<P> Similarly, on moving rightward a group, the elements become progressively more covalent, less basic and more electronegative, whereas on moving down a period the elements become more ionic, more basic and less electronegative . Thus, on both descending a period and crossing a group by one element, the changes "cancel" each other out, and elements with similar properties which have similar chemistry are often found--the atomic size, electronegativity, properties of compounds (and so forth) of the diagonal members are similar . </P> <P> It is found that the chemistry of a first - row (second period) element often has similarities to the chemistry of the second - row (third period) element being one column to the right of it in the periodic table . Thus, the chemistry of Li has similarities to that of Mg, the chemistry of Be has similarities to that of Al, and the chemistry of B has similarities to that of Si . These are called diagonal relationships . (It is not as noticeable after B and Si .) The reasons for the existence of diagonal relationships are not fully understood, but charge density is a factor . For example, Li is a small cation with a + 1 charge and Mg is somewhat larger with a + 2 charge, so the charge density on each ion is roughly the same . Using the Li--Mg pair: (under room temperature and pressure) </P> <Ol> <Li> Li and Mg form only normal oxides whereas Na forms peroxide and metals below Na, in addition, form superoxides . </Li> <Li> Li is the only Group 1 element which forms a stable nitride, Li N. Mg, as well as other Group 2 elements, also form nitrides . </Li> <Li> Lithium carbonate, phosphate and fluoride are sparingly soluble in water . The corresponding Group 2 salts are insoluble . (Think lattice and solvation energies). </Li> <Li> Both Li and Mg form covalent organometallic compounds . LiMe and MgMe (cf . Grignard reagents) are both valuable synthetic reagents . The other Group 1 and Group 2 analogues are ionic and extremely reactive (and hence difficult to manipulate). </Li> <Li> Chlorides of both Li and Mg are deliquescent (absorb moisture from surroundings) and soluble in alcohol and pyridine . Lithium chloride, like magnesium chloride (MgCl. 6H O) separates out from hydrated crystal LiCl. 2H O . </Li> </Ol> <Li> Li and Mg form only normal oxides whereas Na forms peroxide and metals below Na, in addition, form superoxides . </Li>

Similarities in chemical behavior between lithium and magnesium
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