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Molecular Compounds
- None of these compounds contain any ion - they are made up of molecules. A molecule forms
when two or more non-metallic atoms bond together. It can be made up of atoms of different
elements or of atoms of all the same element. For example, CH OH (methanol) and O (oxygen
gas) are both molecules.
- Recall that the formula unit of an ionic compound represents a ratio of ions in a crystal lattice.
In a solid ionic compound, this lattice extends in all directions. A formula unit is not an
independent unit - it is just one part of a crystal lattice. Molecules are independent units made
up of fixed numbers of atoms bonded together.
- Unlike ionic compounds, molecular substances can be solid, liquid, or gas at room
temperature. They tend to be poor conductors of electricity, even in solution. Many do not
dissolve in water very well.
- Sharing Electrons - Covalent Bonds
- The atoms in a molecule are joined together by Covalent bonds that form when atoms share
electrons. Each pair of shared electrons forms one covalent bond. Electrons are not transferred
from one atom to another as they are in ionic bonds.
- Chlorine gas is an example of a substance that has molecules formed of only one element.
Each chlorine molecule is made up of two chlorine atoms joined by a covalent bond. Recall
that a chlorine atom has seven valence electrons in its outer energy level. For this outer energy
level to be filled, an additional electron is needed. A molecule of chlorine gas is created when
two atoms of chlorine each share an electron to form a covalent bond. In some covalent
compounds, the atoms share more than two electrons.
- For example, nitrogen gas occurs in the form N . (Nitrogen gas makes up 78% of our
atmosphere.) An atom of nitrogen has five valence electrons. To form N , two nitrogen atoms
share three pairs of electrons. In carbon dioxide (CO ), all atoms share two pairs of electrons.
- For example, nitrogen gas occurs in the form N . (Nitrogen gas makes up 78% of our
atmosphere.) An atom of nitrogen has five valence electrons. To form N , two nitrogen atoms
share three pairs of electrons. In carbon dioxide (CO ), all atoms share two pairs of electrons.
- Chlorine gas is an example of a substance that has molecules formed of only one element.
Each chlorine molecule is made up of two chlorine atoms joined by a covalent bond. Recall
that a chlorine atom has seven valence electrons in its outer energy level. For this outer energy
level to be filled, an additional electron is needed. A molecule of chlorine gas is created when
two atoms of chlorine each share an electron to form a covalent bond. In some covalent
compounds, the atoms share more than two electrons.
- The atoms in a molecule are joined together by Covalent bonds that form when atoms share
electrons. Each pair of shared electrons forms one covalent bond. Electrons are not transferred
from one atom to another as they are in ionic bonds.
- Sharing Electrons - Covalent Bonds
- Unlike ionic compounds, molecular substances can be solid, liquid, or gas at room
temperature. They tend to be poor conductors of electricity, even in solution. Many do not
dissolve in water very well.
- Recall that the formula unit of an ionic compound represents a ratio of ions in a crystal lattice.
In a solid ionic compound, this lattice extends in all directions. A formula unit is not an
independent unit - it is just one part of a crystal lattice. Molecules are independent units made
up of fixed numbers of atoms bonded together.
- Molecular Compounds That Do Not Contain Hydrogen
- A binary compound contains two elements. Some of these compounds contain hydrogen, and some
do not. IUPAC rules for naming binary molecular compounds not containing hydrogen are similar to the
rules for naming ionic compounds. For molecules, Greek prefixes are used to indicate how many
atoms of each element are present in the compound. For example, P O is called tetraphosphorus
decaoxide: "tetra-" means "4," and "deca-" means"10."
- Any compound that does not have a metal or an ammonium ion in its formula is molecular.
- The format for naming binary molecular compounds not containing hydrogen is:
- Prefix + First element followed by prefix + Second element ending in "-ide"
- Note that the prefix "mono-" is not used when the first element is only one atom. When the prefix
"mono-" is required before "oxide," the last "o" in the prefix is usually dropped. For example, it is
"monoxide," not "monooxide."
- Molecular Compounds That Contain Hydrogen
- Hydrogen is unique in many ways, and this is reflected in naming systems. Many compounds
containing hydrogen have simply been given names. The name "water," for example,was chosen by
IUPAC to be the official name for H O. These names have to be memorized. Note, in particular, that
the prefix "mono-" is omitted in H S , which is named hydrogen sulfide.
- Writing the formulas for molecular compounds is easy because the prefixes in the names indicate the
number of each element. However, predicting formulas when elements combine is difficult because
more than one combination is possible; for example, CO or CO .
- Writing the formulas for molecular compounds is easy because the prefixes in the names indicate the
number of each element. However, predicting formulas when elements combine is difficult because
more than one combination is possible; for example, CO or CO .
- Hydrogen is unique in many ways, and this is reflected in naming systems. Many compounds
containing hydrogen have simply been given names. The name "water," for example,was chosen by
IUPAC to be the official name for H O. These names have to be memorized. Note, in particular, that
the prefix "mono-" is omitted in H S , which is named hydrogen sulfide.
- Molecular Compounds That Contain Hydrogen
- Note that the prefix "mono-" is not used when the first element is only one atom. When the prefix
"mono-" is required before "oxide," the last "o" in the prefix is usually dropped. For example, it is
"monoxide," not "monooxide."
- Prefix + First element followed by prefix + Second element ending in "-ide"
- The format for naming binary molecular compounds not containing hydrogen is:
- Any compound that does not have a metal or an ammonium ion in its formula is molecular.
- A binary compound contains two elements. Some of these compounds contain hydrogen, and some
do not. IUPAC rules for naming binary molecular compounds not containing hydrogen are similar to the
rules for naming ionic compounds. For molecules, Greek prefixes are used to indicate how many
atoms of each element are present in the compound. For example, P O is called tetraphosphorus
decaoxide: "tetra-" means "4," and "deca-" means"10."
- Molecular Element
- Nitrogen is known as a Molecular Element because it forms molecules made up of
only one type of atom. Its molecules are Diatomic, which means each one is
composed of only two atoms: N ("di-" means "two"). Some elements form polyatomic
molecules. For example, sulfur forms a ring of eight atoms and has the formula S .
Other elements are monatomic. Their atoms can exist on their own. Carbon, for
example, is written as C , although in both diamond and graphite it is connected to
other carbon atoms in very large arrays.
- It can be very helpful to have the diatomic elements memorized, particular when
writing chemical reactions. One way to remember them is that the diatomic
elements are the "gens": hydrogen, nitrogen, oxygen, and the halogens.
- It can be very helpful to have the diatomic elements memorized, particular when
writing chemical reactions. One way to remember them is that the diatomic
elements are the "gens": hydrogen, nitrogen, oxygen, and the halogens.
- Nitrogen is known as a Molecular Element because it forms molecules made up of
only one type of atom. Its molecules are Diatomic, which means each one is
composed of only two atoms: N ("di-" means "two"). Some elements form polyatomic
molecules. For example, sulfur forms a ring of eight atoms and has the formula S .
Other elements are monatomic. Their atoms can exist on their own. Carbon, for
example, is written as C , although in both diamond and graphite it is connected to
other carbon atoms in very large arrays.
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