Final answer:
Lithium has one valence electron which is crucial for its reactivity, such as reacting with oxygen to form lithium oxide. Its reactivity increases down the group due to decreasing ionization energy. The crisscross method can be used to find the ionic formula for compounds like lithium peroxide, Li2O2.
Step-by-step explanation:
Lithium is an alkali metal that belongs to the first group of the periodic table. Each lithium atom has one valence electron in its outer shell, which is in the 2s orbital. This valence electron is responsible for lithium's chemical properties, including its reactivity with oxygen to form lithium oxide (Li2O). The formation of a lithium molecule (Li2) involves the combination of two lithium atoms, each contributing one valence electron to the bonding molecular orbital, suggesting that the molecule would be stable at high temperatures.
The reactivity of lithium and other alkali metals increases with atomic number due to the ease of losing that lone valence electron. Alkali metals are easier to oxidize than hydrogen, as evidenced by their vigorous reaction with water, which forms hydrogen gas and a basic solution of the metal hydroxide.
Applying the crisscross method to predict the formula of the ionic compound between lithium ions and the peroxide ion, O22-, reveals that two lithium ions (Li+) are needed to balance out the negative charge of one peroxide ion, resulting in the formula Li2O2 for lithium peroxide.