Video Transcript
In this video, we will learn how to
write electronic configurations for atoms or ions and identify an atom from its
electronic configuration.
As we will learn how to write
electronic configurations in this video, we should first define electronic
configuration. The electronic configuration is the
distribution of electrons in energy levels, where energy levels are defined as
distances from the nucleus that describe the location of electrons. The four lowest energy levels of an
atom are called K, L, M, and N. The electrons of an atom tend to
occupy all possible states in the lowest energy levels before they start to fill
higher levels. But each energy level holds a
different number of electrons. The first level can hold a maximum
of two electrons. The second can hold a maximum of
eight electrons. The third can hold a maximum of 18
electrons. And the fourth can hold a maximum
of 32 electrons.
Let’s take a look at the electronic
configuration of nitrogen. The chemical notation for nitrogen
tells us that nitrogen has an atomic number of seven. Thus, a neutral atom of nitrogen
will have seven electrons. We can show which energy levels
these electrons will fill using an energy level diagram. The electrons of an atom tend to
occupy all possible states in the lowest energy levels before they start to fill
higher levels. So the electrons will fill energy
level K first. And as K holds a maximum of two
electrons, nitrogen has two electrons in the energy level K. As seven minus two is five, there
are five electrons left after filling energy level K. The remaining five electrons fill
the second lowest energy level, L.
We have now drawn an energy level
diagram. The diagram tells us that the
electronic configuration of a neutral nitrogen atom is 2,5. Electronic configurations can also
be written in a list form. So the electronic configuration for
nitrogen can be written as two comma five. The occupied energy levels for an
atom of nitrogen are K and L.
Let’s now look at the electronic
configuration for sodium. The chemical notation for sodium
tells us that it has an atomic number of 11. Thus, a neutral atom will have 11
electrons. The first two fill energy level
K. As 11 minus two is nine, we are
left with nine electrons. But energy level L can contain a
maximum of eight electrons. So even with L full, there is one
electron remaining. This remaining electron goes in
energy level M. Therefore, sodium has an electronic
configuration of 2,8,1.
Not only can we calculate the
electronic configuration from knowing the identify of an atom, and its atomic
number, but we can also do the reverse and identify an atom from its electronic
configuration. Let’s look at an example where the
electronic configuration of an atom is 2,8,3.
First of all, we need to add the
numbers in the electronic configuration together. This gives a total of 13. Assuming the electronic
configuration is for a neutral atom, as the number of electrons present is 13, the
atom will have an atomic number of 13. The atom with atomic number 13 is
aluminum, which has the chemical symbol Al. So the identity of the element with
electronic configuration 2,8,3 is aluminum.
Now that we’ve looked at how to
calculate the electronic configuration for atoms, let’s quickly look at how to
calculate electronic configuration for ions.
An ion is a chemical species with
an unequal number of protons and electrons. To help us further understand what
an ion is, let’s look at an example of the electronic configuration of an atom and
see how that atom can be converted to an ion. An atom of lithium has atomic
number three. This means that a neutral atom of
lithium has three protons and three electrons. Two of these electrons will fill
energy level K. The remaining one electron goes in
energy level L. So an atom of a lithium has the
electronic configuration 2,1. Lithium tends to form a one plus
ion, which means the ion forms after the atom loses one electron. So a lithium ion has three protons
but two electrons. This matches the definition of an
ion, which states that an ion has an unequal number of protons and electrons.
The electron will be lost from the
outer energy level. As there is only one electron in
the outer energy level of the atom, there will be no electrons in this energy level
for the ion. Thus, the electronic configuration
for the lithium ion is just two.
Now that we have looked at how to
write electronic configurations for atoms and ions, let’s look at the impact that
electronic configuration has on reactivity. Elements tend to be chemically
active if they have fewer than eight electrons in their outer energy level. This is the case for oxygen and
fluorine, for example. They have six and seven electrons
in their outer energy level, respectively. Oxygen and fluorine react with many
other substances because they do not have eight outer-level electrons. They are chemically active. There are also similarities between
the way that fluorine reacts and the way that chlorine reacts.
The chemical notation for chlorine
tells us that it has an atomic number of 17. So the first two electrons fill
energy level K. As 17 minus two is 15, there are 15
electrons remaining. Eight of these 15 electrons fill
energy level L, as energy level L holds a maximum of eight electrons. As 15 minus eight is seven, there
are seven electrons remaining. These seven electrons fill energy
level M. So although fluorine atoms contain
nine electrons and chlorine atoms contain 17 electrons, both fluorine and chlorine
have seven outer shell electrons. This is the key reason as to why
fluorine and chlorine are quite reactive and react in similar ways.
However, most noble gas atoms are
essentially inert because they have eight electrons in their outer energy level. An example of this is neon. Neon is a noble gas with eight
electrons in its outer energy level. Neon gas is chemically stable and
cannot react with other substances unless the reaction conditions are incredibly
unusual. The case is similar for argon,
another noble gas. Each neutral atom has 18
electrons. 10 of them fill the lowest energy
levels K and L. The remaining eight electrons are
found in energy level M. Argon atoms are chemically stable
and essentially inert. They don’t tend to react with any
other substances because they have eight outer shell electrons.
Helium, however, is a noble gas but
only has two outer shell electrons. Despite this, helium atoms are
chemically inert. Although helium does not have eight
electrons in its outer energy level, it does have a full outer energy level. Helium atoms are light, small, and
incredibly stable. They do not usually react with any
other substances.
Now that we have learnt how to
write electronic configurations for atoms and learnt how this affects their
reactivity, let’s look at some example questions.
In total, how many electrons are
needed for both the K and L energy levels to be full?
Energy levels are defined as
distances from the nucleus that describe the location of electrons. The first four energy levels are
called K, L, M, and N. Each energy level holds a different
maximum number of electrons. The maximum number of electrons for
energy level K is two, L is eight, M is 18, and N is 32.
The question asks how many
electrons are needed for both the K and L energy levels to be full. As two electrons are needed for K
to be full and eight electrons are needed for L to be full, then two plus eight
electrons are needed for K and L to be full. Two electrons plus eight electrons
equals 10 electrons. Thus, the answer to the question
“how many electrons are needed for both the K and L energy levels to be full?” is 10
electrons.
An atom contains five
electrons. How many of these electrons occupy
the L energy level?
Energy levels are distances from
the nucleus that describe the location of electrons. The first energy level is called K,
the second energy level is L, and the third and fourth are M and N,
respectively. Energy level K can hold a maximum
of two electrons, L holds a maximum of eight, and M and N hold a maximum of 18 and
32, respectively. Electrons fill from the lowest
energy level first, which is K.
The question tells us that the atom
contains five electrons. Two of these electrons fill energy
level K. The remaining three electrons fill
the second lowest energy level L. Thus, the answer to the question
“How many of these electrons occupy the L energy level?” is three electrons.
An atom of oxygen has eight protons
in its nucleus. Which energy level diagram shows
the correct electronic configuration for oxygen?
We’ve been given five energy level
diagrams labeled from (A) to (E). We need to identify which diagram
shows the correct electronic configuration for oxygen. And we are told in the question
that oxygen has eight protons. So a neutral atom of oxygen will
also have eight electrons. We can immediately eliminate answer
choice (E) as the diagram shows that oxygen has 10 electrons, rather than eight.
The other answer choices have the
correct number of electrons, but the electrons aren’t necessarily in the correct
energy levels. Each energy level has a different
maximum number of electrons. K is two, L is eight, M is 18, and
N is 32. The maximum number of electrons
found in energy level K is two. Thus, answer choices (C) and (D)
must be incorrect. Additionally, electrons fill the
lowest energy level first, which is K. So two of oxygen’s electrons will
be found in energy level K. And the remaining six will be found
in the second lowest energy level L. As answer choice (A) has an
electronic configuration of 2,6, it is the correct answer. The energy level diagram which
shows the correct electronic configuration for oxygen is answer choice (A).
Which of the following energy level
diagrams corresponds to the least reactive atom?
We’ve been given five answer
choices labeled from (A) to (E). Energy levels are distances from
the nucleus that describe the location of electrons. The first four energy levels are
called K, L, M, and N, where K is the closest energy level to the nucleus. Electrons fill this energy level
first. They then fill energy level L.
All of the answer choices have two
electrons in K and eight in L. The difference lies in the number
of electrons in energy level M, their outer energy level. Elements tend to be chemically
active if they have fewer than eight electrons in their outer energy level. So the atoms in (A), (B), (D), and
(E) are likely to be chemically active. However, elements tend to be
chemically stable and inert if they have eight electrons in their outer energy
level. Thus, it’s likely that atom (C) is
inert. Therefore, the energy level diagram
which corresponds to the least reactive atom is answer choice (C).
Let’s summarize the key points of
this video. Electrons tend to fill energy
levels in order of increasing energy state so would fill energy level K first, then
L, then M, and so on. The electronic configuration is the
distribution of electrons in energy levels. Atoms are ordinarily active if they
do not have eight outer-level electrons. Most noble gas atoms are inert
because they have eight outer-level electrons.
