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electrons are always
shared in pairs
even though the bonding orbital is one big shared shell, there are still
two nuclei inside of it
if the nuclei are identical, the electrons are
shared equally
the hydrogen atom and the fluorine atom in HF share
one pair of electrons
because the fluorine nucleus has so many more protons, it
strongly attracts the electrons
this attraction distorts the shape of the bonding orbital, and the result is that
the electrons spend more time on the fluorine side of the molecule
whatever atom the electrons spend less time around, that side will becomes
slightly/partially negatively charged
whatever atom the electrons spend more time around, that side will becomes
slightly/partially positively charged
bonds that share un-equally are called
polar covalent bonds
bonds that share equally are called
non-polar covalent bonds
the atoms in a non-polar covalent bond do not
have partial charges
the ability of an atom’s nucleus to attract electrons is called
electronegativity
if the difference between each atom is less than or equal to 0.5, it is
non-polar
if the difference between each atom is greater than 0.5, it is
polar
in a polar bond, the atom with higher electronegativity is
partially negative
in a polar bond, the atom with lower electronegativity is
partially positive
water is a molecule built from
polar covalent bonds
the oxygen atom in water is partially
negative
the two hydrogen atoms in water are partially
positive
polarity allows water to form a special kind of association with other molecules:
a hydrogen bond
water can form hydrogen bonds with any
ionic or polar covalent molecule
when water forms hydrogen bonds, that molecule becomes
dissolved in water
(ex: NaCl)
ions in ionic bonds are held together by the
electrical attraction between the positive ion and the negative health
when ionic compounds dissolve in water,
hydrogen bonds will weaken the electrical attraction between the two atoms
the individual atoms in the ionic compound become attracted to
the water instead of each other
metals found in cells are almost always
charged ions
polar covalent molecules also
dissolve in water
covalent bonds typically do
not break apart in water
hydrogen bonding with water still results in the
polar covalent molecule being dissolved
in biology, the term used for any molecule that can dissolve in water is
hydrophilic
(means “water-loving”)
water is not able to form hydrogen bonds with
non-polar covalent molecules
non-polar molecules
(water and oil)
do not dissolve in water
(these molecules are hydrophobic)
in living cells, most non--polar molecules are
fats or fat-based
polar molecules can dissolve other polar molecules, while non-polar molecules can issolve
other non-polar molecules
(cannot cross paths)
water is not the only molecule that
forms hydrogen bonds
hydrogen bonds are about 1-% as strong as
a covalent bond
compared to covalent bonds, hydrogen bonds
are weak
hydrogen bonds can form and break
easily
(this is not a bad thing)
because hydrogen bonds are easy to form and break, cells can use them to
temporarily stick molecules together and then unstick them
(ex: DNA - the two strands are held together by many hydrogen bonds)
the cell can break the hydrogen bonds in DNA when
it needs to use it
the cell can re-form new hydrogen bonds when
it is done using the DNA
acids will release hydrogen ions when
interacting with water
(ex: carbonic acid)
water forms hydrogen bonds with
carbonic acid
the hydrogen bond is strong enough to
weaken and break the oxygen-hydrogen covalent bond
when this happens, the acid keeps the shared electron and becomes
negatively charged
hydrogen is released as a
positively charged ion
hydrogen ions are often simply referred to as
protons
bases will attract and absorb
hydrogen ions from water
negative bases have atoms with
lolne pairs of electrons
a lone pair is a pair of electrons not
in a covalent bond
when they absorb hydrogen ions, neutral bases become
positiviely charged
we measure how acidic or basic something is using the
pH scale
pure water (nothing dissolved in it) has a
pH of 7
acids cause pH to
decrease
the more acid there is in a solution, the lower the
pH is
bases cause pH to
increase
the omre base there is in a solution, the
higher the pH is
