The Chemical Context of Life
Chapter 2
Elements, Molecules and Compounds
____________
____________
• Substances that can _______ be
• _______ or ____________ atoms held
broken down to other substances
together by chemical bonds
• May be same or different elements
by a chemical reaction
92 occur in nature
____________
• Two or more elements that occur in a
25 essential for life
____________ ____________
____________
• Smallest unit of an element
Elements
Have a one or two letter ____________
4 make up 96% of living organisms
• ____________ (C)
• ____________ (H)
• ____________ (O)
• ____________ (N)
Parts of the Atom
____________– ____________ charged
• ____________ – ____________ charged
• ____________– _______ charge
____________– ____________ charged
Atomic Number
Atomic Number = ____________ of ____________
• Unique to that element
Atomic number written as ____________ to ____________ of symbol
• e.g. 2He
Chapter 2 - The Chemical Context of Life
Atomic Weight
Measured in ____________ (1 dalton ≈ 1.7 x 10-24 g)
Protons weigh _________ dalton
Neutrons weigh _________ dalton
Electrons weigh 1/2000 of a dalton
Atomic weight (mass) = weight of ____________ + weight of ____________
• Ignore the mass of ____________ in the total mass of an atom
Atomic weight is written as a ____________ to ____________ of symbol
• e.g. 42He
Parts of the Atom
____________ –____________ charged
• Protons – positively charged
• Neutrons – no charge
____________ –____________ charged
Charged Particles
Nucleus (core) has net positive charge
• ____________ and ____________
Electrons are ____________ to the positively charged nucleus
• Electrons circle outside of the nucleus at the speed of light
Atom is ____________ in charge if protons = electrons
Atomic Number, Weight, and Charge
Atomic number = number of protons
Atomic weight = number of neutrons + number of protons
When # protons = # electrons, then there is no (____________) charge
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Chapter 2 - The Chemical Context of Life
Examples
Boron
Aluminum
• Atomic number 5
• Atomic number of 13
• Atomic mass 11
• Atomic mass of 27
• How many protons? __________
• How many protons? __________
Electrons? __________
Electrons? __________
Neutrons? __________
Neutrons? __________
Electron __________– Position in Relation to Nucleus
__________charged __________electrons attracted to __________ charged protons
More __________ energy if farther away
Potential energy in electrons changes in steps
__________ shell __________in potential energy
__________shell __________in energy
Electron ____________________
st
• 1 shell: one orbital – __________ electrons
nd
• 2
shell: four orbitals –__________ electrons
rd
• 3 shell: four orbitals –__________ electrons
Electrons __________ the __________shells completely first
No more than __________electrons in the same orbital
Unpaired electrons are ____________________
Valence Shells
Outer shell of electrons = __________ shell
Number of __________in valence shell determines how __________an atom is
Atom with a ____________ valence shell is _____________ (“inert” or “noble element”)
Valence Shells of 1st 18 Elements of the Periodic Table
Arranged in rows according to how many _____________ in the valence shell
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Chapter 2 - The Chemical Context of Life
Chemical _____________
How atoms stay together to form _____________
• Only _____________ interact to make bonds
Bonds occur by _____________ or _____________ valence electrons
• To complete their valence shell
_____________ _____________ = _____________
• Electrons needed to fill the valence shell
H: 1
N: 3
O: 2
C: 4
Electronegativity
The strength of the pull on electrons by the atom’s nucleus
Shared electrons pulled by both nuclei
• Tug of war for electrons
Atoms of different elements have different _____________________
_____________ one of the _____________ electronegative of the 92 atoms
• Often has unequal electron sharing
Types of Bonds
________________– strongest
• _____________ -polar
• _____________
_____________– medium in strength
_____________– weak
_________ der _____________interactions
• Very weak – not even a real bond
Covalent Bonds
Sharing a pair of valence electrons
Very _____________
e.g. Oxygen
e.g. Hydrogen
6 valence electrons
1 valence electron
Valence capacity is 8
Valence capacity is 2
Can share __ pairs to make double bond
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Chapter 2 - The Chemical Context of Life
Polar & Non-Polar Covalent Bonds
Electronegativity
• How strong atom’s nucleus pulls on electrons
_____________ - _____________ bonds
• _____________ electronegativity _____________electrons _____________
_____________ bonds
• _____________ electronegative atoms pull electrons _____________
Non-Polar Covalent Bonds
Atoms of pure elements have _____________electronegativity
• H2 and O2
Atoms of compounds may have _____________electronegativity
• Methane – CH4
1 valence electron in H
4 valence electrons in C
Four single covalent bonds
Polar Bonds
Highly electronegative atoms pull electrons closer
e.g. Oxygen
• _____________ electronegative
Water – H2O
1 valence electron in H
6 valence electrons in O
Two single covalent bonds
• Electrons stay _____________ to Oxygen
• Oxygen has _____________ negative charge
• Each hydrogen has _____________ positive charge
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Chapter 2 - The Chemical Context of Life
Ions and Ionic Bonds
When electronegativity is ________________ different
One atom _____________ an electron from it’s partner
• Results in _____________ atoms
_____________: Charged atom or molecule (+ or -)
_____________ _____________: opposite charge ions attracted
• _____________: A negatively charged ion (1+ electrons gained)
• _____________: A positively charged ion (1+ electrons lost)
Hydrogen Bonds
_____________ - Charge Attractions
• H atom with _____________ charge also attracted to another electronegative atom
• Attraction between charged portions of polar molecules
Opposites attract
_____________ bonds
• Important in properties of _____________
Van der Waals Interactions
Electrons in motion sometimes asymmetrically distributed
Changing _____________-_____________ of _____________
Allow molecules and atoms to stick together
Very weak
• Fall apart really fast
Single bonds
Hydrogen
• One valence electron
• Structural formula (bonding shown)
_____________
• Molecular formula (no bonding shown)
_____________
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Chapter 2 - The Chemical Context of Life
Double bonds
Oxygen
• Six valence electrons
• Structural formula (bonding shown)
_____________
• Molecular formula (no bonding shown)
_____________
Formulas
_____________ formula
• Atoms _____________ bonds represented
• e.g. Water: H—O—H
_____________ formula
• _____________ shown (no bonding shown) H2O
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