Lecture 2Atoms and Molecules

Every atom has the same basic structure Core nucleus of protons and neutrons Orbiting cloud of electrons

Atoms

Matter is any substance in the universe that has mass and occupies space

All matter is composed of extremely small particles called atoms

Atoms

Atomic number Number of protons

Atomic mass Number of protons and

neutrons

Element A substance that cannot be

broken down by ordinary chemical means

Isotopes

Isotopes are atoms with the same number of protons but different numbers of neutrons

Different atomic mass

Same atomic number

99% of all carbon

Radioactive isotope dating

Radioactive Decay

The nucleus of an unstable isotope breaks down into particles with lower atomic numbers

Radioactive isotopes are used in 1. Medicine

Tracers are taken up and used by the body

Emissions are detected using special lab equipment

2. Dating fossils The rate of decay of a

radioactive element is constant

The amount of decay can be used to date fossils

Energy

The capacity to do work (put matter into motion)

Types of energy Kinetic – energy in action Potential – energy of position; stored (inactive) energy

Energy ConceptsPLAYPLAY

Electrons have energy due to their relative orbital position (potential energy)

Electrons

Electron shells, or energy levels, surround the nucleus of an atom

Valence shell – outermost energy level containing chemically active electrons

Bonds are formed using the electrons in the outermost energy level

The Octet Rule

Inert elements have their outermost energy level fully occupied by electrons

Reactive elements do not have their outermost energy level fully occupied by electrons

Octet rule – except for the first shell which is full with two electrons, atoms interact in a manner to have eight electrons

in their valence shell

Molecules

A molecule is a group of atoms held together by energy

The holding force is called a chemical bond

There are three kinds of chemical bonds1. Ionic bonds2. Covalent bonds3. Hydrogen bonds

Ionic Bonds

Ionic bonds form between atoms by the transfer of one or more electrons

Ionic compounds form crystals instead of individual molecules Example: NaCl (sodium chloride)

Two key properties1. Strong: But not as strong as covalent bonds2. Not directional: They are not formed between particular ions in the

compound

Ionic Bonds

Covalent Bonds

Water molecules contain two covalent bonds

Covalent bonds are formed by the sharing of two or more electrons

Electron sharing produces molecules

Two key properties

1. Strong: The strength increases with the number of shared electrons

2. Very directional: They are formed between two specific atoms

Comparison of Bonds

Electrons shared equally between atoms produce nonpolar molecules

Electrons shared unequally produces polar molecules

Atoms with six or seven valence shell electrons are electronegative

Atoms with one or two valence shell electrons are electropositive

Hydrogen Bonds

Hydrogen bonding in water molecules

Formed by the attraction of opposite partial electric charges between two polar molecules

Too weak to bind atoms together

Common in dipoles such as water

Responsible for surface tension in water

Important as intramolecular bonds, giving the molecule a three-dimensional shape

Chemical Reactions

Occur when chemical bonds are formed, rearranged, or broken

Are written in symbolic form using chemical equations

Chemical equations contain: Number and type of reacting substances, and products

produced Relative amounts of reactants and products

Patterns of Chemical Reactions

Combination reactions: Synthesis reactions which always involve bond formation

A + B AB

Decomposition reactions: Molecules are broken down into smaller molecules

AB A + B

Exchange reactions: Bonds are both made and brokenAB + C AC + B

All chemical reactions are theoretically reversibleA + B ABAB A + B

If neither a forward nor reverse reaction is dominant, chemical equilibrium is reached

Oxidation-Reduction (Redox) Reactions

Reactants losing electrons are electron donors and are oxidized

Reactants taking up electrons are electron acceptors and become reduced

Generally, the atom that is reduced contains the most energy

Energy Flow in Chemical Reactions

Exergonic reactions – reactions that release energy

Endergonic reactions – reactions whose products contain more potential energy than did its reactants

Factors Influencing Rate of Chemical Reactions

Temperature – chemical reactions proceed quicker at higher temperatures

Particle size – the smaller the particle the faster the chemical reaction

Concentration – higher reacting particle concentrations produce faster reactions

Catalysts – increase the rate of a reaction without being chemically changed

Enzymes – biological catalysts

Hydrogen Bonds Give Water Unique Properties

Water molecules are polar molecules They can thus form hydrogen bonds with each other and with other polar

molecules

Each hydrogen bond is very weak However, the cumulative effect of enormous numbers can make them quite

strong

Hydrogen bonding is responsible for many of the physical properties of water

Heat Storage A large input of thermal energy is required to disrupt the organization of

liquid water This minimizes temperature changes

Ice Formation At low temperatures, hydrogen bonds don’t break Water forms a regular crystal structure that floats

High Heat of Vaporization At high temperatures, hydrogen bonds do break Water is changed into vapor

Water TransportPLAYPLAY

Hydrogen Bonds Give Water Unique Properties

Cohesion Attraction of water molecules to other

water molecules Example: Surface tension

Adhesion Attraction of water molecules to other

polar molecules Example: Capillary action

Water strider

Hydrogen Bonds Give Water Unique Properties

High Polarity Polar molecules are termed hydrophilic

Water-loving All polar molecules that dissolve in water are termed

soluble

Nonpolar molecules are termed hydrophobic Water-fearing These do not form hydrogen bonds and are therefore

not water soluble

Water Ionizes

Covalent bonds within a water molecule sometimes break spontaneously

H2O +OH–

hydroxide ion

H+

hydrogen ion

This process of spontaneous ion formation is called ionization It is not common because of the strength of covalent bonds

A convenient way to express the hydrogen ion concentration of a solution

pH

pH = log [H+]_

The pH scale is logarithmic A difference of one unit

represents a ten-fold change in H+ concentration

Acid: Dissociates in water to increase H+ concentration

Base: Combines with H+ when dissolved in water

Hydrogen ion reservoirs that take up or release H+ as needed

The key buffer in blood is an acid-base pair

Buffers