Video Transcript
What do elements in the same group
of the periodic table contain an equal number of?
A group is a collection of elements
with similar chemical or electronic properties. The periodic table is organized in
such a way that elements with similar chemical or electronic properties are found in
the same column. Thus, each column of the periodic
table represents a group. In order to determine what elements
in the same group contain an equal number of, let’s examine three elements which are
all found in group one.
The number written above the
element symbol in each cell on the periodic table is the atomic number. The atomic number represents the
number of protons in the nucleus of all atoms or ions of that particular element and
also indicates the number of electrons an atom of that element contains. We can see that hydrogen, lithium,
and sodium all have a different atomic number. So elements in the same group do
not have the same atomic number and therefore do not contain an equal number of
protons or an equal number of electrons. The value which appears below the
element symbol in each cell of the periodic table is the relative atomic mass. The relative atomic mass is the
weighted average of the atomic masses of the isotopes of an element. We can see that hydrogen, lithium,
and sodium do not have the same relative atomic mass.
We’ve now compared all of the
information about hydrogen, lithium, and sodium that we can gather directly from the
periodic table. So now let’s take a closer look at
how the electrons in an atom of each element are arranged using the Bohr model of
the atom. In the Bohr model, each circle
around the nucleus represents an electron shell. Each electron shell can hold a
certain number of electrons. The first electron shell can hold a
maximum of two electrons. The second electron shell can hold
a maximum of eight electrons. And the third electron shell can
hold a maximum of 18 electrons. However, 10 of those electrons are
only added to this electron shell when the atoms get bigger. So we sometimes say that this
electron shell can only hold a maximum of eight electrons.
In the diagram, one blue dot
represents an electron, so we can complete a Bohr model of an atom by drawing blue
dots on each circle to represent the electrons in each electron shell. With this in mind, we can now
construct the Bohr model of a hydrogen atom. An atom of hydrogen only has one
electron, so we draw a single blue dot on the first electron shell. An atom of lithium has three
electrons. We start by drawing two dots on the
first electron shell, which is the maximum number of electrons this shell can
hold. We can then draw lithium’s third
electron on the second electron shell. An atom of sodium has 11
electrons. We can draw the first two electrons
on the first electron shell and the next eight electrons on the second electron
shell, which is the maximum number of electrons the second shell can hold.
To complete the Bohr model of an
atom of sodium, we need to a draw the 11th electron in the third electron shell. We can see from the Bohr models
that an atom of hydrogen, lithium, and sodium do not have the same number of
electron shells. But all three atoms only have one
outermost electron. We call the electrons in the
outermost electron shell the valence electrons. So an atom of hydrogen, lithium,
and sodium, all elements found in group one of the periodic table, contain the same
number of valence electrons. So we can conclude that elements in
the same group of the periodic table contain an equal number of valence
electrons.
