CHAPTER 2CHAPTER 2

ATOMS, MOLECULES, ATOMS, MOLECULES, AND CHEMICAL BONDSAND CHEMICAL BONDS

Matter

•Chemistry is the study of matter

•Matter takes up space and has mass

•Matter comes in many different forms

Chemical Elements•Cannot be broken down•92 elements occurring in nature•Symbol (different languages) 2 or

3 letters•25 elements are essential to life•C, O, H, N

•Ca, P, K, S

•Fe, I

These are themost common.

Compounds

•Two or more elements combined in a fixed ratio

•More common than pure elements WHY?

•Example: Na reacts?Cl reacts?NaCl reacts?

Sulfuric Acid molecule

Isotopes- different atomic forms of the same element just different # of neutrons Radioactive

isotopes – unstable because the nucleus decays spontaneously by giving off particles and energy

Helpful: biological tracers (plant studies, medicine, PET: positron-emission tomography)

Harmful: damages cellular molecules (DNA)

Half-life – time it takes for 50% of the radioactive atoms to decay

Energy Levels

•Why does an atom behave like it does?

•How much energy will an atom possess?

•Electron shells – different states of potential energy due to position in relation to nucleus (closer to nucleus, lower energy)

Orbital – 3-D shape where an electron is found

How many in the 1st electron shell?

How many in the outermost shell?

So the chemical properties of an atom depend on the number of electrons in the outermost shell.

Hydrogen Bonds

-a hydrogen atom covalently bonds to one electronegative atom and is attracted to another electronegative atom

Ex: water to water

H2O

H2O

Cohesiveness

Cohesion – water molecules stick together because of hydrogen bonding

Surface tension – difficult to stretch or break the surface of water (beads up)

Adhesion – clinging of one type of substance to another type

Hydrophilic – water loving (clings to water)

Capillary action – rising up a tube made of hydrophilic material (adhesion and cohesion together)

Imbibition – water soaks into a porous material

Moderating Temperature

Specific heat – amount of energy it takes to raise 1 Kg of a substance 1oC

--hydrogen bonds allow water to resist quick temperature changes

Heat – amount of energy associated with the movement of atoms and molecules

Temperature – measures intensity of heat (average speed of atoms and molecules)

•Heated water breaks hydrogen bonds

--Molecules move faster and water absorbs a lot of heat (but temp. only warms a few degrees)

•Cooled water reforms hydrogen bonds

--Molecules move slower and water releases a lot of heat (but only cools a few degrees)

•Water requires a lot of heat to completely break all hydrogen bonds of a single water molecule so this decreases water’s tendency to evaporate

--This is what gives water a high boiling point--When water does evaporate (heat energy is carried off), the remaining liquid water is cooled. Ex: sweating

Freezing of Water

•Why is water less dense as a solid?

•What is the significance of 4 degrees Celsius in water?

--So water freezes when its molecules no longer move enough to break the hydrogen bonds

Acids, Bases, and pH

When water molecules bond, the H-bond shifts around from one molecule to another allowing for a Hydrogen Ion and a Hydroxide Ion

Dissociation – separation of a water molecule

• An acid adds [H+] to a solution

• A base reduces [H+] in a solution so now it has more [OH-]The product is the pH scale (potential hydrogen), which compresses the range of [H+] and [OH-] by using logarithms.

• An acid adds [H+] to a solution

• A base reduces [H+] in a solution so now it has more [OH-]The product is the pH scale (potential hydrogen), which compresses the range of [H+] and [OH-] by using logarithms.

7 is neutral, below 7 is acidic, above 7 is basic

Buffers minimize changes in [H+] and [OH-] and are usually a weak acid or a weak base

Ex: Carbonic Acid acts as a buffer in our blood.

Acid Rain

•Below pH 5.6•Caused by a reaction of water

and sulfur oxides and nitrogen oxides in the atmosphere

•Harmful effects on terrestrial and fresh water ecosystems

Chemical Reactions

•The process of making and breaking bonds

•What are the 5 types?

Synthesis

DecompositionSingle Replacement

Double ReplacementCombustion