Chapter 4 - Chemistry

Chapter 3 - Chemistry

Matter

Matter occupies space and has weight.

It can exist as a solid, liquid, or gas.

It may be possible to break some kinds of matter down into other kinds of matter with different properties. For example, water (H2O) can be broken down into hydrogen and oxygen.

Hydrogen and oxygen in the above example cannot be broken down any further because they are elements.

Elements

Elements cannot be broken down into substances with different properties. For example, water (H2O) is not an element because it can be broken down into hydrogen (H) and oxygen (O).

The smallest particle of an element is an atom.

Elements are substances made up of only one kind of atom.

There are 92 naturally occurring elements. Matter is therefore composed of 92 different kinds of elements.

The following elements make up 98% of the body weight of organisms: Carbon, Hydrogen, Oxygen, Nitrogen, Sulfur, Phosphorus.

Atoms

Atoms are composed protons, neutrons, and electrons.

Protons and neutrons are located in a central area called the nucleus.

Electrons move about the nucleus. The number of electrons is equal to the number of protons.

It is more accurate to represent the space occupied by electrons as a cloud. The electrons are likely to be located somewhere within the cloud.

Characteristics of Subatomic Particles

The mass of subatomic particles is too small to be conveniently measured in grams so atomic mass units (amu) are used instead. Atomic mass units are also called daltons. One amu (or dalton) is approximately 1.7 X 10-24g. 

Protons and neutrons have a mass of approximately 1 amu. The mass of an electron is much less. The total mass of an atom is due mostly to the mass of protons and neutrons.

 

mass

 charge 

protons

approx. 1 amu

+

neutrons

approx. 1 amu

0

electrons

  1/1800 amu 

-

Protons and electrons have an electrical charge. Protons have a positive charge and electrons have a negative charge. Particles with positive charges are attracted to particles with negative charges. 

Two particles with the same charge (both positive or both negative) will repel each other.

Atomic Weight

The atomic weight is calculated as the sum of the mass of protons, electrons, and neutrons.

The mass of electrons is small enough that we can generally disregard it in our calculations.

Most of an atom is empty space.

Atomic Number

The atomic number is the number of protons.

All atoms of an element have the same atomic number.

The number of protons contributes to the physical properties of an element.

Atoms are neutral, therefore the number of electrons is equal to the number of protons.

Atomic Symbols

Isotopes

Isotopes are atoms that have the same number of protons and differ only in the number of neutrons. Three different forms of hydrogen are shown below.

Most isotopes are stable but radioactive isotopes are unstable and break down into more stable forms by emitting radiation.

Radiation can be detected, so radioactive isotopes are useful as labels in scientific research and medical diagnostic procedures.

Energy

Energy is the ability to do work.  For example, it takes energy  to move matter.

Potential energy is stored energy.

Electrons at greater distances from the nucleus contain more stored energy.

Example - Many of the chemical reactions that are associated with energy (ex: photosynthesis) involve electrons moving to higher or lower energy levels.

Distribution of Electrons

Electron Shells

The different energy levels are also called electron shells.  The first shell is the K shell, the next is the L, followed by M, N, etc.

Electrons in the K shell have the least energy.  Electrons that are further from the nucleus have more potential energy.

The maximum number of electrons in each shell is given by the formula 2N2 where N is the shell number. From this formula, the maximum number in each shell is 2, 8, 18, 32, etc. The maximum number of electrons in the last (outer) shell is 8.

Orbitals

Shells are subdivided into orbitals. An orbital can hold 2 electrons.

The first shell has one spherical orbital.

The second shell has 4 orbitals. One is spherical and the other three are dumbbell-shaped and at right angles to each other. The arrangement of three dumbbell-shaped orbitals can be in the model shown below.

The Outer Shell

Atoms with a complete outer shell do not react. An atom with only one shell requires two electrons. If an atom has only one shell, it is complete with two electrons, otherwise it is complete with eight electrons.

Atoms with incomplete shells react with others in a way that allows it to complete the outer shell.  Atoms react to give up, receive, or share electrons to produce a completed outer shell.

Chemical bonds form when atoms react to fill their outer shells with electrons.

A compound is two or more elements joined together by chemical bonds.

Periodic Table of the Elements

The periodic table (below) is a table showing the atomic symbol, atomic weight, and atomic number of all of the elements. The elements are arranged according to the number of electrons in their outer electron shell.

A more detailed, interactive periodic table can be seen by clicking the link below.

http://www.dreamwv.com/primer/page/s_pertab.html

Ionic Bonding

Ionic bonding is the transfer of electrons from one atom to another.

Atoms that have lost or gained electrons are called ions. The gain or loss of electrons results in ions having a positive or negative charge.

Example:

A sodium atom (Na) and a chlorine atom (Cl) are shown below. A single circle represents the nucleus (protons and neutrons) of the atoms. Dots represent the electrons. The sodium atom has a total of 11 electrons and one electron in its outer shell. Chlorine has a total of 17 electrons with seven in its outer shell.

From this, we can calculate that the atomic number (number of protons) of sodium is 11 because the number of protons in an atom equals the number of electrons. Refer to the periodic table to check this calculation.

When sodium chloride (NaCl) is formed, one electron from sodium is transferred to chlorine.

Ions have a charge and are written with a plus (+) or a minus (-) sign. For example, calcium loses two electrons to form a calcium ion. The chemical symbol for a calcium ion is therefore Ca++ or Ca+2.

The ions in a compound are attracted to each other due to opposite charges.

The electrical attraction between sodium ions (+) and chloride ions (-) produces the regular arrangement diagramed below.

The crystalline structure of sodium chloride discussed above can be seen in the photograph below.

Ionic bonds are weak and the ions can be separated in water (discussed later).

Activity

Draw a calcium atom. Draw a chlorine atom. Use circles to represent electrons and tell how many protons and neutrons are in the nucleus. Draw calcium chloride.

Covalent Bonds

Covalent bonds form when atoms share electrons.

Hydrogen atoms contain one electron and one proton.

In the diagram below, two hydrogen atoms are bonded by a single covalent bond. The two atoms each share a pair of electrons.

Molecules

Molecules are two or more atoms that have bonded together.  They may be composed of atoms of the same element or of different elements.

A molecule is the smallest part of a compound that still has the properties of that compound.

Example: Methane

Carbon needs four electrons, each hydrogen needs one.

Below: CH4

The shorthand method for writing methane is CH4. It may also be written as shown on the right side of the diagram above.

Double and Triple Bonds

In a double bond, 2 atoms share 2 pairs of electrons (4 electrons).

In a triple bond, 2 atoms share 3 pairs of electrons (6 electrons).

Double and triple bonds are stronger than single bonds.

A Shorthand Method for Drawing Covalent Bonds

Straight lines can be used to represent a covalent bond between two atoms. A single line is used to represent a single bond, two lines are used to represent a double bond and three lines represent a triple bond. Some single, double, and triple bonds are shown below.

Polar Covalent Bonds

When the atoms share the electrons equally, the molecule is nonpolar.

Polar molecules are formed when two different size atoms are covalently bonded. The molecule is polar because the electrons are not shared equally between the two atoms; the electrons spend more time with the larger atom.

In the drawing below, Hydrogen shares one pair of electrons with chlorine by a single covalent bond. The electrons are not shared equally because chlorine is much larger than hydrogen.

Shading is used in the drawing below to indicate where the shared electrons are found. The dark area around the chlorine atom indicates that the electrons are found most often with the chlorine atom. This results in a partial positive charge on the hydrogen end of the molecule and a slight negative charge on the chlorine end. 

A water molecule (H2O) is an example of a polar molecule formed by polar covalent bonds.  Unequal sharing of electrons results in oxygen being negatively charged and the hydrogens being positively charged.

Hydrogen Bonds

Hydrogen bonds are an attraction between hydrogen (positively charged due to its small size) and the negative charge on another molecule.

Hydrogen bonds are weak.

The drawing below shows hydrogen bonds between water molecules..

Activity

Draw two atoms that are bonded by a single covalent bond. Draw 2 atoms bonded by a double covalent bond and 2 bonded by a triple covalent bond. Finally, draw two atoms that are bonded by a polar covalent bond. Use any hypothetical atoms. After your diagrams are complete, identify the atom. Use the periodic table if necessary.

Chemical Equations

The chemical equation for producing water from hydrogen and oxygen is:

2H2 + O2   2H2O

In chemical equations, reactants are written on the left and products on the right. In the equation above, hydrogen (H2) and oxygen (O2) are reactants; water (H2O) is the product.

The equation below (bottom of diagram) is not balanced because the number of atoms on the left side of the arrow is not equal to the number on the right side.

In a balanced chemical equation, the number of atoms of each element on the left side of the equation is the same as the number on the right.

Important Concepts Regarding Covalent Bonds

Energy and Covalent Bonds

Energy is required to form a covalent bond and energy is released when a Covalent bond is broken.  Covalent bonds can therefore be used to store energy.

Oxidation-Reduction Reactions

Oxidation is the loss of elecrons or the loss of hydrogen atoms from an atom or molecule. As a result, energy is released.

Reduction is the gain of electrons or the gain of hydrogen atoms. This process stores energy.

Oxidation and reduction occur together. When a atom or molecule is oxidized, another must be reduced.

Example: Na + Cl   Na+Cl-  - The Na is oxidized; the Cl is reduced.

Electrons (or H) function as energy carriers. An atom or molecule that is reduced (gains electrons or hydrogen atoms) also gains energy as a result of the oxidation. Similarly, oxidation is associated with the loss of energy.

Reactions in which one atom or compound is reduced and another is oxidized are called redox reactions.

Water

Life evolved in water. Living things are 70-90% water. In nature, water is a solvent for many kinds of chemical reactions.

Water is a polar molecule because the hydrogen atoms are much smaller than the oxygen atoms and also because the molecule is arranged so that the two hydrogen atoms are on one side while the oxygen is on the other.

 

Oxygen has six electrons in its outer energy level. Two of the orbitals are filled; they have two electrons each. The other two have only one electron each. These two partially- filled orbitals are drawn at the right.

One hydrogen bonds to each of the unfilled orbitals. Notice that they form a 90-degree angle.

When hydrogen bonds to oxygen, however, the positive charge on hydrogen repels the other, pushing the hydrogen atoms apart to approximately 105 degrees.

Ions Separate (Dissolve) in Water

Because water is polar, it bonds to ions and polar molecules. The attraction between water molecules and ions may be strong enough to separate the ions, causing the ions to become suspended (dissolved) in the water.

Below: Note that the orientation of the water molecules is depends on the charge of the ion.

Water molecules adhere to polar molecules.

Water tends to form hydrogen bonds with polar molecules including other water molecules.

The meniscus shown below forms when water adheres to the sides of the glass container.

Its ability to flow freely while hydrogen-bonded to other molecules and ions makes it an excellent transport medium.

The temperature of water changes more slowly than that of other liquids.

Water resists changing temperature. Compared to other substances, it takes a relatively large amount of heat to raise the temperature of water. Normally, adding heat energy to a liquid causes increased motion of the molecules. The hydrogen bonds between water molecules, however, cause resistance to increased motion; additional heat energy is needed to break the bonds. Similarly, a large amount of heat is released as water cools.

It takes 1 calorie of heat to raise the temperature of 1 gram of water 1° C. To raise the temperature of an equal volume of air 1° C takes 0.0003 calories. Water requires 3000 times more energy than air.

This property protects organisms from rapid temperature changes.

Water resists changing to a solid or a gas.

In order to change 0° water to ice without changing the temperature, 80 calories/gram of heat energy must be removed from the water.  Once frozen, removing additional calories will lower the temperature of the ice.

Similarly, 540 calories/gram must be added to change 100° water to water vapor without changing the temperature. Once vaporized, additional calories will raise the temperature of the vapor.

Ice is less dense than water.

Water is most dense at 4 degrees and as it warms, it becomes less dense due to increased molecular motion associated with temperature increases.

Hydrogen bonds  hold water molecules farther apart than they would be without the bonds. The normal motion of liquid water molecules causes some of the hydrogen bonds between water molecules to break, enabling them to become packed closer together. As water gets colder than 4 degrees C, there is less movement of the molecules and therefore less breaking of hydrogen bonds. Increased hydrogen bonding results in a greater average distance between water molecules. The water becomes less dense because a given volume contains fewer molecules. As ice forms, the number of hydrogen bonds becomes maximal (4 per water molecule) causing ice to be less dense than water.

Ice floats on water because it is less dense than water. Other compounds are more dense at lower temperatures and the solid form does not float on the liquid form.

Ionization of Water

Water tends to ionize: H2O   H+ + OH-

It forms equal numbers of hydrogen ions and hydroxyl ions.

The process is reversible; hydrogen and hydroxyl ions can combine to form water.

Acids and Bases

Some molecules form ions when they are dissolved in water. For example, the HCl molecule comes apart (it dissociates) and produces H+ and Cl-. The electron that was normally with the H remains with the Cl. The H now has a positive charge because it no longer has an electron. Similarly, the Cl has a negative charge because it has the electron from the H atom.

HCl    H+ + Cl-

Acids are substances that dissociate to produce hydrogen ions and a negative ion. HCl is therefore an acid.

A hydrogen atom that has lost an electron becomes a hydrogen ion (H+). Because a hydrogen atom normally has only one electron and one proton, a hydrogen ion is a proton. The words "hydrogen ion" and "proton" can therefore be used interchangeably. The word "proton" can be abbreviated "H+."

NaOH dissociates when it is dissolved in water. An electron from the sodium atom remains with the OH. This produces a sodium ion (Na+) and a hydroxyl ion (OH-). is a base because when it dissociates, OH ions are produced.

NaOH    Na+ + OH-

Bases are substances that combine with hydrogen ions, thus lowering the concentration of hydrogen ions. NaOH (above) is a base because OH- is capable of combining with H+ to form water. Bases are therefore proton acceptors.

OH-  +  H+ 

Most bases dissociate to produce OH-.

Water molecules have a slight tendency to dissociate, forming both H+ and OH- as shown below. Water is neutral when it ionizes because the number of H+ equals the number of OH-.

H2O    H+ + OH-

pH

The measure of the strength of an acid or base is called the pH. It is a measure of the concentration of hydrogen ions (H+).

The pH ranges from 0 to 14. An acid has a pH less than 7. A base has a pH greater than 7. A pH of 7 is neutral.