Chapter
7
Fatal
Attraction: Compounds & Bonding
MIXTURES & COMPOUNDS
Mixtures and
compounds both consist of more than one type of atom (element) but they are
different types of substances.
Mixtures consist of
combinations of pure substances that remain separate from each other, but share
the same physical space. Mixtures do not have their own unique characteristic
properties. Instead, the substances
that make up mixtures retain their own properties. Those pure substances simply share the same physical space,
and do not react or combine with each other.
This differs from a compound. As you know, the substances in a compound cannot be separated
from each other by physical means. They
can by chemical means however. Separating
by chemical means changes the identity of the substance.
For example, separating water into hydrogen and oxygen requires
electrolysis. Other ways to
separate a compound into its elements include chemical reactions.
Compounds also contain the same proportion of elements. Water
is always two hydrogen atoms bound to one oxygen atom.
Mixtures do not always contain the same proportion of the substances that
make them up.
COMPOUNDS, MOLECULES, &
CHEMICAL BONDS
Elements are pure substances that are
made up of only one type of element. For
instance, if you hold a hunk of carbon in your hand, that hunk consists only of
carbon atoms. If you have a balloon
full of helium, only helium atoms are moving around inside the space.
Compounds are also pure substances, but they are made of more than one
kind of atom. Compounds consist of
different types elements that are chemically combined.
They cannot be separated into their elements by physical means, but they
can be via a chemical reaction. Because
they are chemically combined, compounds are pure substances that retain their
own characteristic properties. The
properties of a compound are distinct from the properties of the individual
elements that make them up.
Earlier, we used water as an example.
Water is a compound that consists of the elements hydrogen and oxygen.
By itself, the element hydrogen is a very light gas that is extremely
flammable. By itself, the element
oxygen is a flammable gas that easily reacts with other substances.
Those elements combine to form a pure substance that is a liquid (water)
at room temperature, has a density of 1 g / mL, and has the ability to dissolve
many other substances. That liquid
also constitutes 75% of living things. The
characteristic properties of water differ dramatically from the characteristic
properties of the elements (hydrogen & oxygen) that make up the compound.
Another example details the compound
sodium chloride. Sodium chloride
consists of the elements sodium and chlorine.
By itself, chlorine is a poisonous, green colored gas that kills living
things. By itself, sodium is a soft
metal solid that is explosive in water. Together
however, sodium and chlorine form a white, crystal-shaped solid compound that
dissolves in water, and is tasty on your food.
Another word for sodium chloride is table salt!
Examples of other compounds include
glucose (simple sugar), which consists of the elements carbon, hydrogen, and
oxygen; hydrochloric acid (hydrogen and chlorine); carbon dioxide (carbon and
oxygen); and potassium iodide (potassium and iodine).
Table 7.1: Common
Compounds
The smallest unit of an element is an
atom. The smallest unit of most
compounds is called a molecule. A
molecule is the smallest particle of a compound that retains the characteristic
properties of the substance. A
water molecule consists of two hydrogen atoms connected to a central oxygen atom
in a shape that resembles a silhouette of Mickey Mouse – the mouse ears
representing the hydrogen atoms. In
a molecule, the individual atoms and elements are held together by a chemical
bond.
There are different types of chemical
bonds. Chemical bonds are the
forces that hold atoms together in compounds and form according to specific
rules that are determined by the structure of the atom.
Those rules are based on the number of electrons that are found in the
outer energy level of an atom. It
is only these electrons that are able to form chemical bonds –those found in
the outer energy level, that are involved with chemical bonds.
Because of that, those electrons are called valence electrons. Before
detailing chemical bonds however, you need to understand what is meant by the
stability of the atom. In addition,
you must understand the concept of an ion.
ATOMIC
STABILITY
Remember electron
configuration diagrams and electron dot diagrams?
They are useful to help understand atomic stability and ions.
In addition, it is useful to introduce yourself to a magic number of
chemistry – the number eight. The
specific information that electron configurations and electron dot diagrams
provide is the number of electrons found in the outer energy level.
That is the key to understanding atomic stability – or atomic
happiness. An atom is stable when
it is “happy” with its current state. In
other words, it has no need to change from its current state.
Atoms are stable – happy, when their outer energy levels are filled
with the maximum number of electrons. In
other words, the outer most energy level is full.
Some elements are
stable – happy, as individual atoms. Other
atoms attain stability – a form of atomic nirvana, as they join with other
elements to form compounds. The key
to atomic nirvana is having the desired number of electrons in the atom’s
outer energy level. With the
exception of hydrogen helium, Lithium, and Beryllium (atomic numbers 1 – 4)
that desired number is the magic number – 8.
In other words, most atoms are ‘happy’ when they have 8 electrons in
their outer most energy level.
A group of elements that
have attained “atomic nirvana” are the noble gasses.
The noble gasses are found at the far right column on the periodic table,
and include the elements helium, neon, argon, krypton, xenon, and radon.
They are very stable elements that tend not to form chemical bonds with
other elements. The reason they do
not react well with other elements is that they are “happy” as single atoms
since they all exist with their outer energy levels full of electrons.
With the exception of helium, all noble gasses have 8 electrons in their
outer energy level. Helium has 2 electrons in its only energy level, which is the
maximum number for the 1st energy level.
Figure 7.1: Electron Dot Diagrams of the Noble Gasses
I need to interrupt for a
brief moment before proceeding. In
reality, atoms have no conscience thought and thus cannot be “happy” in the
same way that you or I can be happy. The
terms “happy” and “atomic nirvana” are simply ways of expressing atomic
stability. Don’t worry about
hurting an atom’s feelings!
All atoms aspire to be stable
– or happy. In order to be stable
– “happy”, they need to fill their outer most energy level with electrons. Electron configurations and electron dot diagrams can tell
how many electrons are in the outer most levels.
If the outer energy level is not full, as in the case of most elements,
atoms can gain electrons, lose electrons, or share electrons with other atoms.
That is the key to understanding how elements form chemical bonds with
other elements in order to form compounds.
We are first going to look at how atoms gain or lose electrons to form
ions.
IONS & CHARGES
As we learned earlier, atoms
of elements that have different number of neutrons are called isotopes. We also earlier learned that the atomic number of an element
is the same as the number of protons for that element and is the same as the
number of electrons for that element. You
may also recall how we would modify that rule later.
By introducing the concept of ions, later is now!
In reality, atoms can have a number of electrons that is greater than or
less than the element’s atomic number. Atoms
of the same elements that have different number of electrons are called ions.
Ions of elements form so that
its atoms become “happy”. Remember,
an atom’s ‘happiness’ or stability depends upon filling the atom’s outer
energy levels with electrons. One
way to fill the outer energy level is for the atom to gain the necessary number
of electrons so it equals 8 and is full. Another
way to fill the outer energy level is for the atom to loss the necessary number
of electrons so it sheds an entire energy level revealing the energy level that
is already completely filled. Since
all inner energy levels must be filled before electrons are placed in outer
energy levels, the ‘revealed’ energy level already has 8 electrons and is
filled. Thus, the atom is
“happy”.
Ions of elements form as an
atom gains or loses electrons. Because
it gains or loses an electron, it becomes a charged particle.
Technically, science textbooks define the word ‘ion’ as a charged
particle. To be able to measure the charge of an atom, you must remember the
relationship between an atom’s protons and electrons. Protons have a positive (+) charge and electrons have a
negative (-) charge. When an atom
has an equal number of protons as electrons, the charge of the entire atom is
neutral. The total charge of the
protons, counteract the total charge of the protons.
Even though protons and electrons differ in mass, they have equal
strength with regards to their charges. When
an atom has more electrons than protons, the atom has a negative charge.
Atoms have more electrons than protons when they gain electrons.
Thus, ions that form by gaining electrons have a negative charge. The reverse is true when atoms lose electrons.
When an atom loses electrons, it has more protons than neutrons and thus
a positive charge.
As far as atoms are
considered, it doesn’t matter whether or not they are charged.
What does matter is whether or not the outer energy levels are full with
electrons. Atoms gaining or losing
electrons are means atoms use to fill their outer levels in order to become
“happy”. For ions, a
consequence for becoming “happy” is that they become charged.
Charges are measured.
A charge can be measured by calculating the difference between the number
of protons and electrons. For
example, chlorine (atomic number 17) gains an electron in order to fill its
outer energy level. It now has 18
electrons and 17 protons. Because
it has one more negatively charged particle than positively charged particles,
its overall charge is –1. Oxygen
(atomic number 8) gains two electrons in order to fill its outer energy level.
It now has 10 electrons and 8 protons for an overall charge of –2.
The element sodium tends to
lose an electron. Sodium’s atomic
number is 11. It has 11 protons but
it has 10 electrons as an ion. It
loses an electron in order to shed its outer most energy level and reveal a new
outer most level that is full with electrons.
Because sodium has one more proton than electron, its overall charge is
+1. Because of this charge, sodium
is an element that is very reactive. By
itself, it is unstable and unhappy. By
losing its electrons, it becomes “happy” but is reactive with elements
around it. That’s why, as you
know well from 7th grade science, sodium explodes when placed in
water!
This brings us to another
similarity that is characteristic of groups of elements on the periodic table.
All elements in the same column – group, have the same number of
electrons in their outer level, and all have the same charges when they form
ions. Sodium is in the same group
as potassium. Both explode in
water, and both forms ions whose charge is +1.
The elements fluorine, chlorine, bromine, and iodine are all in the same
group. Those elements all have 7
electrons in the outer level, and form ions that have a charge of –1.
Elements that easily form ions easily bond with other elements and are
frequently found in compounds. Because
they are ions and have a charge, the type of chemical bonds that are found in
these compounds are called ionic bonds.
IONIC BONDS
A
chemical bond is the force that holds elements in a compound together.
An ionic bond is a type of chemical bond.
In ionic bonds, the force that holds the atoms together results from the
charges of those atoms found in the compound.
Remember the phrase “opposite charges attract, like charges repel”?
That phrase will help you understand how ionic bonds form and create some
compounds. Compounds that are held
together by ionic bonds are called ionic compounds.
Ionic bonds typically form between metals and non-metals.
Elements from group #1 and #2 (left hand columns of the periodic table)
tend to form ionic bonds with elements from groups #7 (right hand side of the
table. Ionic compounds are often
crystal-solids with high melting points.
Let’s
use the example of sodium and chlorine. Sodium
has 1 electron in its outer energy level. To
become “happy” it loses an electron to become an ion whose charge is +1.
It is “happy” but charged. Chlorine has 7 electrons in its outer shell. To become “happy” it gains an electron to become an ion
whose charge is –1. This sounds
fairly convenient. Since
“opposite charges attract”, sodium with a charge of +1 is attracted to the
chlorine whose charge is –1, as an ionic bond between the two atoms forms. The resulting compound has a neutral charge.
It is called sodium chloride. You
probably put that compound on your food, as the common name for sodium chloride
is table salt.
The elements potassium and iodine make
similar bonds. Potassium with a
charge of +1 forms an ionic bond with iodine, which has a charge of –1.
The two atoms combine together to form the neutrally charged compound,
potassium iodide.
In both cases, it is possible to transfer
one electron from the atom that loses the electron to the atom that gains the
electron. It’s as if the
potassium or sodium atoms have an advertisement that reads: “Available, one
electron.” The iodine or chlorine
atoms have a message that reads: “Wanted, one electron.” They “get
together” or “hook-up” in a compound!
“Getting together” in this way forms an ionic bond.
The resulting ionic compound has a neutral charge because the positive
and negative charges of the ions cancel each other out.
Potassium iodide and sodium chloride are examples of ionic compounds.
Other examples of ionic compounds include magnesium chloride, sodium
fluoride, calcium chloride, and potassium bromide.
Now let’s find out how ionic bonds differ from another type of chemical
bond – covalent bonds.
COVALENT BONDS
One element transferring electrons to
another element to form a bond is a technical definition of “ionic bond.”
It is important to distinguish the “transfer” of electrons from the
“sharing” of electrons. Some
elements do not gain or lose electrons to become “happy”, they share
electrons instead. Sharing electrons is another way that atoms bond with each
other and form compounds. Chemical
bonds that form by sharing electrons are called covalent bonds. Covalent bonds tend to form between atoms that neither gain
nor lose electrons easily. By
sharing electrons, each atom fills up the outer most energy level in order to
become “happy”.
Covalent bonds form between atoms of
different elements to create covalent compounds.
Water is an example of a covalent compound. Oxygen has 6 electrons in its outer shell, and hydrogen has
only 1 electron. Two hydrogen atoms
bind with the oxygen –the oxygen shares one of its electrons with one of the
hydrogen atoms, and another electron with the other hydrogen.
Because there are two hydrogen atoms and one oxygen atom in a water
molecule, in chemical notation the formula for water is expressed as H2O.
Other examples of covalent compounds include hydrochloric acid (HCl),
methane (CH4), ammonia (NH3), carbon dioxide (CO2),
and glucose (C6H12O6).
Covalent bonds often take place between
atoms of the same element. For
example, hydrogen has one electron in its outer shell.
If it shares that electron with another hydrogen atom that has one
electron in its lone energy level, the number of electrons in each atom is two
– full for the 1st energy level.
Hydrogen rarely exists as a single atom.
Usually, there are two hydrogen atoms bound together.
In chemical notation, this is expressed as H2.
The same could be said for nitrogen (N2), chlorine (Cl2)
and oxygen (O2), since both those elements form covalent bonds with
other like atoms. Because hydrogen,
oxygen, chlorine, and nitrogen particles can consist of more than one atom bound
together, they are called diatomic elements, and form molecules.
But since they are not different types of elements, they do not form
compounds. Covalent bonds are very
useful in forming compounds however.
In covalent compounds, single double, or
triple bonds can form. Imagine a
covalent bond to be a rope in a tug of war and the participants on each side to
be the atom. The knot (or hanky) in
the center of the rope represents the shared electrons.
One rope signifies a single bond, two ropes a double bond, and three
ropes a triple bond.
CHEMICAL FORMULAE & NAMING
COMPOUNDS
Scientists use a shorthand known as
chemical notation (formula) instead of writing out the full name of compounds.
You will also become proficient using chemical formulae to denote
compounds. To do so, you need to
rely upon the chemical symbols of the elements.
A chemical formula tells what elements a compound contains, and the exact
number of atoms of each element that are found in a molecule or unit of that
compound. In chemical notation,
water is expressed as H2O. That
means that there are a total of three atoms in a single water molecule.
There are two hydrogen atoms and one oxygen atom.
The ‘2’ means that there are two hydrogen atoms present.
Sodium chloride is expressed as NaCl, meaning that there is one sodium atom and
one chlorine atom in each unit.
To write the chemical formula of a
compound, you need to know the elements that make up the compound and the number
of atoms for each element that are found in the smallest unit of the compound.
To acquire this information, you will need to “look it up”.
Sometimes you will need to make your best inference, instead.
Once you have that information you will use chemical symbols and
subscripts.
For example, lithium and nitrogen combine
to form a compound. There are 3
atoms of lithium for every one atom of nitrogen.
The chemical formula for the compound is Li3N.
This compound is called lithium chloride.
As a general rule, the element that typically forms positive charged ions
is listed first. The second element
uses the ending “-ide” to indicate that it is a compound.
Examples of this rule in writing the formula and naming compounds include
carbon dioxide (CO2), magnesium chloride (MgCL2), sodium
fluoride (NaF) , calcium chloride (CaCL2), and potassium bromide (KI).
Not all
compounds are as simple to name and write formula for.
Some compounds consist of more than two elements and contain polyatomic
ions. A polyatomic ion is a
positively or negatively charged, covalently bonded group of atoms.
Polyatomic ions tend to stay together as if they were a single atom.
Examples of polyatomic ions include ammonium (NH4 ), nitrate
(NO4), nitrite (NO3), carbonate (CO3), sulfate
(SO4 ), sulfite (SO3 ), hydroxide (OH), chlorate (ClO3
), and phosphate (PO4
).
You may notice
that several of the polyatomic ions that are listed have “-ate” or
“-ite” for an ending. Upon
further inspection, you may also notice that each of those polyatomic ions also
contain the element oxygen. The
endings “-ate” and “-ite” means oxygen.
OBJECTIVES:
Students should be able to...
