Chapter 2

BiologySixth Edition

Raven/Johnson

(c) The McGraw-Hill Companies, Inc.

Matter is anything that takes up space and has weight.

All matter, living or nonliving, is made up of elements

Elements contain atoms, the smallest unit of matter that can enter chemical reactions.

Atoms have a central nucleus made up of protons (+) and neutrons (o), and shells around the nucleus in which electrons (-) orbit.

Fig 2.2

The atomic number = number of protons (# of protons determines the element).

• protons bear a positive electrical charge

The atomic weight = number of protons plus number of neutrons.

• neutrons bear no electrical charge • electrons bear a negative electrical charge

An electrically neutral atom means:

number of protons = number of electrons

Atoms have an atomic symbol, atomic number, and atomic mass.

Some atoms differ in their number of neutrons and are called isotopes.

Carbon has 3 isotopes:

• Carbon 12 (most abundant)

• Carbon 13

• Carbon 14 (radioactive - unstable)

Which has more mass?

Fig 2.3

Review of an Atom

1

2

3

4

5

67

8

9

10

2 e- in inner shell8 e- in outer shell10 e- TOTAL

10 neutrons10 protons20 atomic mass units

Element = Ne (Neon)

Atomic number 10

NeAtomic mass 20# of protons determine element

Because an atom’s electrons are relatively far from the nucleus, the nuclei of two atoms never come close enough in nature to interact with each other. Therefore, an atoms electrons determine its chemical behavior.

Chemical Behavior of Atoms

Electron Orbital vs. Energy Level

• Electron Orbital – The area around a nucleus that an electron is most likely to be found.

• Electron Energy Level – The potential energy of an electron is directly related to the distance between an electron and the nucleus.– Energy – the ability to do work

Fig 2.4

Only 2 electrons per orbital

Fig 2.5

K only has ‘s’ orbital (1s)

L and M has ‘s’ and ‘p’ orbitals (L: 2s and 2p; M: 3s and 3p)

Fig 2.7

Elements are arranged in a periodic table

by atomic number and # electrons in outer shell:

Outer Shell Full1 Electron in Outer Shell

Distribution of the Elements Fig 2.8

Important Atom Characteristics

• Electrons in the outermost energy level are called valence electrons.– These are the electrons that determine

the atoms chemistry

• Octet rule (rule of eight) – For most atoms important to life, the outer energy level can contain no more than eight electrons.

Oxidation – The loss of an electron (atom becomes electrically positive)

Reduction – The gain of an electron (atom becomes electrically negative)

Fig 2.6

Chemical Bonds Build Molecules

• Bonds between atoms can either be ionic, covalent, or hydrogen.

• Ionic and covalent bonds are the result of interactions among electrons.

• Hydrogen bonds are attractions between weakly positive and weakly negatively charged particles.

In ionic bonding, atoms either give up or accept electrons, resulting in ions (electrically charged atoms/non-neutral). Ions with opposite charges (- or +) are attracted to each other and form an ionic bond. (Opposite charges attract, + is attracted to -, - is attracted to +).

Fig 2.9

In covalent reactions, atoms share electrons, resulting in covalent bonds.

Fig 2.10

7 e- inoutershells

2 shared e- to complete both outer shells

Covalent Bonds-build stable molecules

Aside from single covalent bonds, double, or triple covalent bonds can form.

Which is stronger?

Why is a molecule formed by covalent bonds stable?

• It has not net charge

• The octet rule is satisfied

• It has no free electrons

Fig 2.12

Fig 2.13

Important Properties of Water

• Cohesion (liquid) / Adhesion (polarity)• High Specific Heat (maintain temperature)• High Heat of Vaporization (cools body)• Lower Density of Ice (water bodies)• High Polarity / Water as a Solvent

Fig 2.14

Fig 2.15

Fig 2.16

Fig 2.17

Water Ionizes

• Water dissociates and releases hydrogen ions (H+) and hydroxide ions (OH-).

• Concentrations of hydrogen ions or hydroxide ions can be represented using the pH scale.

moles/liter1 x 10 –6 [H+] = pH 6 1 x 10 –7 [H+] = pH 7 1 x 10 –8 [H+] = pH 8

*A mole is the atomic weight of a molecule weighed out in grams. Atomic weight of H+ is one.

pH Concentration of H+

pH 0 = 1x10-0 1pH 1 = 1x10-1 0.1pH 2 = 1x10-2 0.01pH 3 = 1x10-3 0.001pH 4 = 1x10-4 0.0001pH 5 = 1x10-5 0.00001pH 6 = 1x10-6 0.000001pH 7 = 1x10-7 0.0000001pH 8 = 1x10-8 0.00000001pH 9 = 1x10-9 0.000000001pH 10 = 1x10-10 0.0000000001pH 11= 1x10-11 0.00000000001pH 12 = 1x10-12 0.000000000001pH 13 = 1x10-13 0.0000000000001pH 14 = 1x10-14 0.00000000000001

• Acids are molecules that release hydrogen ions in solution (usually water).

HCl H+ + Cl-

•Bases are molecules that either take up hydrogen ions or give off hydroxide ions in solution.

NaOH Na+ + OH-

Hydroxide ion

Fig 2.18

• Buffers are substances that help to resist change in pH. Buffers will take-up/remove excess hydrogen ions (H+) or hydroxide (OH-) ions.

When hydrogen ions are added to solution the following reaction occurs:

H+ + HCO3¯ H2CO3

When hydroxide ions are added to solution the following reaction occurs:

OH ¯ + H2CO3 HCO3¯ + H2O

Fig 2.20

Fig

2.19