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Chapter 2

BIOL 150B Chapter Notes - Chapter 2: Ionic Compound, Electronegativity, Hydrogen Bond

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David Creasey

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Atoms, Molecules, & Life
1. Matter: anything that occupies space & has a mass
composed of chemical elements
chemical elements: substances that cannot be broken down into other
-natural elements: essential for life (25 of them)
-trace elements: essential for life, but only in minute amounts
-chemicals can be used for language (ex. by moths & spiders)
-all biological hierarchy starts at the chemical level
compound: a substance containing a fixed ratio of 2 or more elements
-most compounds in organisms contain:
(1) carbon
(2) hydrogen
(3) oxygen
(4) nitrogen
(5) phosphorus
2. Atoms: the smallest unit of matter
pertains the properties of an element
all atoms of a particular element have the same # of protons
an atom’s # of protons make it a unique element
nucleus: (n) the central core of an atom
protons: (p) + electric charge, mass of 1, found in the nucleus
neutrons: (e) no electric charge, mass of 1, found in the nucleus
electrons: - electric charge, no mass, orbiting the nucleus
atomic number: the # of protons in each atom of a particular element
mass number: the sum of the # of protons & neutrons in an atom
3. Isotopes: a variant form of an atom that has the same # of protons, but a
different # of neutrons
most are stable
radioactive isotope: an unstable isotope in which the nucleus decays
spontaneously, giving off particles & energy
Ex. Carbon-12 - naturally occurring 99% of the time (6p, 6n, 6e)#
Carbon-13 - naturally occurring ~1% of the time (6p, 7n, 6e)#
Carbon-14 - naturally occurring minute amounts (6p, 8n, 6e)
4. Electron shells: energy level representing the distance of an electron from
the nucleus of an atom
first shell can hold 2 electrons - rest of shells can hold 8
electron arrangement determines the chemical properties of an atom
an atoms # of electrons determines how it will interact with other atoms
full shell = most stable
electrons absorb energy which elevates them lose energy as light & drops
back down into the low-energy shell
5. Molecules: two or more atoms bonded together
6. Chemical bond: an attraction between 2 atoms
the greater amount of bonds = greater diversity
ionic bond: an electron is transferred between atoms, creating a positive &
negative ions that attract each other, holding the ions together
-ion: an atom or molecule that has gained or lost one or more electron
therefore electrically charged
cation: has a positive charge
anion: has a negative charge
-ionic compound: a compound formed by an ionic bond (ex. sodium
covalent bond: a sharing of outer-shell electrons
-the “need” for an electron isn’t greater in either atom = no greater
pull from ones nucleus to another
-symbolized by a single line between the atoms
-double bond: 2 atoms share 2 electrons
symbolized by 2 lines between the atoms
-electronegativity: tendency of atoms to pull electrons towards itself
differences in electronegativity = unequal sharing of electron pairs in
covalent bonds
# of protons = + charge = attraction of electrons
Oxygen (out of the elements relevant to us) has the greatest
-polar: unequal sharing of electrons due to different electronegativity
shared electrons are pulled closer & spend more time around the
more electronegative atom, making it partially - , & the other
partially +
-non-polar: equal sharing of electrons due to similar electronegativity
hydrogen bond: attraction between nearby polar molecules in which H
(slightly +) attracts & is bonded to O or N (both of which are slightly -)
-very weak & unstable
-break & reform whenever there is movement between molecules
- temperature = molecules move = breaking of hydrogen bonds
7. Water: the simplest example of hydrogen bonding, in which the bonds are
constantly breaking & re-forming due to the molecules moving
contains polar covalent bonds
-O has a + charge than H = electrons spend more time around O
-can therefore break down other polar molecules
cohesion: attraction between molecules of the same kind
-most water molecules are bonded to other water molecules
-water cohesion is stronger than any other liquid
-helps water to reach treetops
-causes surface tension
surface tension: how difficult it is to stretch/break the surface of a liquid
-unusually high in water due to hydrogen bonds
solvent: dissolving agent that dissolves another substance
-completely surrounds & disperses individual atoms of the molecule
solute: substance that is dissolved
solution: a solvent containing one or more dissolved substance
-solvent + solute = solution
-aqueous solution: water solvent
hydrophilic: polar or charged molecules that are soluble in water (ex. salt)
hydrophobic: large non-polar molecules that do not dissolve in water (ex. fat)
-non-polar molecules cluster together because they are excluded from the
water molecules forming hydrogen bonds with each other
moderates temperatures: maintains our core temperature in order to
allow maximum, constant, efficient functioning
-evaporative cooling: surface becomes cooler during evaporation
due to the loss of highly kinetic molecules to a gaseous state
ice: contains fewer molecules than the equal volume of liquid water making
it less dense
-explains why ice floats on water
-has stable hydrogen bonds due to the molecules not moving
-each molecule is bonded to 4 neighbours
water-based solutions can be either basic, acidic or neutral
pH: potential hydrogen
-measure the concentration of H in a solution
pH scale: measures the relative acidity of a solution
-ranges from 0 (most acidic) to 14 (most basic)
acid: releases H = high H concentration = lowers pH
base: removes H = high OH concentration = raises pH
buffer: resists pH changes, maintaining a relatively constant pH
-allows for proteins & enzymes to continue to work
acid precipitation: any type of precipitation with a pH below 5.6
-can damage/destroy non-living & living organisms by acidifying lakes,
streams, oceans & land habitats
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