Chapter 2:The Chemistry of Life

PowerPoint modified from one by Mrs. Fisch

Atom - from Greek “atomos” = unable to be cut

Atomic Structure

• 2 regions:– Nucleus: the center of the atom,

contains nearly all of the mass of the atom

– Electron cloud: region that surrounds the nucleus that contains most of the space in the atom

Nucleus

Electron

Cloud

ATOMIC STRUCTURE

All atoms are made up of subatomic particles called protons, neutrons and

electrons.Particle Charge Mass Location

Proton Positive charge

1 Inside nucleus

Neutron No charge

1 Inside nucleus

Electron Negative charge

Close to 0

Orbiting nucleus

Nucleus

+N

N

+-

-

proton

Electron neutronNeutron

Where are these particles located?

Proton

Electron Cloud

Nucleus

+N

N

+-

-

proton

Electron neutronNeutron

Where are these particles located?

Proton

Electron Cloud

• Atomic number= number of protons in an atom of an element.

• All atoms of an element have the same atomic number.

• Example: Every hydrogen atom has one proton in its nucleus. Hydrogen is assigned the atomic number 1.

A The atomic number of sulfur (S) is 16.

B The atomic number of iron (Fe) is 26.

C The atomic number of silver (Ag) is 47.

How many protons?

A The atomic number of sulfur (S) is 16. Sulfur has 16 protons in its nucleus

B The atomic number of iron (Fe) is 26. Iron has 26 protons in its nucleus

C The atomic number of silver (Ag) is 47. Silver has 47 protons in its nucleus

How many protons?

Atoms are neutral, so each positive charge in an atom is balanced by a

negative charge.

So the atomic number equals the number of electrons.

Mass NumberThe mass number= the sum of the protons + neutrons of that atom.

Question: Aluminum’s atomic number is 13. It’s mass number is 27. How many neutrons does Aluminum have?

Mass NumberThe mass number= the sum of the protons + neutrons of that atom.

Question: Aluminum’s atomic number is 13. It’s mass number is 27. How many neutrons does Aluminum have? Answer: 14

ATOMIC STRUCTURE

the number of protons in an atomHe

2

4 Mass number

Atomic number

number of electrons = number of protons

How many protons?

How many neutrons?

How many electrons?

EXAMPLE

55Cs

1. What is the atomic number? 55Atomic number = # of protons and # of electrons

2. How many protons? 553. How many electrons? 554. What is the mass number? 133Mass number = sum of protons and neutrons

5. How many neutrons? 78 (133 – 55 = 78)

133

EXAMPLE

55Cs

1. What is the atomic number? 55Atomic number = # of protons and # of electrons

2. How many protons? 55 3. How many electrons? 554. What is the mass number? 133Mass number = sum of protons and neutrons

5. How many neutrons? 78 (133 – 55 = 78)

133

element – pure substance that consists of only 1 type of atom.

Isotope – Atoms of an element that have the same number of protons but different numbers of neutrons

RheniumRe

186

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Protons:Electrons:Neutrons:

RheniumRe

186

75

Protons:Electrons:Neutrons:

75

11175

Rhenium IsotopeRe

187

75

Protons:Electrons:Neutrons:

Rhenium IsotopeRe

187

75

Protons:Electrons:Neutrons:

75

11275

Some isotopes are radioactive, meaning that their nuclei are unstable and break down at a constant rate over time

Radioactive isotopes can be used:• to determine the ages of rocks and

fossils.• to treat cancer.• to kill bacteria that cause food to spoil.• as labels or “tracers” to follow the

movement of substances within an organism.

chemical compound - a substance formed by the chemical combination of two or more elements in definite proportions.

• A pure substance always has the same composition.

• 2 Types:–Elements - cannot be

broken down chemically into simpler substances

–Compounds - can be chemically broken down into elements Water is a compound. All the

components are the same—H2O molecules.

• Mixture: a blend of two or more kinds of matter, each of which retains its own identity and properties.

• Homogeneous (solutions): Uniform in composition (Saltwater, air, milk, alloys)

• Heterogeneous: not uniform throughout (can be separated) (pizza, concrete, salad)

Chemical BondsThe atoms in compounds are held

together by chemical bonds.

The electrons that are available to form bonds are called valence electrons.

The main types of chemical bonds are: • ionic bonds •covalent bonds

Ionic Bonds

An ionic bond is formed when one or more electrons are transferred from one atom to another.

These positively and negatively charged atoms are known as ions.

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Covalent BondsSometimes electrons are shared by atoms instead of being transferred.

A covalent bond forms when electrons are shared between atoms.

• single covalent bond - one pair of electrons is shared

• double bond - two pairs are shared• triple bond - three pairs are shared

The structure that results when atoms are joined together by covalent bonds is called a molecule.

1. Hydrogen bonds. Attraction between a hydrogen (slightly positive) and the electrons on another atom (usually oxygen or nitrogen).Examples: water (section 2); DNA and RNA (section 3)

Some weaker types of bonds

www.education.mrsec.wisc.edu

wikipedia

2. Van der Waals ForcesWhen molecules are

close together, a slight attraction can develop between the oppositely charged regions of nearby molecules.

Ex. - molecules in a gecko’s foot attract molecules in a wall

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THE PROPERTIES OF WATER : Water has some unique properties that make this molecule

unlike ANY other. And we are made up of about 70% water!

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SECTION 2. PROPERTIES OF WATERYour body is made up of mostly of water. Approximately 85% of your brain, 80% of your blood and 70% of your muscle is water. Every cell in your body needs water to live.

OHH

_

++

• Water is a polar molecule (slightly charged)– Electrons are unevenly shared in the

covalent bonds (polar covalent bonds). Oxygen is slightly negative, hydrogen slightly positive.

– Allows it to attract charges on other molecules

• Hydrogen bonds between the hydrogen in one molecule and the oxygen of another– Responsible for 3 special properties:

1. Cohesion – an attraction between molecules of the same substance-causes water’s tendency to stick together

http://www.youtube.com/watch?v=s63JXdsL5LU&feature=endscreen&NR=1

Cohesion …

Helps insects walk across water

Surface Tension – water molecules on the surface do not have other molecules on all sides so they stick to the ones closest. Helps keep the surface of the water intact.

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WATER: HIGH SURFACE TENSION

Pressure applied to water surface

“V”-shaped water molecules are held together by hydrogen bonds. The bonds are just strong enough to give water a surface tension with net-like properties.

Hydrogenbond

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2. Adhesion – attraction between water molecules and different substances- capillary action causes water to rise in a narrow tube against the

force of gravity- allows water to rise upward in plants

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WATER: STRONG COHESIVENESS

Because of the cohesive properties of water, trees such as the giant sequoia are able to transport water molecules from the soil to their leaves 300 ft. above.

As each water molecule evaporates, it pulls additional water up through the tree because of the “sticky-ness” of the hydrogen bonds that link the water molecules.

Water molecule released into the atmosphere

300 ft.

Water molecule pulled into root system

Water molecules pulled upward

6-ft.-tall man

3. Water has a high ability to hold or give off heat (specific heat). A large amount of energy is needed to change the temperature.Good for regulating temperature (homeostasis)

Water has a high heat of vaporization. It takes a lot of energy to change from liquid to gas.

This is why sweating can cool you down (heat removed from body to evaporate the sweat)

Water has a high heat of fusion. A lot of energy must be removed to freeze it. This makes it difficult to freeze an organism living in cold water.

Solute – substance dissolved in a solvent to form a solution

Solvent – fluid that dissolves solutes

Example: Ice Tea – water is the solvent and tea and sugar the solutes

Water is a powerful solvent, capable of dissolving a variety of substances

Water can dissolve ions and polar molecules. The slightly negative oxygen atoms are attracted to positive ions and atoms, and the slightly positive hydrogen atoms are attracted to negative ions and atoms.

When a crystal of table salt is placed in warm water, sodium and chloride ions are attracted to the polar water molecules.

Na+

Na+

Cl -

Cl -

WaterWater

Density – water is unusual because it is less dense as a solid (ice) than as a liquid.

Two effects of this:

1. Lakes and streams freeze from the top. The ice is less dense so it stays at the top and insulates the water below. This allows organisms to survive in the water.

2. If a cell freezes, the water will expand, causing the cell to burst. Ex. - vegetables that have frozen and thawed are less crunchy.

Other unique properties of waterOnly substance that exists free in nature in all

three states (solid, liquid, and gas)High boiling pointLow freezing pointViscosity increases as its temperature is

loweredA relatively high density to support animals

with no or delicate skeletal systems

Acids, Bases and pH

Hydrogen Ion Hydroxide Ion Acid Base

H2O H+ + OH-

A water molecule can react to form ions (positively or negatively charged atoms)

pH scale expresses hydrogen ion (H+) concentration in a solution.– ranges from 0 to 14

• neutral = 7 (H+ = OH -)• below 7 = acid (more H+ than OH -)• Above 7 = base (more OH- than H+)

– Each step represents a factor of 10(ex. pH 4 has 10 times higher concentration of H+

than pH 5)

Acid – compound that forms H+ ions in solution.–Sour taste–Strong acids pH 1-3–ex. HCl hydrochloric acid or stomach

acid

• Base – compound that produces OH- ions in solution.–Also called alkaline–Bitter taste–Strong bases have pH 11-14–Ex. NaOH = sodium hydroxide

• An acid releases a hydrogen ion when it dissolves in water.– high H+ concentration– pH less than 7

more acidic

stomach acid pH between 1 and 3

7

• A base removes hydrogen ions from a solution.

– low H+ concentration– pH greater than 7

bile pH between 8 and 9

more basic

8

• A neutral solution has a pH of 7.

pure water pH 7

9

Bases

Acids

pH

Buffers – weak acids or bases that react with strong acids or bases to prevent sharp, sudden changes in pH.–Offer protection from extreme pH levels–Produced naturally by organisms:

• Organisms can’t tolerate much pH change

• Cells function best within a narrow pH range

• Ex. Saliva and blood

Questions

1. As you go up the pH scale (toward 14), are there more or less H+ ions?

2. How many times more acidic is pH 5 compared to pH 7?

3. Would pH 8 be a strong acid, strong base, weak acid or weak base?