The Chemistry of Life

Some Simple Chemistry, Water

Topics

• The Structure of Atoms• The Octet Rule• Isotopes and Ions• Chemical Bonds• Water• Acids and bases• The pH Scale• Buffers

The Structure of AtomsSome Definitions

• MatterMatter is anything that occupies space. is anything that occupies space.

• ElementElement is matter composed of atoms of is matter composed of atoms of only one kindonly one kind

• AtomsAtoms are the smallest particles into are the smallest particles into which an element can be divided. For which an element can be divided. For example, carbon is an element composed example, carbon is an element composed of only carbon atoms.of only carbon atoms.

An atom is made up of 3 subatomic An atom is made up of 3 subatomic particles: protons, neutrons, and electrons.particles: protons, neutrons, and electrons.

• Protons:Protons: have a positive electrical charge. have a positive electrical charge. The number of protons in each atom isThe number of protons in each atom is uniqueunique. The proton number is called the . The proton number is called the atomic numberatomic number. Carbon has 6 protons . Carbon has 6 protons (only carbon has 6 protons), nitrogen – 7, (only carbon has 6 protons), nitrogen – 7, oxygen – 8, sodium – 11, chlorine – 17 oxygen – 8, sodium – 11, chlorine – 17 ….etc.….etc.

• NeutronsNeutrons: have no electrical charge. : have no electrical charge. Protons and neutrons in an atom form the Protons and neutrons in an atom form the a central nucleus. The sum of both a central nucleus. The sum of both numbers is the numbers is the atomic massatomic mass (weight). (weight). Carbon – C has 6 protons and 6 neutrons, Carbon – C has 6 protons and 6 neutrons, so the atomic mass is 12. Nitrogen – N 7 so the atomic mass is 12. Nitrogen – N 7 and 7 – 14, sodium Na 11 and 12 – 23, and 7 – 14, sodium Na 11 and 12 – 23, chlorine Cl 17 and 18 – 35.chlorine Cl 17 and 18 – 35.

• ElectronsElectrons: have a negative electrical : have a negative electrical charge. charge. The number of electrons in anThe number of electrons in an atom is equal to the number of protons.atom is equal to the number of protons. For example, carbon 6 protons and 6 For example, carbon 6 protons and 6 electrons, oxygen 8 and 8, sodium 11 and electrons, oxygen 8 and 8, sodium 11 and 11, chlorine 17 and 17.11, chlorine 17 and 17.

Electrons determine the chemical behavior of atoms

• Electrons are associated with energy

– electrons have energy of position, called potential energy

– the field of energy around an atom is arranged as levels called electron shells

Electron shells have specific numbers of orbitals that may be filled with electrons

– atoms that have incomplete electron orbitals tend to be more reactive

– atoms will lose, gain, or share electrons in order to fill completely their outermost electron shell

– these actions are the basis of chemical bonding

as electrons move to a lower energy level, closer to the nucleus, energy is released

moving electrons to energy levels farther out from the nucleus requires energy

Distribution of ElectronsThe Octet Rule

• The shell nearest the nucleus can hold either one electron or two.

• The second shell can accommodate up to 8 electrons, and will be filled before any electrons appear in the third shell (which also becomes stable when it contains 8 electrons) and so on in subsequent shells.

Distribution of Electrons

Element Symbol # of electrons 1st shell 2nd 3rd 4th

Hydrogen H 1 1 - - -

Carbon C 6 2 4 - -

Nitrogen N 7 2 5 - -

Oxygen O 8 2 6 - -

Neon Ne 10 2 8 - -

Sodium Na 11 2 8 1 -

Chlorine Cl 17 2 8 7 -

Calcium Ca 20 2 8 8 2

Isotopes

Atoms that have the same number of protons but different numbers of neutrons. Example:carbon-13 and carbon-14

Ions

• Ions – atoms that have gained or lost one or more electrons.

• When sodium loses one electron it becomes Na+ because it has 11 positive protons and 10 negative electrons.

• When chlorine gains one electron it becomes Cl- because it has 17 positive protons and 18 negative electrons.

Chemical Bonds

– the energy holding two atoms together is called a chemical bond

– there are 3 principal types of chemical bonds1. ionic

2. covalent

3. hydrogen

Ionic Bonds

• Ionic bonds involve the attraction of opposite electrical charges

• Example: the bond holding sodium and chlorine in sodium chloride (salt)

Covalent Bonds

- Form between two atoms when they share electrons– the number of electrons shared varies

depending on how many the atom needs to fill its outermost electron shell

– covalent bonds are stronger than ionic bonds

Water molecules contain two covalent bonds

Hydrogen Bonds

• Hydrogen bonds are weak bonds that form due to covalent bonds where one nucleus attracts the shared electrons more than another nucleus

• Hydrogen bonds form in association with polar molecules– each atom with a partial charge acts like a

magnet to bond weakly to another polar atom with an opposite charge

Water molecules stick to each other by hydrogen bonds

Hydrogen Bonds Give Water Unique Properties

• Water is the cradle of life. • Water is essential for life

– the chemistry of life is water chemistry

• Water is a polar molecule

– water can form hydrogen bonds– hydrogen bonding confers on water many different

special properties

• Water is a compound made of two hydrogen atoms and one oxygen atom (H2O).

• Hydrogen bonds cause the attraction of water molecules.

• Water has a high heat capacity.

• Water freezes at 0 C.

• Water boils at 100 C.

• Water is a solvent. ……

• Evaporating water has a cooling effect.

• Many of the substances that dissolve in water also pollute it.

• Solid water (ice) is less dense than liquid water.

• Ice floats on water.

• Water molecules are sticky– cohesion – when one water molecule is

attracted to another water molecule– adhesion – when polar molecules other than

water stick to a water molecule

Acids

• Acid – any substance that dissociates in water and increases the [H+]– acidic solutions have pH values below 7– acid is a H+ donor– Hydrochloric acid HCl is an acid. Why?

Bases

• Base – any substance that combines with [H+] when dissolved in water– basic solutions have pH values above 7– base is a H+ acceptor– Sodium hydroxide NaOH is a base. Why?

The pH Scale

pH is an abbreviation for“power of hydrogen”

The pH scale ranges from 1 to 14

Water has the neutral value of 7

Below 7 solutions are more and more acidic. Ex. a solution

with a pH of 4 is more acidic than a solution with a pH of 5

Above 7 solutions are more and more basic. Example, a solution

with a pH of 9 is more basicthan a solution with a pH of 8

The pH Scale

• Each unit on the pH scale represents a 10-fold change in H+ concentration.

• This means that a solution with a pH of 4 has 10 times the concentration of H+ present in one with a pH of 5, and 100 times present in one with a pH of 6, and so on.

Buffers

• Buffer is a substance that acts as a reservoir for hydrogen ions, donating them to the solution when their concentration falls and taking them from the solution when the H+ concentration rises.

• Buffers keep pH of blood constant between 7.35 and 7.45