Chapter 7: Chemical Compounds
Putting Atoms Together

Most things are made up of individual atoms. Atoms can be chemically joined together with other
atoms to form small units called molecules.

Atoms combine to become more stable.

The most stable elements on the periodic table are the noble gases (group #18). They are
considered to be the most stable because they have the maximum number of electrons in their
outermost (valence) shell – Helium has 2 electrons in its valence shell, and all other noble gases
have 8 electrons in their valence shell.

Elements that do not have the maximum number of electrons in their valence shell combine with
other elements to obtain this maximum number of electrons.

Atoms become stable by gaining, losing, or sharing electrons.

Molecules are two or more atoms of the same or different elements that are chemically joined
together in a unit. For example) Air is composed of many kinds of molecules: 80% nitrogen
molecules, 20% oxygen molecules, and small amounts of water and carbon dioxide molecules.

A chemical
formula is a notation that indicates the type and number of atoms in a pure
substance. For example) Nitrogen gas is written as N2 – the subscript “2” indicates that there
are two nitrogen atoms joined together to create the molecule of nitrogen gas.
Molecular Elements

A molecular
element is a molecule consisting of atoms of the same element.

There are 7 elements that form molecules that consist of two atoms. These molecular elements
are commonly called diatomic
molecules (the prefix “di” indicates two).

The 7 diatomic molecules are: H2, N2, O2, F2, Cl2, Br2, and I2.

A non-sensical word to remember the diatomic molecules is: HOF BrINCl (it rhymes with
Bullwinkle).
Molecular Compounds (Non-metal + Non-metal)

A molecular
compound is a molecule that consists of two or more different elements.

Molecular compounds are formed between two non-metal elements.

For example) H2O is water. It is made up of two hydrogen atoms and one oxygen atom.

For example) CH4 is methane. It is made up of one carbon atom and four hydrogen atoms.

For example) C12H22O11 is table sugar. It is made up of 12 carbon atoms, 22 hydrogen atoms, and
11 oxygen atoms.

Molecular compounds are created when a covalent bond forms between two or more non-metals.

A covalent

The combining capacities (the number of covalent bonds the element will form) for the non-metal
bond is a bond formed when non-metals share their valence electrons.
elements are:
1
2
3
4
F
Cl
Br
I
O
S
Se
N
P
As
C
Si
Practice Problems for Creating Molecular Compounds:
a) Nitrogen gas
N2
d) Ammonia
NH3
b) Oxygen gas
O2
e) Methane
CH4
c) Water
H2 O
f) Carbon dioxide
CO2
Alloys (Metals and Metals)

Metals form mixtures with other metals.

These mixtures are called alloys.

Alloys are created by melting two or more metals and then mixing these hot liquids. After
mixing, the alloy is allowed to solidify.

Alloys are different from compounds because in compounds, atoms join together chemically in a
specific ratio to form pure substances. Alloys are solutions of metals. The metals do not
combine chemically.

For example) Sterling silver is a solution of 92.5% silver and 7.5% copper.
Ionic Compounds (Metals + Non-metals)

Some compounds are made up of charged particles called ions.

An ion is a particle that has either a positive or a negative charge.

Ions form when an atom loses or gains one or more electrons without changing its number of
protons.

A Cation is a positively charged ion. A cation is always a metal. Metals always have 1, 2, or 3
electrons in their valence shell. Remember that a full shell has 8 electrons. It is easier (takes
less energy) to lose 1, 2, or 3 electrons and have a full inner shell than it is to find another 5, 6,
or 7 more electrons in order to fill its original valence shell. Since the metal element lost its
valence electrons to become stable, it will now have more protons than it does electrons. And,
since protons are positively charged, the metal will be a positively charged ion.

For example) Calcium is in group #2 which means that it has 2 electrons in its valence
shell. Calcium has 20 protons and 20 electrons. It is easier for calcium to lose these 2
electrons and have a full inner shell than it is to find 6 more electrons to fill its valence
shell. If calcium loses its 2 valence electrons, it will now have 18 electrons, while still
having 20 protons. Since protons are positively charged and electrons are negatively
charged, calcium will now have a +2 overall charge (+20 and -18 = +2)

An Anion is a negatively charged ion. An anion is always a non-metal. Non-metals always have 5,
6, or 7 electrons in their valence shell. Remember that a full shell has 8 electrons. It is easier
(takes less energy) to gain 1, 2, or 3 electrons and fill that valence shell than it is to lose all 5, 6,
or 7 electrons in order to have a full inner shell. Since the non-metal element gains more
electrons to its valence shell to become stable, it will now have more electrons than it does
protons. And, since electrons are negatively charged, the non-metal will be a negatively charged
ion.

For example) Fluorine is in group #17 which means that it has 7 electrons in its valence
shell. Fluorine has 9 protons and 9 electrons. It is easier for fluorine to gain 1 other
electron and have a full valence shell than it is to lose all 7 valence electrons in order to
have a full inner shell. If fluorine gains another electron, it will now have 10 electrons,
while still having 9 protons. Since protons are positively charged and electrons are
negatively charged, chlorine will now have a -1 overall charge (+9 and -10 = -1)

An Ionic
Compound is a compound that consists of positively and negatively charged ions.

Remember that metals lose electrons and become cations (positively charged ion) while nonmetals gain electrons and become anions (negatively charged ion). Cations and anions attract each
other because they have opposite charges.

An ionic compound is formed when a positively charged metal gives a negatively charged non-metal
its valence electrons. An ionic
bond is formed by the giving and taking of valence electrons
between a metal and a non-metal.
How to Write and Create Ionic Compounds

Always write the chemical symbol of the metal first, then the non-metal.

Write the metal’s charge number above its symbol; do the same for the non-metal.

“Criss-cross” the charge numbers to create the subscripts of the ionic compound. The charge
number of the metal will become the subscript of the non-metal and visa versa.

Do not write a subscript of “1”.

If necessary, reduce any subscripts. For example) Mg2O2 can be reduced to MgO

When naming an ionic compound, write the name of the metal first. Then, write the name of the
non-metal with its ending changed to “-ide”. For example) Magnesium oxide.
Practice Problems for Ionic Compounds……
1) Combine the following elements to create an ionic compound: Provide the name and formula.
a) Potassium and Oxygen
b) Aluminum and Nitrogen
c) Beryllium and Chlorine
2) Provide the name of the ionic compounds.
a) Li2S
b) NaF
c) Mg3P2
3) Provide the chemical formula for the ionic compounds.
a) Calcium nitride
b) Aluminum oxide
c) Potassium fluoride