The Chemical Basis of Life Chapter 2 Elements, Atoms, and Compounds
Living organisms are composed of matter, which is anything that occupies space and has mass (weight).
Matter is composed of chemical elements.
An element is a substance that cannot be broken down to other substances by ordinary chemical means.
There are 92 elements in nature—only a few exist in a pure state.
Of the 92 naturally occurring elements, which are essential for life?
Humans need 25 elements
Only four elements ---
oxygen, carbon, hydrogen and nitrogen make up 96.3% of all living matter
Trace elements are essential but are only needed in minute quantities.
A compound is a substance consisting of two or more different elements in a fixed ratio.
Compounds are more common than pure elements.
Sodium chloride, table salt, is a common compound of equal parts of sodium (Na) and chlorine (Cl).
Atoms consist of protons, neutrons, and electrons
Each element consists of one kind of atom.
An atom is the smallest unit of matter that still retains the properties of an element.
Three subatomic particles in atoms are relevant to our discussion of the properties of elements.
Protons are positively charged.
Electrons are negatively charged.
Neutrons are electrically neutral.
Neutrons and protons are packed into an atom’s nucleus.
Electrons orbit the nucleus.
The negative charge of electrons and the positive charge of protons keep electrons near the nucleus.
What makes the atoms of different elements unique?
All atoms of a particular element have the same number of protons.
The number of protons is the atom’s atomic number.
An atom’s mass number is the sum of the number of protons and neutrons in the nucleus.
The atomic mass is approximately equal to its mass number.
Although all atoms of an element have the same atomic number, some differ in mass number.
Different isotopes of an element have the same number of protons, but different numbers of neutrons.
Different isotopes of an element behave identically in chemical reactions.
Carbon’s atomic number (the number of protons) is 6.
Carbon-12 (named for its mass number, the sum of protons and neutrons in the nucleus) accounts for about 99% of naturally occurring carbon.
Carbon-13 has 7 neutrons and carbon-14 has 8 neutrons.
Carbon-14 is an unstable radioactive isotope, its nucleus decays spontaneously and gives off particles and energy.
Chemical Bonds
The distribution of electrons determines an atom’s chemical properties
Of the three subatomic particles—protons, neutrons, and electrons—only electrons are directly involved in the chemical activity of an atom.
Electrons can be located in different electron shells, each with a characteristic distance from the nucleus.
An atom may have one, two, or more electron shells.
Information about the distribution of electrons is found in the periodic table of the elements
Helium has two electrons in its electron shell
An electron distribution model of carbon
The electron distribution diagrams of the first 18 elements in the periodic table
The number of electrons in the outermost shell, called the valence shell, determines the chemical properties of the atom.
Atoms whose outer shells are not full tend to interact with other atoms in ways that enable them to complete or fill their valence shells.
When two atoms with incomplete outer shells react, each atom will share, donate, or receive electrons, so that both partners end up with completed outer shells.
These interactions usually result in atoms staying close together, held by attractions called chemical bonds.
Covalent bonds join atoms into molecules through electron sharing
In a covalent bond, two atoms, each with an unpaired electron in its outer shell, actually share a pair of electrons.
Two or more atoms held together by covalent bonds form a molecule.
A covalent bond connects two hydrogen atoms in a molecule of the gas H2.
There are four alternative ways to represent common molecules.
The hydrogen atoms in H2 are held together by a pair of shared electrons.
In an oxygen molecule (O2), the two oxygen atoms share two pairs of electrons, forming a double bond, indicated in a structural formula by a pair of lines.
H2 and O2 are molecules composed of only one element.
Methane (CH4) and water (H2O) are compounds, substances composed of two or more different elements.
Atoms in a covalently bonded molecule continually compete for shared electrons.
The attraction (pull) for shared electrons is called electronegativity.
More electronegative atoms pull harder.
In molecules of only one element, the pull toward each atom is equal, because each atom has the same electronegativity.
The bonds formed are called nonpolar covalent bonds.
Water has atoms with different electronegativities.
Oxygen attracts the shared electrons more strongly than hydrogen.
So the shared electrons spend more time near oxygen.
The oxygen atom has a slightly negative charge and the hydrogen atoms have a slightly positive charge.
The result is a polar covalent bond.
Ionic bonds are attractions between ions of opposite charge
An ion is an atom or molecule with an electrical charge resulting from the gain or loss of one or more electrons.
When an electron is lost, a positive charge results: cation
When an electron is gained, a negative charge results: anion
Two ions with opposite charges attract each other.
When the attraction holds the ions together, it is called an ionic bond.
Salt is a synonym for an ionic compound.
Hydrogen bonds are weak bonds important in the chemistry of life
In living organisms, most of the strong chemical bonds are covalent, linking atoms to form a cell’s molecules.
Crucial to the functioning of a cell are weaker bonds within and between molecules.
One of the most important types of weak bonds is the hydrogen bond, which is best illustrated with water molecules.
Recall that the hydrogen atoms of a water molecule are attached to oxygen by polar covalent bonds.
Because of these polar bonds and the wide V shape of the molecule, water is a polar molecule—that is, it has an unequal distribution of charges.
This partial positive charge allows each hydrogen to be attracted to a nearby atom that has a partial negative charge.
Weak hydrogen bonds form between water molecules.
Each hydrogen atom of a water molecule can form a hydrogen bond with a nearby partially negative oxygen atom of another water molecule.
The negative (oxygen) pole of a water molecule can form hydrogen bonds to two hydrogen atoms.
Thus, each H2O molecule can hydrogen-bond to as many as four partners.
Chemical reactions make and break chemical bonds
Remember that the structure of atoms and molecules determines the way they behave.
Atoms combine to form molecules.
Hydrogen and oxygen can react to form water:
2 H2 + O2 2 H2O
The formation of water from hydrogen and oxygen is an example of a chemical reaction.
The reactants (H2 and O2) are converted to H2O, the product.
Chemical reactions do not create or destroy matter.
Chemical reactions only rearrange matter.
Water’s Life-Supporting Properties
Hydrogen bonds make liquid water cohesive
The tendency of molecules of the same kind to stick together is cohesion.
Cohesion is much stronger for water than for other liquids.
Most plants depend upon cohesion to help transport water and nutrients from their roots to their leaves.
The tendency of two kinds of molecules to stick together is adhesion.
Cohesion is related to surface tension—a measure of how difficult it is to break the surface of a liquid.
Hydrogen bonds give water high surface tension, making it behave as if it were coated with an invisible film.
Water striders stand on water without breaking the water surface.
Water’s hydrogen bonds moderate temperature
Thermal energy is the energy associated with the random movement of atoms and molecules.
Thermal energy in transfer from a warmer to a cooler body of matter is defined as heat.
Temperature measures the intensity of heat—that is, the average speed of molecules in a body of matter.
Heat must be absorbed to break hydrogen bonds.
Heat is released when hydrogen bonds form.
To raise the temperature of water, hydrogen bonds between water molecules must be broken before the molecules can move faster. Thus,
when warming up, water absorbs a large amount of heat and
when water cools, water molecules slow down, more hydrogen bonds form, and a considerable amount of heat is released.
Earth’s giant water supply moderates temperatures, helping to keep temperatures within limits that permit life.
Water’s resistance to temperature change also stabilizes ocean temperatures, creating a favorable environment for marine life.
When a substance evaporates, the surface of the liquid that remains behind cools down; this is the process of evaporative cooling.
This cooling occurs because the molecules with the greatest energy leave the surface.
Ice floats because it is less dense than liquid water
Water can exist as a gas, liquid, or solid.
Water is less dense as a solid than a liquid because of hydrogen bonding.
When water freezes, each molecule forms a stable hydrogen bond with its neighbors.
As ice crystals form, the molecules are less densely packed than in liquid water.
Because ice is less dense than water, it floats.
Water is the solvent of life
A solution is a liquid consisting of a uniform mixture of two or more substances.
The dissolving agent is the solvent.
The substance that is dissolved is the solute.
An aqueous solution is one in which water is the solvent.
Water’s versatility as a solvent results from the polarity of its molecules.
Polar or charged solutes dissolve when water molecules surround them, forming aqueous solutions.
Table salt is an example of a solute that will go into solution in water.
The chemistry of life is sensitive to acidic and basic conditions
In liquid water, a small percentage of water molecules break apart into ions.
Some are hydrogen ions (H+).
Some are hydroxide ions (OH–).
Both types are very reactive.
A substance that donates hydrogen ions to solutions is called an acid.
A base is a substance that reduces the hydrogen ion concentration of a solution.
The pH scale describes how acidic or basic a solution is.
The pH scale ranges from 0 to 14, with 0 the most acidic and 14 the most basic.
Each pH unit represents a 10-fold change in the concentration of H+ in a solution.
A buffer is a substance that minimizes changes in pH.
Buffers accept H+ when it is in excess and
Buffers donate H+ when it is depleted.