Lecture 5
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Chemistry
How matter (stuff) is put together and taken apart
Chemistry
Atom: basic unit of matter
Life Science: typically deals with atoms and its larger, subatomic parts
Life Science: dance of matter and energy
Chemical energy results from the movement of matter
 Groups of atoms
 Electrons
The Atom
 Matter is composed of many very small units called atoms
 100 million carbon atoms, lined end to end, would stretch only about 3 centimeters
 1 inch = 2.54 centimeters
 Atoms are composed of smaller parts
 “Subatomic particles”
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The Atom
Protons
Neutrons
Electrons
The Atom
Atoms possess positive charges in the form of protons
Atoms possess negative charges in the form of electrons
The number of protons is typically equal to the number of electrons
As a result, the atom is uncharged
The Atom
Charge Position
 Protons
+
nucleus
 Neutrons
0
nucleus
 Electrons
outside nucleus
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 Protons and neutrons are packed together to form an atom’s nucleus
 The much smaller electrons exist outside of the nucleus
 Most elements possess multiple isotopes
The Atom
 e.g., Hydrogen, carbon, iodine, etc.
 Some of these isotopes are radioactive
 e.g., Hydrogen-3 (tritium), carbon-14, etc.
 Some of these isotopes are used in medicine
The Atom
 An atom’s electrons are involved in combining the atom with other atoms
 “Chemical bonds”
Chemical Bonding
Why form chemical bonds?
 When atoms bond with each other, they can move to a lower, more stable energy state
 Such reactions will tend to occur
 “Energy always seeks its lowest state”
Chemical Bonding
 Electrons reside in well-defined “energy levels” or “energy shells”
 The first shell can hold 2 electrons
 Each of the next few shells can hold 8 electrons
Chemical Bonding
Chemical bonding in water (H2O)
 One oxygen and two hydrogen atoms combine to form water
 Oxygen atom: 6 electrons in outer shell
 Hydrogen atoms: 1 electron in outer shell
Chemical Bonding
Chemical bonding in water (H2O)
 Oxygen shares one pair of electrons with a hydrogen atom and another pair of electrons with
a second hydrogen atom
 All electron shells are full
 The sharing of a pair of electrons is called a covalent bond
Chemical Bonding
 When two or more atoms bond, a molecule is formed
 e.g., One oxygen atom and two hydrogen atoms form a single water molecule
 e.g., Sucrose = C12H22O11
Chemical Bonding
 Molecules are held together by covalent bonds
 Sharing of a pair of electrons
 This sharing of electrons may be equal
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e.g., Two hydrogen atoms form H2
“Nonpolar covalent bond”
Chemical Bonding
 This sharing of electrons may be unequal
 e.g., H-O-H (H2O)
 The electrons spend most of their time near the oxygen atom
Oxygen has a partial negative charge
Hydrogens have partial positive charge
 “Polar covalent bond”
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Chemical Bonding
 A chlorine atom has seven electrons in its outer shell
 If it gains an electron
 The outer shell is completely full
 The atom is more stable
 The atom has a negative electrical charge
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Negative “ion”
Cl-
Chemical Bonding
 Individual hydrogen bonds are relatively weak
 Large numbers of hydrogen bonds can have a major effect on the shapes of molecules
 e.g., Hydrogen bonds hold the two strands of a DNA molecule together
Shapes of Molecules
 Molecules depicted on paper appear to be two-dimensional
 These molecules actually possess a three-dimensional shape
Chemical Bonding
Water molecules have polar covalent bonds
 Hydrogen atoms: partial positive charges
 Oxygen atoms: partial negative charges
 A partial positive charge on one water molecule can interact with a partial negative charge on
another water molecule
 “Hydrogen bond”
 Other molecules with partial charges can also form hydrogen bonds
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Shapes of Molecules
 Molecules depicted on paper appear to be two-dimensional
 These molecules actually possess a three-dimensional shape
Shapes of Molecules
 The three-dimensional shape of a molecule can be important in their function
 Particularly when this molecule interacts with another molecule
 Key fitting into lock
Chemistry Symbols
 Each chemical element has its own letter-based symbol
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H = hydrogen
He = helium
C = carbon
O = oxygen
Pt = platinum
Etc.
Chemistry Symbols
 A molecular formula indicates how many atoms of each type are present
 O2 = an oxygen molecule, consisting of two oxygen atoms covalently connected
 3O2 = 3 oxygen molecules
 6H2O = six water molecules, each of which is made of one oxygen atom and two hydrogen
atoms
 Etc.
Chemistry Symbols
 A molecular formula does not show the arrangement of atoms within a molecule
 A structural formula also shows the arrangement of atoms within a molecule
 Solid lines represent covalent bonds
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Sharing of one pair of electrons
 Double-solid lines indicate double covalent bonds
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How many electrons are shared here?
Chemistry Symbols
 Structural formulas represent a molecule in two dimensions
 Ball-and-stick models and space-filling models represent a molecule in three dimensions
pH
 Power of hydrogen
 Measure H+ in solution: greater number of H+, lower pH
Acids release H+
Bases release ions that react with H+, H+ acceptors
pH
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Buffers: combine or release H+
 Prevent drastic pH swings
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Salt
 Ionic compound formed when acid and base react
 HCl + NaOH -> NaCl + H2O