Regents Biology
The Chemical Basis of Life
Sections 2.6 – 2.9Chemical Bonding
Today we will learnabout how atoms
bond together
Regents Biology
Chemical Bonding Properties Bonds are made with
electrons electrons determine
chemical “behavior” of atom
depends on numberof electrons in atom’s outermost shell valence electrons
How many valenceelectrons do you see?
How many valenceelectrons do you see?
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Elements & their valence shells
Elements in the same row have the same number of shellsElements in the same row have the same number of shells
Moving from left to right, each element has a sequential addition of electrons (& protons)Moving from left to right, each element has a sequential addition of electrons (& protons)
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Elements & their valence shells
Elements in the same column have the same valence & similar chemical properties
Elements in the same column have the same valence & similar chemical properties
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Bonding Properties
Effect of electrons Atoms like to have maximum number of
valence electrons!!!!!
What’s themagic number?
How does this atom behave?How does this atom behave? How does this atom behave?How does this atom behave?
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Chemical Reactivity So atoms tend to complete a partially
filled valence shell or empty a partially filled valence shell
This tendency drives chemical reactions…This tendency drives chemical reactions…
and creates bondsand creates bonds
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H2 (hydrogen gas)
Covalent bond
Bonds in Biology Ionic Bonds (atoms to ions) Covalent Bonds (shares e-)
nonpolar & polar
Hydrogen Bonds
Hydrogen bond
H2O
H2O
Ionic bond
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LE 2-7a-1
Transfer ofelectron
NaSodium atom
ClChlorine atom
Ionic Bonds
Ion – any atom that has lost or gained an electron.
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Covalent bonds Covalent bonds are strong bonds
two atoms share a pair of electrons both atoms holding onto the electrons very stable
Forms molecules
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H2 (hydrogen gas)H2 (hydrogen gas)
H — HH2O (water)H2O (water)
H
H
Oxygen
H
HO
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Multiple covalent bonds 2 atoms can share >1 pair of electrons
double bonds 2 pairs of electrons
triple bonds 3 pairs of electrons
Very strong bonds
H
H–C–H
H
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More isbetter!
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Nonpolar covalent bonds Pair of electrons shared equally by 2 atoms
example: methane (CH4 )
H
H–C–H
H
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H
H
Oxygen
Polar covalent bonds
Pair of electrons shared unequally by 2 atoms example: water = H2O
oxygen has stronger “attraction” for the electrons than hydrogen
water is a polar molecule
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