Video Transcript
In this video, we will learn how to
describe the formation of ionic bonds in simple binary compounds. We’ll review how ions are formed
and look at some examples of different ionic compounds.
Before we can learn about ionic
compounds, we need to understand ions. And to understand ions, let’s start
with an atom. Atoms are electrically neutral. This is because an atom contains
the same number of positively charged protons and negatively charged electrons.
During a chemical reaction, atoms
can gain or lose electrons. After gaining or losing electrons,
most atoms end up with eight electrons in the outermost energy level. When atoms gain or lose electrons,
the number of protons and the number of electrons will no longer be equal. This species that has an unequal
number of protons and electrons is called an ion.
As ions do not have the same number
of positively charged protons and negatively charged electrons, they have an overall
electric charge. The charge of an ion depends on the
number of electrons the atom gains or loses. Metal atoms tend to lose their
outermost electrons during a chemical reaction. This means that there will be more
positively charged protons than negatively charged electrons in a metal ion.
So, when metal atoms lose
electrons, they form positive ions. For example, a sodium atom is a
metal atom that has 11 electrons. Two electrons are in the K energy
level, eight are in the L energy level, and one is in the M energy level. A sodium atom can lose the single
electron in the M energy level. The species produced is a sodium
ion. The sodium ion has eight electrons
in the outermost energy level and a charge of one plus as one electron was lost.
Nonmetal atoms tend to gain
electrons during a chemical reaction. This means that there will be more
negatively charged electrons than positively charged protons in a nonmetal ion. So, when nonmetal atoms gain
electrons, they form negative ions. For example, a chlorine atom is a
nonmetal atom that has 17 electrons. Two electrons are in the K energy
level, eight are in the L energy level, and seven are in the M energy level. A chlorine atom can gain an
electron in the M energy level. The species produced is a chlorine
ion, more commonly called a chloride ion. This ion has eight electrons in the
outermost energy level and a charge of one minus as one electron was gained.
Atoms gain or lose electrons during
a chemical reaction when electrons are transferred between two or more atoms. To better understand this transfer
of electrons, let’s look at a sodium atom and a chlorine atom in another way.
In these diagrams, the pink dot
represents the nucleus. The blue circles represent the
energy levels. And the blue dots represent the
electrons in each energy level. During a chemical reaction, the
outermost electron of a sodium atom can be transferred to a chlorine atom. In doing so, each species now has
eight electrons in its outer energy level. This transfer of electrons between
a metal atom and a nonmetal atom produces a positive ion and a negative ion. These ions have opposite charges,
and opposite charges attract.
The electrostatic attraction
between positive ions and negative ions is called an ionic bond. Positively and negatively charged
ions that are ionically bonded together form an ionic compound. Even though ionic compounds are
composed of positively and negatively charged ions, the compound must be neutral
overall. In this example, the sodium ion has
a one plus charge and the chloride ion has a one minus charge. These charges are opposite and
equal. So an ionic compound containing one
sodium ion and one chloride ion will be neutral.
We can represent this with the
chemical formula NaCl, indicating that just one sodium ion and one chloride ion are
needed in order to produce a neutral ionic compound. NaCl is the chemical formula of
sodium chloride, also known as table salt.
Let’s take a look at another
example of an ionic compound. For this example, let’s consider
what ionic compound would be formed when magnesium reacts with oxygen. Magnesium is a metal. So we know that it can lose
electrons during a chemical reaction and form a positive ion. Oxygen is a nonmetal. So, during a chemical reaction, it
can gain electrons and form a negative ion.
Let’s take a look at the electronic
configuration of each atom. We can see that an atom of
magnesium can lose two electrons and produce an ion that has eight electrons in the
outermost energy level. As the magnesium atom lost two
electrons, the magnesium ion will have a charge of two plus. The oxygen atom can gain two
electrons and produce an ion that has eight electrons in its outer energy level. As the oxygen atom gained two
electrons, the oxide ion will have a charge of two minus.
So, during the reaction between
magnesium and oxygen, two of the magnesium atom’s electrons will be transferred to
the oxygen atom. This produces two oppositely
charged ions that are electrostatically attracted, forming an ionic bond. As the charges are equal and
opposite, only one magnesium ion and one oxide ion are needed to produce a neutral
ionic compound. We represent this with the chemical
formula MgO. This ionic compound is magnesium
oxide.
So far, we’ve seen two examples
where one positive ion and one negative ion form a neutral ionic compound. But this isn’t always the case. Let’s consider an ionic compound
that contains the elements magnesium and chlorine.
We know that a magnesium atom can
lose two electrons and form an Mg2+ ion. We also know that a chlorine atom
ordinarily only gains one electron and forms a Cl− ion. So, in this reaction, two chlorine
atoms will be needed for each magnesium atom. Each of the chlorine atoms can gain
one of the electrons that the magnesium atom loses. This will produce one magnesium two
plus ion and two chloride minus ions.
The charge of a magnesium ion and a
chloride ion are opposite but not equal. However, the combination of one
magnesium ion and two chloride ions will produce an ionic compound that is neutral
overall. We represent this ionic compound
with the chemical formula MgCl2, which indicates that there are two chloride ions
for every magnesium ion in the compound. This is the ionic compound
magnesium chloride.
We can use electronic
configurations or diagrams like the one shown here to determine the charge of the
metal and nonmetal ions and work out the ionic compound formed. But we can also determine the
charge state of an ion from its periodic table group number. Atoms of metallic elements found in
column one, like the sodium atom shown here, all have one electron in their
outermost energy level.
During a chemical reaction, the
outermost electron can be lost. This forms an ion with a one plus
charge. So metals in group one tend to lose
one electron during a chemical reaction and form metal ions with a one plus
charge. The metals in group two, like the
magnesium atom shown, tend to lose two electrons during a chemical reaction. So atoms of these elements tend to
form ions with a charge of two plus.
Atoms of metallic elements in
column 13 tend to lose three electrons and form ions with a charge of three
plus. And atoms of metallic elements in
column 14 tend to lose four electrons and form ions with a four plus charge. Atoms of nonmetallic elements in
columns 15, 16, and 17 tend to gain electrons and form negative ions. Nonmetallic elements in column 15
tend to gain three electrons and form ions with a charge of three minus. Atoms of nonmetallic elements in
column 16 tend to gain two electrons and form ions with a charge of two minus. And the atoms of elements found in
group 17 tend to gain one electron and form ions with a charge of one minus. The inert gases, also called the
noble gases, found in group 18 do not form ions. And atoms of elements found in the
middle section of the periodic table can form ions that have a variety of charges
that are not easy to predict.
We can use this information to
quickly determine the formula of an ionic compound. For example, lithium is in group
one and fluorine is in group 17. Lithium atoms can form lithium ions
with a charge of one plus. And fluorine atoms can form
fluoride ions with a charge of one minus. The charges of these ions are
opposite and equal. This means that only one lithium
ion and one fluoride ion are needed to make a neutral ionic compound. So the chemical formula is LiF.
Sulfur is in group 16. So sulfur atoms tend to form
sulfide ions with a charge of two minus. In order for a neutral ionic
compound to form between lithium and sulfur, two lithium ions will need to combine
with one sulfide ion. So the chemical formula of lithium
sulfide will be Li2S.
So far, in all of our examples,
we’ve focused on how many positively charged ions and negatively charged ions are
needed to produce a neutral ionic compound. But the structure of ionic
compounds is slightly more complicated. Let’s consider the ionic compound
sodium chloride.
We know that in order to be neutral
overall, one sodium ion and one chloride ion must form an ionic bond. But during a chemical reaction,
there are many, many ions involved, not just one of each type. All of these oppositely charged
ions are attracted to each other. These newly added ions are
attracted to even more ions. Layers of ions build up in all
directions, creating a uniform structure called a lattice. Here we’ve represented the lattice
in two dimensions. But the structure is actually
three-dimensional.
Even though the ionic compound
sodium chloride contains many, many sodium and chloride ions, the chemical formula
NaCl, representing only one ion of each, is still correct. This is because the chemical
formula represents the simplest combination of ions that will produce a neutral
compound.
We’ve mentioned several ionic
compounds in this video. These compounds can be broadly
classified into two categories.
Oxide compounds are compounds that
contain oxygen and another chemical element. Oxide compounds are ionic if the
element bonded to the oxygen is a metal. Sodium oxide, aluminum oxide, and
magnesium oxide are all examples of ionic oxide compounds. All other ionic compounds that do
not contain the oxide ion can be called salts.
Salts have a wide variety of
different properties and solubilities. Sodium chloride, lead iodide, and
copper bromide are all examples of salts. We can see that these salts are
different colors and some dissolve in water while others do not.
We’ve learned a lot about ionic
compounds. But before we wrap up this video,
let’s take a look at a question.
The following diagram is a
representation of the electronic configuration of a metal atom. This metal can react with the
element oxygen to form an oxide. Which of the following diagrams
would be a similar representation of the ion of the same metal formed during the
reaction?
This question describes the
reaction between a metal and the element oxygen. Oxygen is a nonmetal. When a metal reacts with a
nonmetal, they can form an ionic compound. An ionic compound is a neutral
compound composed of positively and negatively charged ions.
During the reaction to produce an
ionic compound, the metal atom loses electrons and forms a positive ion. At the same time, the nonmetal atom
gains electrons and forms a negative ion. The oppositely charged ions are
attracted to one another, forming the ionic compound.
In this question, we’ve been given
the electronic configuration of a metal atom. We need to determine which of the
electronic configuration diagrams below represents the ion of this metal formed
during the reaction with oxygen. We know that the metal will lose
electrons and form a positive ion. The number of electrons the atom
will lose depends on the electronic configuration.
When atoms lose electrons, they
tend to lose outer electrons and produce a species that has eight electrons in the
outermost energy level. In this atom, there are two
electrons in the outermost energy level. If these electrons are lost, then
the second energy level becomes the outermost energy level. And this energy level has eight
electrons. So the diagram of the metal ion
should have two energy levels, with the energy level closest to the nucleus
containing two electrons and the outermost energy level containing eight
electrons.
Therefore, the diagram that is a
representation of the ion of the given metal that is formed during the reaction with
oxygen is the diagram shown in answer choice (A).
Now let’s summarize what we’ve
learned with the key points. Ions are a chemical species with an
unequal number of protons and electrons. During a chemical reaction, metal
atoms lose electrons and form positive ions, while nonmetal atoms gain electrons and
form negative ions. An ionic bond is an electrostatic
attraction between oppositely charged ions. An ionic compound is a neutral
compound composed of positively and negatively charged ions that are ionically
bonded together.
The charge state of the ions in an
ionic compound depends on how many electrons are lost or gained by the atoms. The charge state can be quickly
determined by looking at the group numbers on the periodic table. Ionic compounds have a giant
lattice structure that consists of many, many positively and negatively charged
ions. Ionic compounds that contain the
element oxygen are called oxides, while the remaining compounds are called
salts.
