unit 4: chemical bonding -how elements get together- 3 types of bonding
TRANSCRIPT
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Unit 4: Chemical Bonding
-How Elements Get Together-
3 types of bonding
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What is Chemical Bonding?• The joining of atoms to form new
substances• The properties of these “substances”
are completely different from the properties of the original elements
• An interaction that holds 2 atoms together is called a chemical bond
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Why revisit the Bohr Model?• Scientists and their
theories are best explained with models
• Remember: we use models to explain what we can not see or directly observe
• Based on theories
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Do Atoms Bond Only through their Electrons?
• Yes and where are the electrons?• The electrons are located around the
nucleus in various energy levels• Remember:
– The atomic number is the number of protons in an atom
– the atomic number also represents the number of electrons in the atom.
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What are the energy levels of an atom? p.311
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What are the outer Electrons called?p.311
• Valence Electrons are the outer most orbital or “ring” of an atom
• ALL bonding occurs at this energy level • This energy level is the highest
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How do you know how many Valance Electrons an atom has?
p.312
3 4 5 6 78
2
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What is the “Octet Rule”?• The octet rule refers to the tendency of
atoms to prefer to have eight electrons in the valence shell.
• When atoms have fewer than eight electrons, they tend to react and form more stable compounds
• The exceptions are with Hydrogen (H) & Helium (He) that need only 2 in their outer shell.
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Do All Atoms Bond?• Not necessarily • Group 18 atoms VERY
rarely form bonds• The Noble Gases have a
full outer electron shell - a full octet
• 8 valence electrons = stable; no bonding
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How do Atoms fill their Outermost level?
• All Atoms are LAZY!• Some atoms accept or
take electrons – If they have 4 or more
valence they accept more to reach 8
• Some atoms give up electrons– If they have 1, 2 or 3
they give them up and drop down to their lower energy level
Sulfur
Magnesium
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What are the Rules for determining what type of bond is
occurring?• Ionic Bonds
– ONLY with groups 1, 2 & sometimes 3 bonding with any nonmetal (groups 15-17)
• Covalent Bonds– ONLY between nonmetals
• Metallic Bonds– ONLY between metals
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What are Ionic Bonds?• A bond that forms when electrons are
transferred from one atom to another atom.
• What is an Ion?– Charged particles that form when atoms
gain or lose electrons.– Can be a positive ion (+) or negative ion
(-)– Positive ion = more protons (+) than
electrons (-)– Negative ion = more electrons (-) than
protons (+)
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How are Positive Ions Formed?
• Metal atoms tend to lose their valence electrons
• Sodium loses its 1 valence electron and drops to the second energy level
• Groups 1 & 2 lose their electrons very easily
Before After
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How are Negative Ions Formed?
• Nonmetal atoms tend to accept electrons easily
• Chlorine accepts a single electron to complete its octet or outer energy level
• Group 17 atoms give off the most energy when they bond
Before After
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Sodium + Chlorine = Salt
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What are Salts?
• An ionic compound formed from the positive ion of Groups 1 or 2 and the negative ion of Groups 15 - 17.
• Not necessarily NaCl (table salt).
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What are some uses of Salts?
• NaCl – sodium chloride or table salt
• NaSO4 – sodium hydroxide or lye
• CaSO4
– calcium sulfate or wallboard
• NaNO3 – sodium nitrate or food preservative
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Elements That FormIonic Bonds
Groups 1,2 or 3 with Groups 15 - 17
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What are Properties of Ionic Compounds?
• The + or - charges that are exchanged results in a neutral compound
• They form a crystal lattice– A 3-dimensional pattern
• Brittle, with high melting & boiling points
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What are Lewis Dot Structures?
• Models that focus only on valence electrons• The element symbol is centered among the
dots representing the element’s valence electrons as shown below
Na Cl
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What are Lewis Dot Structures?
• Models that focus only on valence electrons• The element symbol is centered among the
dots representing the element’s valence electrons as shown below
Na Cl
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Ne
1. Draw the element symbol
2. Determine the number of valence electrons
3. Arrange the “dots” around the symbol as follows…
O
Be
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A Quick Assignment• You need the small dry-erase board
at your table & a dry-erase marker• Turn to your book’s Periodic table• Draw the Lewis-dot structures for
the following Elements:• Hold your board up when finished
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PotassiumStrontium
PhosphorousTin
GalliumSelenium
XenonSilicon
BerylliumLead
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What are Covalent Bonds?
• When atoms share electrons • Mostly happen between nonmetals
on the periodic table• Properties include
– Low melting & boiling points– Brittle only in a solid state– Examples: water, sugar, oxygen, wood,
etc…
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What are Covalent Bonds?p.318
H2
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What does a Covalent bond of water look like?
The Lewis Dot model of water looks like….
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The Lewis Dot Model looks like….
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What are the Diatomic Molecules?
• The simplest molecule• Made up of two
bonded atoms• Elements found in
nature as diatomic molecules are called diatomic elements
• H2, O2, N2, & the Halogens - F2, Cl2, Br2 & I2 are diatomic elements
F2
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What are the More-Complex Molecules?
• Typically have Carbon involved in some way
• Each carbon atom needs to make 4 covalent bonds to reach the octet (8 valence electrons)
• Hydrocarbons are good examples Molecule of Propane
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What are Metallic Bonds?
• A bond formed by the attraction between positively charged metal ions and the electrons in the metal
• When metal atoms are close enough their valence electrons overlap
• The result is a solid (mostly) with electrons that flow freely around the material
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What are the properties of Metallic
Bonds?• It is these bonds
that give all metals their particular characteristics
• Electrical conductivity– Electricity means
negative charges (electrons) moving through the metal
• Malleability & ductility
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Malleability & Ductility?• Because the
electrons (-) move freely atoms can easily be rearranged
• Bent, hammered into sheets = malleability
• Pulled into wire = ductility
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Bending without breaking?
• Because the electrons (-) move about fixed positive ions (+) – The compounds
can bend instead of break
• All metals have this property