Video Transcript
In which period of the periodic
table is the fourth electron shell of an atom the valence shell?
Let’s begin by looking at the
periodic table of elements. The horizontal rows on the periodic
table are known as periods. Periods are collections of elements
with the same highest occupied electron shell, or valence shell. Every atom consists of a nucleus
surrounded by a cloud of electrons. In a simple model, we can describe
electrons as occupying electron shells at various distances from the nucleus. These distances represent different
energy levels.
Now let’s look at a few examples to
help us answer the question. An atom of hydrogen has only one
electron, and we usually find that electron in the first electron shell. So, hydrogen is found in the first
period of the periodic table. Helium is also in period one
because atoms of helium only have two electrons, which sit in the first electron
shell. Lithium, however, is in period
two. Atoms of lithium have three
electrons, and there’s only space in the first shell for two electrons. So, the third electron goes in the
second electron shell. We see something similar when we
get to sodium. The highest occupied electron shell
for sodium atoms is the third electron shell.
This pattern continues all the way
down the periodic table. So, we can use the periodic table
and electron shell diagrams to convert the period number to the shell number of the
highest occupied electron shell. With this in mind, atoms in which
the fourth electron shell is the valence shell will be found in period four.
In which period of the periodic
table is the fourth electron shell of an atom the valence shell? The answer is period four.
