Ch 2

– Life primarily consists of C, H,
O, N
– Rest are present in small amounts called trace elements
Table 2.1

• Essential to life
• Occur in minute amounts
• common additives to food and water
• Deficiencies can cause various physiological conditions

Protons- positive charge
Found in the nucleus
Neutrons- neutral charge
Protons and neutrons occupy the nucleus
Electrons- negative charge
Arranged in electron shells around the nucleus
– –
+
+
2 + Protons
2
2 –
Neutrons
Electrons
Mass number = 4
2e –
+
+
Figure 2.4A
Electron cloud
Nucleus

Atoms of each element always have the same number of protons
Electron cloud
6e –
+ +
Atomic Number=# protons
Nucleus
Mass Number (atomic weight) = Protons + neutrons
6 +
6
6 –
Protons
Neutrons
Electrons
Mass #
= 12
Atoms have no overall charge, thus…
Figure 2.4B

Number of protons (+ charge) must be equal to the number of electrons (-)
For any element:
• Number of Protons = Atomic Number
• Number of Electrons = Number of
Protons = Atomic Number
• Number of Neutrons = Mass Number -
Atomic Number

Krypton
Atomic # = 36
Atomic weight = 84
# Protons=?
# Neutrons=?
# Electrons=?
Kr

Chemical elements combine in fixed ratios to form compounds
Different combinations of atoms determine the unique properties of each compound
Sodium
Figure 2.3
Chlorine Sodium Chloride

The number of neutrons in an atom may vary
• Variant forms of an element are called isotopes
• Some isotopes are radioactive
Table 2.4

Isotope having an unstable nucleus
-nucleus decays emitting particles until stable
-Ex: Tritium (hydrogen 3) strontium 90 phosphorus 32 uranium 238

• Irradiation
– A process of disinfestation, pasteurization or sterilization of food products.
– Bacterial spores and viruses not inactivated by irradiation
– Could still be contaminated by food-service workers after irradiation

• Research
– Radioactive tracers enable researchers to follow particular molecules as they undergo change such as through photosynthesis
• Medical
– Kidney disorders
– Medical Imaging
• PET scans
PET scan image of
Parkinson's diseased brain
Brown.edu 2008

• The active part of the atom
– Only part directly involved in chemical activity
• Interactions between electrons form chemical bonds
• Electron shells
– Arranged in shells, which may contain different numbers of electrons
– Shells are “Full” with either 2 or 8 electrons

Electron arrangement determines the chemical properties of an atom
– Electrons in an atom are arranged in shells, which may contain different numbers of electrons
Outermost electron shell (can hold 8 electrons)
First electron shell (can hold 2 electrons)
Electron
Hydrogen (H)
Atomic number = 1
Figure 2.6
Carbon (C)
Atomic number = 6
Nitrogen (N)
Atomic number = 7
Oxygen (O)
Atomic number = 8

• Valence electron shell
– Outermost shell of electrons
– These electrons interact with other atoms
Outermost electron shell (can hold 8 electrons)
First electron shell (can hold 2 electrons)
Electron
Hydrogen (H)
Atomic number = 1
Figure 2.6
Carbon (C)
Atomic number = 6
Nitrogen (N)
Atomic number = 7
Oxygen (O)
Atomic number = 8

• Atom’s attraction for shared electrons
– Greater the electronegativity, stronger the pull on e-

•Pairs of e- shared between pairs of atoms
–# of bonds is equal to additional enecessary to fill the outer shell
•Polar and Non-polar
Covalent bond Diamond with 4 covalent bonds info.lu.farmingdale.edu/.../atomicbonds.html

• Non-polar covalent- When the atoms exert a equal pull on the e-
– Overall charge of 0
• Polar covalent- when the atoms exert an unequal pull on the e-, bond is
– Overall charge on the more electronegative atom is partially negative and the less electronegative atom is partially positive

(
–) • Unequal e- sharing produces polar bonds
O
(
–)
H
H
(+)
Figure 2.9
Water Molecule
(+)

• Created when an atom gains or looses and electron
• Results in an net positive or negative charge depending on if the atom gained or lost the e-
• Anion has neg chrge
• Cation has positive charge

Attractions between ions of opposite charge
– An electrical attraction between ions with opposite charges results in an ionic bond
+
–
–
Transfer of electron
–
Cl Na Na Cl
Na
Sodium atom
Figure 2.7A
Cl
Chlorine atom
Na +
Sodium ion
Cl
–
Chloride ion
Sodium chloride (NaCl)

• Na has 1 e- in the outer shell
• Cl has 7 e- in the outer shell
• Na “donates” its e- to Cl, leaving Na with only 2 shells and Cl with a full outer shell

The atom is now an ion
– Recall that an ion is an atom or molecule with an electrical charge form either gaining or loosing electrons

• Partial + charge allows shared attractions with – charged molecules
• + charged region always H
( –)
Hydrogen bond
(+)
H
(+)
( –)
(+)
(
–)
O
H
(+)
( –)
Figure 2.10

1. H2O is asymmetric
2. Each of the two covalent bonds are highly polarized
3. All three atoms are adept at forming H bonds

• Cohesion between H
2
0 molecules
– Droplet formation
– Transportation
• Adhesion to substrate
• Surface tension
• Temperature moderation (body and climatic)
– When heated, most thermal E is consumed in disrupting the bonds rather than contributing to molecular motion
• Density
– Ice vs. liquid water

Molecules can be represented in many ways
Table 2.8

• Water is the solvent of life dissolving any charged or polar substances
• Solution
• Aqueous solution
• Solvent
– Dissolving agent
• Solute
– Dissolved substance

• Some molecules disassociate and release
H + or OH -
– pH is measure of H + ions

• A compound that donates H + ions in solution is an acid
– Increases the net H + ions in solution
• A compound that accepts H + ions in solution is a base
– Increases the net OH ions in solution

H +
H +
H +
OH –
OH –
H +
H +
H +
H +
H +
Acidic solution
OH
–
OH –
OH –
H +
OH –
H +
OH
–
H +
H +
H +
Neutral solution
OH –
OH –
OH
–
H + OH
–
OH
– OH –
OH
–
H +
Basic solution
0 pH scale
1
2 Lemon juice, gastric juice
3 Grapefruit juice, soft drink
4 Tomato juice
NEUTRAL
[H + ]=[OH – ]
5
6 Human urine
7 Pure water
Human blood
8
Seawater
9
10
Milk of magnesia
11
12
Household ammonia
Household bleach
13
Oven cleaner
14
Each unit is 10 fold change in concentration of H + ions
• Buffers act to minimize Ph changes
– React with free H+ of OH ions
– Often weak acid/base

• Maintain pH stability by alternately donating and accepting ions

Chemical reactions change the composition of matter reactants products
Figure 2.17A
2 H
2
O
2
2 H
2
O

• Atoms
• http://www.youtube.com/user/greatpacificmedia# p/u/48/pV822HfqT44
• Ionic and Covalent bonding http://www.youtube.com/watch?v=QqjcCvzWww w
• http://www.youtube.com/user/greatpacificmedia# p/u/49/UR4eG60jjQQ
• Hydrogen Bonding
• http://www.youtube.com/watch?v=lkl5cbfqFRM& feature=related
• http://www.youtube.com/watch?v=cgiNk94XyaI