Video Transcript
In this video, we will learn about
ions. We will learn how to describe ions
and write their symbols. We will also learn how to determine
whether an atom will form a positive or negative ion.
Before we discuss what an ion is,
we first need to know that ions form from atoms. Let’s remind ourselves what atoms
are. An atom is the smallest unit of
matter that forms an element, and it contains different types of subatomic
particles. Examples of different types of
atoms are oxygen atoms, lithium atoms, hydrogen atoms, iron atoms, carbon atoms, and
so on.
Atoms have positively charged
protons and neutral neutrons in the nucleus. Negatively charged electrons are
found outside the nucleus. These three types of particles
which make up atoms are called subatomic particles. In an atom, the number of protons
in the nucleus is equal to the number of electrons outside the nucleus. Therefore, an atom has no overall
charge. Atoms are neutral overall.
Now, each type of atom, or we could
say atoms of different elements, have a different number of protons in their
nucleus. For example, oxygen has eight
protons in its nucleus, and lithium has three protons. Since atoms are neutral and since
there are eight protons in an oxygen atom, we can deduce that there are also eight
electrons present outside the nucleus. In the same way, since an atom of
lithium contains three protons, we can deduce that it must have three electrons. The net effect is that the positive
charges and negative charges cancel out, causing the atoms to have a neutral state
overall. However, atoms can lose or gain
electrons during chemical reactions, and this upsets the charge balance.
Here is an example. The lithium atom, we said, has
three protons and three electrons. When a lithium atom reacts with
another atom, it tends to lose one of its electrons. This results in a particle which
still contains three protons but now has only two electrons. The particle of lithium has an
overall positive charge. We call this a lithium ion. An ion is a particle with an
unequal number of protons and electrons. The lithium ion is a positively
charged ion. All positively charged ions have a
higher number of protons than electrons.
Now, here is an example of an atom
which can form a negatively charged ion when it reacts. We said that an atom of oxygen has
eight protons and eight electrons. When an atom of oxygen reacts with
another atom, it tends to gain two electrons. So, an oxygen ion, which happens to
be called the oxide ion, has eight protons but 10 electrons, giving it an overall
negative charge. All negative ions have a higher
number of electrons than protons.
Now that we know what an ion is and
how positive and negative ions form, let’s learn how to write the symbols for
ions.
Scientists represent ions by
writing the chemical symbol of an element with a charge sign next to it written as a
superscript. The symbol for the lithium ion is
Li superscript one plus to show that it has one more proton than electrons. However, we need not show the
number one. A superscript plus sign by itself
indicates that the charge is positive one. The oxygen, or oxide ion, is
written as O superscript two minus. We said that an oxide ion forms
when an oxygen atom gains two electrons. Since the oxide ion has two extra
electrons, we need to indicate that there are two extra negative charges by writing
the number two in front of the negative sign. Now we know how to write the
symbols for positive and negative ions.
We also saw from these two examples
that positive ions form when an electron or electrons are lost during a chemical
reaction and that negative ions are formed when an electron or electrons are gained
during a chemical reaction. You may have noticed that this
positive ion is a metal ion. And perhaps you noticed that this
negative ion is a nonmetal ion. In fact, these are just two
examples of a general rule.
In general, metal atoms tend to
form positive metal ions. And usually nonmetal atoms tend to
form negative nonmetal ions. We have looked at lithium and
oxygen as examples. Let’s now look at some other metal
and nonmetal atoms and how their ions form.
An atom of the metal magnesium has
12 protons in its nucleus and 12 electrons arranged into energy levels. It also has neutrons in its
nucleus, but we have not shown these as neutrons do not influence ion charge. During a chemical reaction, a
magnesium atom tends to lose the two electrons in its outer electron shell. A magnesium ion, symbol Mg2+,
forms, with 12 protons in its nucleus but only 10 electrons. Overall, it has a plus two
charge. The magnesium ion is very
stable. In fact, it is more stable than the
magnesium atom, since it has the same electronic configuration as a noble gas
atom. This electronic configuration of
2,8 is the same as that of neon. In general, metal atoms tend to
have one, two, or three outer electrons, and they tend to lose these electrons
during a reaction and end up with eight outer-shell electrons.
Now, here is an example of a
nonmetal atom forming an ion. This time we have chlorine. A chlorine atom has 17 protons and
17 electrons. Its electronic configuration is
2,8,7. Now, nonmetals tend to gain
electrons and form negative ions. But how many electrons will a
chlorine atom gain when it reacts? Well, we know that atoms tend to
gain or lose electrons and get the same electronic configuration as the closest
noble gas and become stable. And we know that noble gases,
except for helium, have eight electrons in their outer shell.
We can deduce that the chlorine
atom will gain just one electron and end up with eight electrons in its outer
shell. This electronic configuration of
2,8,8 is the same as that of the noble gas argon. The ion which forms has the symbol
Cl−, and it has 17 protons and 18 electrons. In general, nonmetals tend to have
five, six, or seven outer-shell electrons, and they tend to gain electrons and end
up with eight outer-shell electrons.
Now, we are going to remove some
information on this screen and see if we can work out missing information using only
the electronic configurations.
We can easily fill in the missing
information about atoms and ions if we examine their electronic configurations. We can start by identifying the
atoms. We know atoms have the same number
of electrons and protons. And we know that the number of
protons, or atomic number, indicates the identity of the element.
In the top example, there are 12
electrons, so there are 12 protons. The element with an atomic number
of 12 is magnesium. We can see that the electronic
configuration changes from 2,8,2 in a neutral atom to 2,8. We can see that the two outer
electrons have been lost. So, we could write “loses two
electrons” above the arrow. Also, we know that it is metal
atoms which tend to lose electrons. So we can confirm the atom as being
a metal atom. Finally, since the neutral atom has
lost two electrons, it must form a positive magnesium ion with a two plus
charge.
Now, let’s look at the bottom
example. There are 17 electrons in the
atom. So there must be 17 protons. The element with an atomic number
of 17 is chlorine. Since the electronic configuration
changes from 2,8,7 in the neutral atom to 2,8,8, we know that one electron has been
gained. And this electron went into the
outer shell. So, we could write “gains one
electron” above the arrow. It is nonmetal atoms which tend to
gain electrons, so we can confirm the atom as being a nonmetal atom. Finally, since the neutral chlorine
atom has gained an electron, it must form a negative chloride ion with a one minus
charge.
It is now time to practice what we
have learnt.
Typically, how might an element
with three electrons in its outermost energy level behave in a reaction?
We are asked about an element which
has three electrons in its outermost energy level. We need to describe how such an
element would behave in a chemical reaction. Let’s start by drawing a diagram to
show the electron placement or electronic configuration of an atom of this
element. We know atoms have a nucleus with
protons and neutrons. In this case, we do not know how
many of each subatomic particle there are in the nucleus, nor do we know how many
electrons an atom of this element has or how many electron shells are filled. So, for simplicity, let us draw
just three energy levels.
We do, however, know that an atom
has an equal number of protons and electrons, making it neutral. And we know that there are three
electrons in its outer shell. We know that all inner shells are
full. The full electronic configuration
of this atom is shown here. But remember, there are other
possible electronic configurations if we add more inner shells. Again, we do not know how many
inner shells there are in this atom. We have just chosen to work with
two inner shells.
Now, metal atoms tend to have one,
two, or three electrons in their outer shell, and nonmetal atoms tend to have five,
six, or seven electrons in their outer shell. Atoms that have less than eight
electrons in their outer shell are unstable. Helium, however, is stable even
though it has less than eight electrons, since its outer shell is full with two
electrons. We can deduce that the element in
question must be a metal since we are told it has three outer electrons. Except for the noble gas atoms,
which tend to be unreactive, atoms tend to lose or gain electrons in a chemical
reaction and usually end up with eight electrons in their outer shell. When there are eight electrons in
the outer shell, an atom is stable, like the noble gases.
In this question, we have a metal
atom. Metal atoms tend to lose all their
outer electrons in chemical reactions and end up with the electronic configuration
of a noble gas. Nonmetal atoms do the opposite;
they gain electrons. So, we know this metal atom will
lose outer electrons. But how many electrons will an atom
of this element lose? We said they will lose all their
outer electrons. So, it will lose all three of its
outer electrons.
Now, the number of protons is no
longer equal to the number of electrons, but there are three more protons than
electrons. A positively charged particle will
form, with a plus three charge. We call this particle an ion. An ion is a particle with an
unequal number of protons and electrons. So, how does an element with three
electrons in its outer energy level behave? The answer is it loses its three
electrons and changes to a positive ion.
Now, let’s review the key points of
this video. We learnt that an ion is a particle
with an unequal number of protons and electrons. Positively charged ions have more
protons than electrons. Negatively charged ions have less
protons than electrons. Metal atoms tend to lose their
outer electrons in a chemical reaction and form positive ions. And nonmetal atoms tend to gain
electrons during a chemical reaction and form negative ions.
