Chapter-1 Modern Periodic Table

·        Mendeleev made a successful effort in grouping elements in the form of his periodic table. The limitations of Mendeleev’s periodic table forced scientists to believe that atomic mass could not be the basis for the classification of elements.

·        In 1913, Henry Moseley demonstrated that atomic number (instead of atomic mass) is a more fundamental property for classifying elements.

·        Atomic number = Number of protons = Number of electrons

·        Atomic Mass: The atomic mass of an element is the sum of neutrons and protons present inside the nucleus of an atom.

·        Atomic mass = Number of protons + Number of neutrons

·        The modern periodic law states that the properties of elements are a periodic function of their atomic numbers, not their atomic masses.

·        The table that is obtained when elements are arranged in the increasing order of their atomic numbers is called the Modern Periodic Table or Long Form of the Periodic Table.

·        In the modern periodic table, which was prepared by Niels Bohr, elements are arranged in rows and columns. These rows and columns are known as periods and groups respectively. The table consists of 7 periods and 18 groups.

·        The number of electrons that a shell can hold can be calculated using the formula 2n2. Here, n represents the number of shells from the nucleus. For example, n is equal to 1, 2, and 3 for K, L, and M shells respectively.

·        Valency is defined as the number of electrons an atom requires to lose, gain, or share in order to complete its valence shell to attain the stable noble gas configuration.

·        Elements show periodicity because of their valence shell configuration. All elements showing periodicity in properties have the same number of electrons in the last or valence shell.

·        Atomic Radius: The atomic radius is usually considered as the distance from the centre of the nucleus to the outermost shell i.e., to a point where the electron density is effectively zero.

·        Atomic radius decreases across a period and increases down the group.

·        Ionisation Potential: It is the energy required to remove one mole of electrons from the valence shell of one mole of isolated gaseous atoms.

·        Across the period, the ionisation potential increases. It also increases down the group.

·        Electronegativity: The tendency of an atom to attract a bonding pair of electron towards itself when combined in a compound is called electronegativity.

·        Across the period, the electronegativity increases while down the group it decreases.

·        Metallic character: It is defined as the tendency of an atom to lose electrons.

·        Across the period, the metallic character decreases while it increases down the group.

·        The atomic radius of halogen atoms goes on increasing on moving down the group.

·        Atomic size is the radius of an atom in its neutral state i.e., the distance of the nucleus from its valence shell in an isolated state.

·        The elements present in the 1st group are the most metallic in their respective periods as they contain only one electron and have the largest atomic size in their respective periods.