Video Transcript
When lithium reacts with water,
fizzing is observed where the lithium sits on the surface of the water, and the
reaction is relatively slow. When potassium reacts with water,
the potassium immediately ignites and moves violently around on the surface of the
water, and the reaction is over quickly. Which statement best describes why
these differences are observed? (A) Lithium is less dense than
potassium. (B) Lithium loses its valence
electrons more easily than potassium. (C) Potassium is more flammable
than lithium. (D) Group one metals increase in
reactivity as you go down the group. (E) The potassium atoms have lost
an electron and formed positive ions.
In this question, we want to know
why potassium reacts more violently with water than lithium does. Let’s clear some space to answer
this question.
Lithium and potassium are group one
metals, and all group one elements have one valence electron, although they have
difference core electron configurations. For example, lithium has two shells
with one valence electron, while cesium, which is near the bottom of the group, has
six shells but also one valence electron. Since their electron configurations
are different and they have a different number of inner or core shells, their
reactivity is also different.
The number of electrons greatly
influences reactivity. Lithium is less reactive, while
cesium further down the group is very reactive. In fact, when a piece of cesium is
added to water, it reacts explosively. There is a definite trend, going
down group one, so the atomic number or number of protons, and thus the number of
electrons in a neutral atom, increases. And thus the reactivity of the
alkali metals increases.
We were asked to identify which
statement best describes the difference in behavior between lithium in water and
potassium in water. Potassium is lower down in group
one, has more inner electrons, and is more reactive. Thus, we can identify that the
correct answer to this question is (D). Group one metals increase in
reactivity as you go down the group.
