• Atoms can attain a
more stable
arrangement of
electrons in their
outermost shell by
interacting with one
another.
• An ionic bond is
formed when
electrons are
transferred from one
atom to the other. A
covalent bond is
formed when
electrons are shared
between atoms.
Molecular Compounds and Ions
1. What does it mean to say that an atom is neutral?
contains equal number of protons and electrons
2. What would happen to the charge on an atom if an electron
were removed? If an electron were added?
have a positive charge, have a negative charge
3. What is the name for an atom that has a charge?
an ion
4. What is a molecule?
A particle that contains more than one atom
5. How do the molecules of elements and compounds differ?
the molecules of a compound contain atoms from more than one
element
With the exception of the noble gases, which exist
as isolated atoms, most elements found in nature
form molecules
• Molecule – smallest electrically neutral unit of a
substance that still has the properties of the
substance. A molecule is made up of two or more
atoms that act as a unit
• Examples:
• H2O
• O3
• CO2
Compounds – Atoms of different elements can combine
chemically to form compounds. Compounds composed
of molecules are called molecular compounds;
Compounds composed of ions are called ionic
compounds
Molecular Compounds
• tend to have relatively low melting and boiling points
• exist as gases or liquids at room temperature
• most molecules of molecular compounds are composed
of the atoms of two or more nonmetals
Ions – not all compounds are molecular, many are
composed of particles called ions. Ions are atoms or
groups of atoms that have a positive or negative
charge. An ion forms when an atom or group of atoms
loses or gains electrons
Ions
• EXAMPLE: NaCl
• net charge on a sodium atom is zero
[11+(-11) = 0]
• in forming a chemical compound, an
atom of sodium tends to lose one of
its electrons, making its number of
electrons no longer equal to the
number of protons
• the atom of sodium becomes an ion,
because there are more positive
charges (protons) than negative
charges (electrons), the sodium ion
has a positive charge
NaCl
Cation – atoms or groups of atoms that have a
positive charge
• atoms of metallic elements tend to form cations
by losing one or more electrons
• cations have fewer electrons than the electrically
neutral atom from which it is formed
• an ionic charge is written as a number followed
by a sign
• What is the charge of a sodium cation?
[1+ (11 protons, 10 electrons)] = Na+ or Na1+
• What is the charge on a magnesium ion?
[2+ 12 protons, 10 electrons)] = Mg2+
• How many electrons does an Aluminum atom
lose to form an aluminum cation?
(3 electrons)
Ions vs. Atoms
• There are many
differences between
the metal and its
respective cation
Example:
• Sodium metal reacts
violently with water,
but sodium cations
are quite unreactive
Anion – atoms or groups of atoms that have a
negative charge
• atoms of nonmetallic elements tend to
form anions by gaining one or more
electrons
• anions have more electrons than the
electrically neutral atom from which it is
formed
• the name of an anion of a nonmetallic
element is not the same as the element
name. Anions typically end in “-ide”
EXAMPLE: NaCl
• net charge on the chlorine atom is zero
[17+ (-17) = 0]
• in forming a chemical bond, a nonmetal
tends to gain electrons to form an anion
• the chloride anion has 17 protons and 18
electrons, it has an ionic charge of 1• written as Cl• Common anions
– Oxide ion, oxygen atom gains 2 electrons,
giving it an ionic charge of 2-, written as O2-
Examples
• Cations are givers (positive)
• Anions are takers (negative)
• sulfur forms a _________ anion
Sulfide S2• bromine forms a __________ anion
Bromide Br-
Ionic Compounds – compounds
composed of cations and anions
• ionic compounds are usually composed of metal
cations and nonmetal anions
• Ionic compounds, even though they are
composed of ions that are electrically charged,
the total positive charge equals the total negative
charge. The overall compound is electrically
neutral
• Usually solid crystals at room temperature, melt
at high temperatures
Examples
• NaCl - sodium
cation & chloride
anion
• CaO (lime) – calcium
cations & oxide
anions
Characteristics of Molecular and Ionic Compounds
Characteristic
Molecular
Compound
Representative
unit
Molecule
Type of
elements
Physical state
Melting point
Nonmetallic
Solid, liquid, or
gas
Low (usually
below 300C)
Ionic Compound
Formula unit
(balance of
oppositely
charge ions)
Metallic
combined with
nonmetallic
Solid
High (usually
above 300C)
Review Questions- Molecular
Compounds & Ions
• Give the name and symbol of the ion
formed when:
• a sulfur atom gains two electrons
sulfide ion S2• an aluminum atom loses three electrons
aluminum ion Al3+
• a calcium atom loses two electrons
calcium ion Ca2+
Review Questions- Molecular
Compounds & Ions
• How many electrons
are lost or gained in
forming each ion?
• What is the symbol
and name for the
cation formed when:
• Ba2+
• a potassium atom
loses one electron
K+
• a zinc atom loses two
electrons
Zn2+
2 lost
• As33 gained
• Cu2+
2 lost
Review Questions- Molecular
Compounds & Ions
• Write the symbol and name for the anion
formed when:
• a fluorine atom gains one electron
F• a sulfur atom gains two electrons
S2-