The reactivity of the elements in Group 1, known as alkali metals, increases as one moves down the group.

Group 1 consists of the following elements: lithium ($\text{Li}$), sodium ($\text{Na}$), potassium ($\text{K}$), rubidium ($\text{Rb}$), caesium ($\text{Cs}$), and francium ($\text{Fr}$). These metals are characterized by their high reactivity, which stems from the presence of a single electron in their outermost electron shell. This lone electron is easily lost, leading to the alkali metals’ pronounced reactivity.

As we progress down the group, the outermost electron is located farther from the nucleus. Consequently, it experiences a reduced attraction to the positively charged protons in the nucleus. This diminished attraction facilitates the loss of the outermost electron, thereby increasing the likelihood of the atom participating in chemical reactions.

Among the alkali metals, lithium ($\text{Li}$) is the least reactive, while francium ($\text{Fr}$) is the most reactive. This trend can be attributed to the increasing atomic radius, which refers to the size of the atom. As the atomic radius grows larger down the group, the outermost electron is positioned further from the nucleus and is less tightly bound by the protons’ positive charge. This makes it increasingly easier for the atom to lose this electron and form a positive ion, which is essential for chemical reactivity.

For instance, when alkali metals react with water, they produce a metal hydroxide and hydrogen gas. The intensity of this reaction escalates as you move down the group. Lithium ($\text{Li}$) reacts slowly with water, sodium ($\text{Na}$) has a more rapid reaction, and potassium ($\text{K}$) reacts very vigorously. This trend continues with rubidium ($\text{Rb}$) and caesium ($\text{Cs}$), which can react explosively with water.

In summary, the reactivity of Group 1 elements increases as one moves down the group. This trend is primarily due to the increasing atomic radius, which makes the outermost electron easier to lose, thus enhancing the atom’s propensity to engage in chemical reactions.