Section: 2.4a
Date:
Essential
Questions
Chemical Bond
Formation
Chemical Bonds
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Formulas
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Single Element
Electron Dot
Diagrams
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Why does chemical bonding occur?
How do electron dot (Lewis dot) diagrams help predict chemical bonding?
How are the names and formulas for compounds written?
Many elements combine chemically to form compounds when the conditions are
right.
Many times the compound formed looks different from the pure substances that
formed it and it has own unique properties.
Atoms form compounds to reach stability.
Stability occurs when an atom reaches 8 valence electrons in its outer energy
level (except for hydrogen and helium which only need 2 valence electrons).
Chemical formulas show what elements a compound contains and the exact
number of atoms of each element in a unit of that compound.
Example – H2O
o H is Hydrogen
o O is Oxygen
o The subscript 2 means there are 2 hydrogen atoms.
o No subscript means there is one oxygen atom.
Electron dot (or Lewis dot) diagrams are used to show the electrons in the outer
energy level of an atom.
To draw an electron dot diagram:
1. Write the element’s symbol.
2. Find the number of valence electrons the element has by looking at the last
number in the upper right corner on the periodic table.
3. Draw dots around the symbol – first one on each side of the symbol (top,
bottom, left, and right) then pair them up with what remains.
Example
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Oxygen
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Mercury
Reaching
Stability to
Forming
Chemical Bonds
Ions
Ionic Bonds
Covalent Bonds
Octet Rule
Electron Dot
Diagrams for
Covalent
Compounds
Example
 The electron dot diagram shows how close an atom is to reaching stability.
 Atoms with unstable outer energy levels can lose, gain, or share electrons to
obtain a stable outer energy level.
 They do this by combining with other atoms that also have partially complete
outer energy levels.
 Elements with 4 or less valence electrons are more likely to give up their
electrons, while those with 5 or more valence electrons are more likely to take
valence electrons to achieve stability.
 Depending on the elements, sometimes electrons will be shared so that stability
is achieved.
 Some atoms lose or gain electrons to gain stability.
 An atom that has gained or lost an electron is called an ion.
 Ions are represented with superscript numbers and symbols.
1. + means a positive charge (lost electrons)
2. – means a negative charge (gained electrons)
 An ionic bond is the force of attraction between opposite charges of the ions in
an ionic bond.
 The number of positive charges must equal the number of negative charges in
order to form a compound with a neutral charge.
 Ionic bonds are usually formed between metals and nonmetals.
 Ionic compounds are typically crystalline solids with high melting points that are
called salts.
 Covalent bonds occur when the attraction that forms between atoms results in
sharing electrons.
 Each covalent bond is composed of two shared electrons.
 Multiple covalent bonds can occur between two atoms:
1. Double bonds occur when two sets of electrons are shared.
2. Triple bonds occur when three sets of electrons are shared.
 Covalent bonds form between nonmetallic elements by following the octet rule.
 The octet rules says that atoms tend to combine in such a way that they end up
with 8 electrons in the valence shell.
 The only exceptions are hydrogen and helium that end up with 2 electrons in the
valence shell.
 Electron dot diagrams can be used for covalent compounds.
 To draw an electron dot diagram for a covalent compound:
1. Find the total valence electrons by adding up the amount for each element in
the compound.
2. Decide which atoms are bonded (Normally the first element will go in the
middle with the exception being hydrogen).
3. Place 2 electrons in each of the bonds.
4. Complete the octet rule for the outside atoms.
5. The electrons left get put on the central atom.
6. If the central atom doesn’t fulfill the octet rule, make double bonds by
sliding a pair or pairs of electrons from the outside atom(s).
 CH4
 CO2