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Chemical Bonds
Chapter 18
17.1 Stability in Bonding
Main Idea: When atoms form compounds,
each atom is more stable in the compound
than it was by itself.
• Examples: (discuss with your table
examples of things that become more stable
when combined)
Combined Elements
Compounds
New Properties
• Pure elements (i.e. copper,
silver, oxygen) combine
chemically to form compounds
in the right conditions.
• Ex: Copper sulfate is one of
the compounds responsible for
the color of the Statue of
Liberty
• Physical and chemical
properties of a compound are
often different than the
properties of the individual
elements that combine to
create the compound.
Formulas
• Every element has a chemical symbol.
• A chemical formula shows what
elements a compound contains and the
exact number of the atoms of each
element in a unit of that compound.
• Subscript – written below
• A symbol with no subscript means there
is only one atom of the element in the
compound.
Chemical Bond Formation
Why do atoms form compounds?
Electron Dot Diagrams (Lewis Dot Structures) refer to WS in NB
• The unique noble gases – an atom is chemically stable when its outer
energy level is complete. H and He are stable with 2 electrons. All
other elements are stable with 8. Helium rarely forms compounds.
• Unfilled and filled energy levels – dot diagrams show if the outer
energy level is filled. A filled outer energy level creates stability.
• Outer levels – getting their fill – atoms with unstable outer energy
levels can lose, gain, or share electrons to obtain a stable outer
energy level. This is achieved through joining with other atoms.
Chemical Bond Formation (cont.)
Why do atoms form compounds?
• When an element loses electrons, the next-to-the last
energy level becomes the new outermost energy level.
• Gaining and losing electrons – NaCl
Chemical Bond Formation (cont.)
Why do atoms form compounds?
Sharing electrons– H2O
• Draw dot structures for hydrogen and
oxygen to show how a water molecule
is created.
Chemical bond formation – a chemical
bond is the force that holds atoms
together in a compound.
Predict whether these atoms would more
likely gain or lose electrons to form an octet.
Section 2 - Types of Bonds
Main Idea: Atoms form ionic bonds by
transferring electrons and form covalent bonds
by sharing electrons.
Ions
• Atoms lose or gain electrons to meet a standard –
stability.
• An ion is a charged particle that has either more or
fewer electrons than protons.
• Losing electrons creates a positively charged ion.
(cation)
• Gaining electrons creates a negatively charged ion.
(anion)
Transfer of electrons
• Superscript- written above
• A superscript is used to
indicate the charge of an
ion.
• The charge is determined
by the number of electrons
gained or lost.
• Ex: KI p. 559
The Ionic Bond
• An ionic bond is the force
of attraction between the
opposite charges of the ions
in an ionic compound. The
number of positive charges
must equal the number of
negative charges in order to
form a compound with a
neutral charge.
Zero net charge
• The compound as a whole is neutral because the sum of
the charges on the ions is zero.
• Ionic bonds are usually formed between metals and
nonmetals.
• Ionic compounds are often formed by elements across
the periodic table from each other.
• They are typically crystalline solids with high melting
points.
Ex: MgCl2
Molecules
• Some atoms of nonmetals are unlikely to lose or gain
electrons.
• Group 14 elements would have to either gain or lose 4
electrons to achieve stability. This would take a great
deal of energy.
• Group 14 elements become more chemically stable by
sharing electrons.
• The neutral particle that forms as a result of electron
sharing is called a molecule.
The Covalent Bond
• The attraction that forms between atoms when they share
electrons is a covalent bond.
• A single covalent bond is composed of two shared electrons.
• A water molecule contains two single bonds.
Multiple covalent bonds
• A covalent bond can contain more than one pair of electrons.
• In a diatomic oxygen molecule two electrons are shared between
the two oxygen atoms.
•
•
•
•
A double bond occurs when atoms share two pairs of electrons.
A triple bond contains three shared pairs of electrons.
Covalent bonds form between nonmetal elements.
Many covalent compounds are solids or gases at room
temperature.
Equal Sharing
• A nonpolar bond is a covalent bond in which electrons are shared
equally by both atoms.
• Ex: diatomic molecules
Unequal Sharing
• In some molecules, electrons are not shared equally and
the electron charge is concentrated more on one end of
the molecule than the other.
• The strength of the attraction of each atom to its
electrons is related to the size of the atom, the charge
of the nucleus, and the total number of electrons the
atom contains.
• More positive charge in a nucleus attracts electrons more
strongly.
Partial Charges
• Ex. HCl page 562
Tug-of-War
• Polar – having opposite ends
• A polar bond is a bond in which electrons are shared unequally.
Polar and nonpolar molecules
• A polar molecule is one in
which the unequal sharing
of electrons results in a
slightly positive end and a
slightly negative end,
although the molecule is
neutral.
• A nonpolar molecule does
not have oppositely charged
ends.
• Polarity and Geometry – The
polarity of the bonds and the
shape of the molecule determine
whether the molecule is polar or
nonpolar.
Writing Formulas and Naming
Compounds – Section 3
Main Idea: For both ionic and covalent compounds, you
can write a name from the chemical formula or a
chemical formula from the name.
Writing Chemical Formulas
Oxidation Numbers
• Oxidation number – a
positive or negative
number that indicates
how many electrons
an atom has gained,
lost, or shared to
become stable.
• For ionic compounds,
the oxidation number
is the same as the
charge on the ion.
Oxidation Numbers and
the Periodic Table
• Many elements have 1
oxidation number.
• Groups 1, 2, 13-18 fit
with the period table
groupings.
• Many of the transition
elements can have
more than one
oxidation number.
Binary Ionic Compounds
• Binary compounds are the
easiest for which to write
formulas.
• Binary compounds –
composed of two elements
• Even though individual ions
carry a charge, the
compound itself is neutral.
• Ex: aluminum oxide (p.
566)
Writing Formulas
1. Write the symbol of the element that has the positive
oxidation number or charge. (all metals and often
Hydrogen)
2. Write the symbol of the element with the negative
oxidation number. (nonmetals and Hydrogen when
bonded with a metal)
3. Find the least common multiple of the charges of each
ion. (The charge, without the sign, of one ion becomes
the subscript of the other ion.) Reduce the subscripts to
the smallest whole numbers that retain the ratio of the
ions.
Examples – Binary Ionic Compounds
Naming
1. Write the name of the positive ion.
2. Check to see whether the positive ion forms more than one
oxidation number. If so, determine the oxidation number of the
ion from the formula of the compound. Write the charge of the
positive ion using roman numerals in parentheses after the ion’s
name.
3. Write the root name of the anion. The root is the first part of
the elements name.
4. Add the ending –ide to the root.
Examples – Naming Binary Ionic Compounds
Compounds with Complex Ions
• Many common compounds contain more than two atoms.
• A polyatomic ion is a positively or negatively charged, covalently bonded
group of atoms.
• Polyatomic ions as a whole contain two or more elements. They act like
individual ions in forming compounds.
Writing Formulas
• To write formulas for compounds containing polyatomic
ions, follow the rules for binary compounds, with one
addition.
• When more than one polyatomic ion is needed to balance
the charges of the ions, place parentheses around the
polyatomic ion before adding the subscript.
• Ex: barium chlorate (p. 569)
Naming
1. Write the name of the positive ion.
2. Write the name of the negative ion.
Naming Binary Covalent Compounds
• Some pairs of nonmetals can form more
than one compound with each other.
• Ex: N2O, NO, NO2, and N2O5
• A different system of naming must be
used for covalent compounds.
Using Prefixes
• Scientists use Greek prefixes to
indicate how many atoms of
each element are in a binary
covalent compound.
• Ex: N2O, NO, NO2, and N2O5
Compounds with Added Water
• A hydrate is a compound that has water chemically attached to its
atoms and written into its chemical formula.
• Anhydrous – without water
Common hydrates
• Hydrate comes from a word that means water.
• When naming hydrates you use the same Greek prefixes
• Ex: p. 570 cobalt chloride