Class 11 Chemistry Chapter 10 |Introduction - s Block Elements Class#11ChemistryNCERT BY @MANDEEPSIR

Class 11 Chemistry Chapter 10 |Introduction - s Block Elements Class#11ChemistryNCERT BY MANDEEP SIR Class 11 Chemistry Chapter 10 |Introduction - s Block Elements Class#11ChemistryNCERT BY MANDEEP SIR[10/31, 3:40 PM] Typing.........: Lithium and beryllium, the first elements of Group 1 and Group 2 respectively exhibit some properties which are different from those of the other members of the respective group. In these anomalous properties they resemble the second element of the following group. Thus, lithium shows similarities to magnesium and beryllium to aluminium in many of their properties. This type of diagonal similarity is commonly referred to as diagonal relationship in the periodic table. The diagonal relationship is due to the similarity in ionic sizes and/or charge/radius ratio of the elements. Monovalent sodium and potassium ions and divalent magnesium and calcium ions are found in large proportions in biological fluids. These ions perform important biological functions such as maintenance of ion balance and nerve impulse conduction. 10.1 GROUP 1 ELEMENTS: ALKALI METALS increase in atomic number, the atom becomes larger. The monovalent ions (M²) are smaller than the parent atom. The atomic and ionic radii of alkali metals increase on moving down the group i.e., they increase in size while going from Li to Cs. 10.1.3 Ionization Enthalpy The ionization enthalpies of the alkali metals are considerably low and decrease down the group from Li to Cs. This is because the effect of increasing size outweighs the increasing nuclear charge, and the outermost electron is very well screened from the nuclear charge. 10.1.4 Hydration Enthalpy. The hydration enthalpies of alkali metal ions decrease with increase in ionic sizes. Li has maximum degree of hydration and for this reason lithium salts are mostly hydrated es LiCI-2H.O [10/31, 3:40 PM] Typing.........: The alkali metals show regular trends in their physical and chemical properties with the increasing atomic number. The atomic. physical and chemical properties of alkali metals are discussed below. 10.1.1 Electronic Configuration All the alkali metals have one valence electron. ns (Table 10.1) outside the noble gas core. The loosely held s-electron in the outermost valence shell of these elements makes them the most electropositive metals. They readily lose electron to give monovalent Mions. Hence they are never found in free state in nature. Element Symbol Electronic configuration Lithium Li Sodium Na Potassium K Rubidium Rb Caesium 10.1.5 Physical Properties All the alkali metals are silvery white, soft and light metals. Because of the large size, these elements have low density which increases down the group from Li to Cs. However, potassium is lighter than sodium. The melting and boiling points of the alkali metals are low indicating weak metallic bonding due to the presence of only a single valence electron in them. The alkali metals and their salts impart characteristic colour to an oxidizing flame. This is because the heat from the flame excites the outermost orbital electron to a higher energy level. When the excited electron comes back to the ground state, there is emission of radiation in the visible region of the spectrum as given below: Alkali metals can therefore, be detected by