group
Need and Genesis of Classification
* A periodic table helps classify all the known elements in rows and columns such that the periodicity in their properties becomes apparent. The periodic law defines this periodicity.
* The classification of elements in a periodic table helps predict the properties of unknown elements.
* Johann Dobereiner was the first scientist to observe trends in the properties of elements in 1817; he stated the law of triads based on his observations.
* The first periodic table that applied to all the known elements was formulated by A.E.B de chancourtois in 1862.
* Chancourtois’s cylindrical periodic table was based on atomic weights of elements and displayed the periodic recurrence in their properties.
* John Alexander Newlands gave the Law of Octaves which states that when the elements are arranged in the increasing order of their atomic weights, the properties of every eighth elements are similar to that of the elements
* Lothar Meyer and Dmitri Mendeleev independently proposed the periodic Law, as we know it today, in 1869.
* According to Mendeleev’s periodic law, the physical and chemical their eventual success, he is considered the father of Modern Periodic Table.
Periodic Trends: Chemical Reactivity
* The trends in the chemical reactivity of elements can be explained based on the trends in fundamental properties like atomic and ionic radii, ionization enthalpy and electron gain enthalpy.
* The alkali, metals and halogens show a higher tendency to combine with oxygen as compared to the elements in the centre. The maximum chemical reactivity among the alkali metals is exhibited by the loss of an electron and in halogens is shown by the gain of electrons.
* The alkali metals and halogens of a period can easily combine with oxygen and form acidic and basic oxides.
* Oxides formed by elements in the centre are amphoteric or neutral.
* Down a group, the chemical reactivity generally increase for the alkali and the alkaline earth elements and decrease for the transition elements.
Periodic Trends: Physical Properties – II
* The elements in a periodic table demonstrate periodicity in terms of physical properties such as atomic and ionic radii, ionization enthalpy, electrons gain enthalpy and electronegativity.
* The radius of an atom is the distance between the nucleus and the outermost orbital of the electrons surrounding the nucleus. It can only be measured approximately as convalent, metallic or van der waals radius.
* Atomic radii generally decrease across a period and increase down a group in a periodical table; Ionic radii exhibit a similar trend.
* Ionic radius of a cation is always smaller than its parent atom and the ionic radius of an anion is larger than of the parent atom.
* The more positive the charge on an ion, the smaller the radius. The more the negativity of the charge, the larger the radius.
* Ionization enthalpy is the energy required to remove the most loosely bound electrons from an isolated gaseous atom (X) in its round state. It increase across a period and decrease down a group in a periodic table.
* Electrons gain enthalpy is the energy associated with the gain of an electron by an isolated gaseous atom (X) in its ground state. Electrons gain enthalpy becomes more negative from left to right in the period and becomes less negative as we move down the group.
* Electronegativity is a qualitative measure of the ability of an atom in a chemical compound to attract shared pair of electrons towards itself.
* Electronegativity increases across a period and decrease down the group.