Chapter 2 Notes

The Chemical Context of Life

Concept 2.1

Organisms are composed of matter: anything that takes up space or has mass

Element: a substance that cannot be broken down by chemical reactions

Compound: substance consisting of two or more elements in a fixed ratio

Concept 2.1

Life requires about 25 elements4 of those make up 96% of living

matterCarbon, Oxygen, Hydrogen, and Nitrogen

Trace Elements are those that are required in only minute quantities-ex. Iron, iodine

Concept 2.1

Concept 2.2

Atom: smallest unit of matter that still retains the properties of an element

Subatomic particles: Protons (+), Neutrons (0), Electrons (-)

Protons and Neutrons have a mass of 1 dalton. Electrons have no mass

Concept 2.2

Atomic number: # of protonsMass number: sum of protons +

neutronsIsotopes: different atomic forms of

an element.-ex. Carbon-12 (99%), Carbon-13 (1%), Carbon-14 (<1%)

Concept 2.2

C-12 and C-13 are stable. C-14 is unstable, and radioactive. It will decay giving off particles and energy.

Carbon-14 will decay into Nitrogen

Concept 2.2

An atom’s electrons vary in the amount of energy they possess.

Energy: the ability to do work.Potential Energy: energy that

matter stores because of its position or location

Electrons have potential energy because of their position in relation to the nucleus.

(a) A ball bouncing down a flight of stairs provides an analogy for energy levels of electrons

Third shell (highest energylevel)

Second shell (higherenergy level)

Energyabsorbed

First shell (lowest energylevel)

Atomicnucleus

Energylost

Concept 2.2

Concept 2.2

The different states of potential energy that electrons have in an atom are called energy levels or electron shells.- the first shell has the lowest energy. The second shell has more than the first, etc.

Valence electrons: those in the outermost shell

Concept 2.2Hydrogen

1H

Lithium

3LiBeryllium

4BeBoron

5BCarbon

6CNitrogen

7NOxygen

8O

Fluorine

9FNeon

10Ne

Helium

2HeAtomic number

Element symbol

Electron-distributiondiagram

Atomic mass

2He

4.00Firstshell

Secondshell

Thirdshell

Sodium

11NaMagnesium

12Mg

Aluminum

13AlSilicon

14SiPhosphorus

15PSulfur

16S

Chlorine

17ClArgon

18Ar

Atoms will bond with others to gain stability

Covalent bonds: sharing of a pair of valence e- by two atoms-ex. Hydrogen atoms will share their electrons. They become H-H

Concept 2.3

Concept 2.3

Concept 2.3

Electronegativity: the attraction of an atom for the electrons of a covalent bond

Nonpolar: when the electrons are shared equally

Polar: when one atom is bonded to a more electronegative atom

Concept 2.3 –

+ +

H H

O

H2O

Concept 2.3Ionic bonds: when two atoms are

so unequal in their attraction for electrons that one atom will strip the electrons from its partner

Ion: a charged atom; cation has a positive charge, anion has a negative charge (ca+ion; a negative ion)

Compounds formed by ionic bonds are salts

Concept 2.3

Na Cl

NaSodium atom Chlorine atom

Cl

Concept 2.3

Na Cl Na Cl

NaSodium atom Chlorine atom

Cl Na+

Sodium ion(a cation)

Cl–Chloride ion

(an anion)

Sodium chloride (NaCl)

Concept 2.3

Na+

Cl–

The advantage of weak bonding is that the contact can be brief

Hydrogen bonds: H is covalently bonded to an electronegative atom and attracted to another electronegative atom.-ex. Water and ammonia

Concept 2.3

Concept 2.3 +

+

+

+

+

Water (H2O)

Ammonia (NH3)

Hydrogen bond

Concept 2.4

Chemical reactions: making and breaking of chemical bonds.- starting material is reactants- ending material is products

Chemical equilibrium: point at which reactions offset one another.

Concept 2.4