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Chapter 2: Basic Chemistry (Outline)
Chemical elements Atoms Isotopes Molecules and compounds
Covalent and ionic bonding/reactions Ionic and Covalent Hydrogen
Properties of Water Define/describe acids and bases
pH scale and buffers
Chemical Elements
Matter refers to anything that has mass and occupies space Only 92 naturally occurring fundamental types of
matter – 92 Elements
Organisms composed primarily (98%) of only six elements Carbon, Hydrogen, Nitrogen, Oxygen,
Phosphorus and Sulfur
CHNOPS
Elements Composition of Earth’s Crust vs. Organisms
Atomic Structure
Atoms - smallest particles of elements Atoms composed of 3 types of subatomic
particles Protons: positively charged particles
within the nucleus Neutrons: electrically neutral charged
particles also found in nucleus Electrons: negatively charged particles
orbiting the nucleus
Atomic Symbol
Each element represented by unique atomic symbol
One or two letters
First letter capitalized
Superscripted number before: Represents mass number
Number of protons & neutrons in nucleus
Subscripted number before: Represents atomic number
Number of protons in nucleus
MassMassNumberNumber
AtomicAtomicNumberNumber
AtomicAtomicSymbolSymbol
126
Carbon
C
Periodic table Elements grouped in periodic table based on
characteristics Vertical columns = groups; chemically similar Horizontal rows = periods; larger and larger
1
H1.008
3
Li6.941
11
Na22.99
19
K39.10
4
Be9.012
12
Mg24.31
20
Ca40.08
5
B10.81
13
Al26.98
21
Ga69.72
6
C12.01
14
Si28.09
22
Ge72.59
7
N14.01
15
P30.97
23
As74.92
8
O16.00
16
S32.07
24
Se78.96
9
F19.00
17
Cl35.45
25
Br79.90
10
Ne20.18
18
Ar39.95
26
Kr83.60
2
He4.003
II
IIII IIIIII IVIV VV VIVI VIIVII
VIIIVIII
11
22
33
44
Groups
Periods
Isotopes
Isotopes: Atoms of the same element that differ in the number of neutrons. Isotopes of an element have the same chemical properties but different weights
Example:
126
Carbon 12C 13
6
Carbon 13C 14
6
Carbon 14C
Electron Shells, Orbitalsand Energy Levels Atoms normally have as many electrons as
protons Opposite charges balance leaving atom neutral Electrons are attracted to the positive nucleus
Revolve around nucleus in orbitals
Can be pushed into higher orbitals with energy
Release that energy when they fall back to lower orbital
Different energy levels referred to as electron shells
The Octet Rule forDistribution of Electrons
Bohr models show electron shells as concentric circles around nucleus Each shell has two or more electron orbitals
Innermost shell has 2 orbitals
Others have 8 or multiples thereof
Atoms with fewer than 8 electrons in outermost shell are chemically reactive If 3 or less – Tendency is to donate electrons
If 5 or more – Tendency is to receive electrons
Periodic Table
Elements and Compounds
Molecule - two or more atoms react or bond together If all atoms in molecule are of the same element
Material is still an element
O2, H2, N2, etc.
If at least one atom is from a different element
Material formed is a compound
CO2, H2O, C6H12O6, etc.
Characteristics of compounds dramatically different from constituent elements
Chemical Bonding
Bonds between atoms are caused by electrons in outermost shells
The process of bond formation is called a reaction
The intensity of simple reactions can be predicted by the periodic table If two elements are horizontally close in the
table, they usually react mildly
If they are horizontally far apart, they usually react vigorously
Types of Bonds:Ionic Bonding
Octet rule Atoms need 8 electrons in outer shell
If have < 4 outers, desire to donate them If have > 4 outers, desire to receive more
Consider two elements from opposite ends of periodic table Element from right side:
Has 7 electrons in outer shell “Desperately needs” one more (7+1=8)
Element from left side: Has only 1 electron in outer shell “Desperately needs” to donate it (1-1=0=8)
Formation of Sodium Chloride
Types of Bonds:Covalent Bonds
When atoms are horizontally closer together in the periodic table
The electrons are not permanently transferred from one atom to the other like in NaCl
A pair of electrons from the outer shell will “time share” with one atom and then the other
This also causes the atoms to remain together
Known as covalent bonding
Covalently Bonded Molecules
Sometimes two pairs of electrons are shared between atoms – a double covalent bond (e.g. O2 )
Three pairs of electrons – a triple bond (e.g. N2)
Covalently Bonded Molecules
Nonpolar Covalent Bonds
Consider two elements that are equidistant from the edges of the periodic table Atoms will have about equal affinity for electrons
One will “want” to donate electron(s)
The other will “want” to receive electron(s)
When bonded covalently:
The bond electrons will spend about equal time with both atoms
Such covalent bonds are said to be nonpolar
Polar Covalent Bonds
Some atoms attract electrons more strongly than others - electronegativity
Atoms will have unequal affinity for electrons When bonded covalently:
The bond electrons will spend more time with one atom than the other The atom that gets the most time with the
electrons will be slightly negative
The other will be slightly positive
Such covalent bonds are said to be polar
Types of Bonds:Hydrogen Bonds
Water (H2O or H–O–H) is a polar molecule Electrons spend more time with the larger oxygen
atom than the smaller hydrogen atom
H becomes slightly positive and O slightly negative
When polar molecules are dissolved in water The H of water molecules are attracted to the
negative parts of the solute molecules
Results in a weak bond – the hydrogen bond
Easily broken, but many together can be quite strong
One of the most important interactions in biological molecules (e.g. proteins, DNA, etc.)
Water Molecule and Hydrogen Bonding
The Chemistry of Water:Heat Capacity Water has a high heat capacity
Temperature = rate of vibration of molecules When water is heated
Hydrogen bonds restrain bouncing Temperature rises more slowly per unit heat
Thermal inertia – resistance to temperature change Because of hydrogen bonding – takes a large heat
loss or gain for each 1C change in temperature. Hydrogen bonds must absorb heat to break, and
they release heat when they form Keeps temperature fluctuations within a range
suitable for life
Properties of Water:Heat of Vaporization
Water has a high heat of vaporization - the energy required to convert 1g of liquid water to a gas To raise water from 99 to 100 ºC; ~1 calorie
To raise water from 100 to 101 ºC; ~540 calories!
Large numbers of hydrogen bonds must be broken to evaporate water
This is why sweating (and panting) cools Evaporative cooling is best when humidity is low
because evaporation occurs rapidly
Great example is when you get out of the shower!
Properties of Water:Heat of Fusion
Heat of fusion (melting) To raise water from -1 to 0 ºC; ~1 calorie To raise water from 0 to 1 ºC; ~80 calories!
This is why ice at 0 ºC keeps stuff cold MUCH longer than water at 1 ºC
This is why ice is used for cooling NOT because ice is cold But because it absorbs so much heat before it
will warm by one degree
Heat Content of Waterat Various Temperatures
Properties of Water:Water as a Solvent
Solution - liquid that is a completely homogenous mixture of two or more substances Solvent - dissolving agent of a solution Solute - substance dissolved in a solution Hydrophilic
Water loving, property of having an affinity for H2O
Ionic and polar compounds
Hydrophobic Water fearing, not water-soluble
Nonpolar compounds
Ionic compounds dissociate in water
Properties of Water:Cohesion & Adhesion
Cohesion – Hydrogen bonds hold water molecules tightly together
Adhesion – Hydrogen bonds for between water and other polar materials
High Surface Tension A measure of how difficult it is
to stretch or break the surface of a liquid
Allows small nonpolar objects (like water strider) to sit on top of water
Cohesion & adhesion are both important because combined together they form an event called capillary action
Capillary action defies gravity and helps move water up the plant to its leaves
Water as a Transport Medium
Properties of Water:Uniqueness of Ice Frozen water is less dense than
liquid water The density of water:
Prevents water from freezing from the bottom up
Ice forms on the surface first – the freezing of the water releases heat to the water below creating insulation
Makes transition between season less abrupt
pH of Water: Acids
Acids Dissociate in water and release hydrogen ions
[H+] Sour to taste DO NOT TASTE ACIDS IN THE LAB!! Turn litmus red Hydrochloric acid (stomach acid) is an inorganic
acid with symbol HCl In water, it dissociates into H+ and Cl-
Dissociation of HCl is almost total, therefore it is a strong acid
pH of Water: Bases
Bases: Either take up hydrogen ions [H+] or release
hydroxide ions [OH-] Feel soapy or slippery Turn litmus blue Bitter to taste DO NOT TASTE BASES IN THE LAB! Sodium hydroxide is a solid with symbol NaOH
In water, it dissociates into Na+ and OH-
Dissociation of NaOH is almost total, therefore it is a strong base
pH Scale
pH scale used to indicate acidity and alkalinity of a solution Values range from 0-14
Acidic: if pH is less than 7 Basic: if pH is greater than 7 Nuetral: is pH is equal to 7
Logarithmic Scale Each unit change in pH represents a change of 10
times pH of 4 is 10 times as acidic as pH of 5 pH of 10 is 100 times more basic than pH of 8
The pH Scale
Increasing pH means the H+ ions are decreasing Decreasing pH means H+ ions are increasing
Buffers and pH
When H+ is added to pure water at pH 7, pH goes down and water becomes acidic
When OH- is added to pure water at pH 7, pH goes up and water becomes alkaline
Buffers are solutes in water that resist change in pH
When H+ is added, buffer may absorb, or counter by adding OH-
When OH- is added, buffer may absorb, or counter by adding H+
Buffers in Nature
Biological buffer system Human blood normally 7.4 (slightly alkaline) Many foods and metabolic processes add or
subtract [H+] or [OH-] Reducing blood pH to 7.0 results in acidosis
Increasing blood pH to 7.8 results in alkalosis
Both life threatening situations
Bicarbonate ion [-HCO3] in blood buffers pH to 7.4