What is diagonal relationship? How do lithium and magnesium show similar chemical behaviour?

When examining the chemical behavior of lithium, an element in the alkali metals group, we see a plethora of distinctive reactivities. As the lightest metal, lithium has an exceptional eagerness to part with its single valence electron, thus forming ionic compounds with a +1 charge.

Lithium's claim to fame is its vigorous reaction with water, leading to the liberation of hydrogen gas and the creation of lithium hydroxide. When dealing with carbonates, lithium again doesn't shy away from a reaction, resulting in a white precipitate of lithium carbonate. Its unique properties don't stop there; despite being an alkali metal, lithium shows a certain reluctance to react with nitrogen directly – a trait it shares with magnesium, an alkaline earth metal.

Moving down the periodic table to magnesium, we detect a more subdued but still notable trend inherent to the alkaline earth metals. Magnesium, with its two valence electrons, is typically more reserved in its reactions compared to the boisterous lithium. However, similar to its diagonal counterpart, magnesium is predisposed to transfer its valence electrons, creating +2 ions.

Magnesium doesn't entertain a rapid reaction with water under normal conditions, but give it a dash of heat, and it'll reluctantly engage, forming magnesium hydroxide and hydrogen gas. Carbonates are not immune to magnesium's reactions; they yield a white precipitate as well. Interestingly, like lithium, magnesium forms a layer of oxide when exposed to air, which inhibits further reaction – a stark contrast to the swift tarnishing seen in most other alkaline earth metals.

Alkali metals, gracing Group 1 of the periodic table, are known for their reactivity and are always found in nature combined with other elements. Each one houses a solitary valence electron that they are all too eager to discard in chemical reactions, resulting in the formation of +1 ions.

The reactivity of these metals increases as we descend the group, thanks to the growing distance of the valence electron from the nucleus, which weakens its hold. They all engage ferociously with water, forming hydroxides and liberating hydrogen gas – a trait that is notably tempered in lithium due to its diagonal relationship with magnesium.

Alkaline earth metals, residing in Group 2 of the periodic table, encompass elements like magnesium that contain two valence electrons. While this group shares a similar willingness to lose their electrons, they form +2 ions instead. Their reactivity, although less intense than their alkali metal cousins, still sees a progressive increase down the group.

Alkaline earth metals react with water at varying rates – beryllium doesn't react at all, whereas magnesium needs some heat, and barium reacts readily. These elements often overshadow the alkali metals in terms of their contribution to Earth's crust but are usually upstaged by their more reactive neighbors when it comes to pure chemical drama.