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Chemical reactivity refers to the ability of a substance to undergo a chemical change. This property varies across the periodic table of elements, particularly within groups such as the halogens. The halogens include fluorine, chlorine, bromine, iodine, and astatine. They are known for their high reactivity, especially with metals, forming a wide range of salts. For example, when a halogen reacts with a metal, a halide (salt) is typically formed.

In the context of reactivity, an 'activity series' is a helpful tool. It ranks elements like halogens based on their ability to displace other halogens from their compounds in chemical reactions. This series is crucial for predicting the outcomes of such reactions. In our textbook exercise, we examined reactions involving bromine and chlorine to establish an activity series for these halogens.

Displacement reactions occur when a more reactive element displaces a less reactive one from a compound. In the case of halogens, a displacement reaction typically involves halogens and halide salts. The halogen displacing another must be higher in the activity series for the reaction to occur.

For instance, we saw that chlorine (Cl₂) reacts with sodium bromide (NaBr) to displace bromine (Br₂) and form sodium chloride (NaCl). This reaction only occurs because chlorine is more reactive than bromine. When teaching these concepts, it's important to clarify that displacement reactions demonstrate the differences in reactivity among elements in the same group. In this case, the ability of one halogen to displace another from its salt is directly related to its position in the activity series.

The reactivity trend of halogens is crucial for understanding why certain halogens can displace others. Within the group of halogens, reactivity decreases as you move down the periodic table. Fluorine is the most reactive, followed by chlorine, bromine, and iodine, with astatine being the least reactive due to its instability and rarity.

This trend is due to the atomic structure of halogens; as you move down the group, each element has more electron shells, which shield the attractive force of the nucleus from the valence electrons. This makes it more difficult for the element to attract additional electrons, hence the decrease in reactivity. In the exercise solutions, this trend is illustrated through the successful displacement of iodine by bromine and bromine by chlorine, confirming that Cl₂ is more reactive than Br₂, and Br₂ more than I₂.