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 by chemical reactions. For example, water (H₂O) is not an element because it can be broken down into hydrogen (H) and oxygen (O).

Substances that are composed of two or more different elements are called compounds. For example, water is a compound because it is composed of hydrogen and oxygen.

The smallest particles of an element that have the characteristics of that element are atoms.

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 96% of the body weight of organisms: Oxygen, Carbon, Hydrogen, Nitrogen.

Atoms

An atom is composed of subatomic particles. Three important kinds of subatomic particles are protons, neutrons, and electrons. Some atoms (ex: hydrogen) do not have neutrons.

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 measured in units called atomic mass units (amu) because grams are too large.

The mass of an electron is much less than that of a proton or a neutron. The total mass of an atom is due mostly to the mass of protons and neutrons.

	mass	charge
protons	1 amu	+1
neutrons	1 amu	0
electrons	1/1836 amu	-1

Charge is a state in which particles are either attracted to each other or they repel each other. Two particles that are attracted to each other have opposite charges (positive and negative). Particles that repel each other have the same charge; they are both either positive or they are both negative.

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.

Atoms are neutral. The number of electrons (negatively charged) is equal to the number of protons (positively charged), therefore the overall charge is zero.

Atomic Mass and Mass Number

The mass of electrons is small enough that we can generally disregard it in our calculations of mass. The mass number is the number of protons and neutrons.

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.

Neutrons

Atoms of an element can vary in the number of neutrons that they contain. Three different forms of hydrogen are shown below.

Electron Shells

Electrons are distributed in spaces surrounding the nucleus called electron shells.

The maximum number of electrons in the first shell is 2. The maximum number in the 2nd shell is 8. Other shells may contain more electrons but the maximum number of electrons in the last (outer) shell is 8.

The Outer Shell

The inner shells of atoms are filled with the maximum number of electrons but the outer shells may or may not be filled with the maximum number.

An atom with only one shell requires two electrons to complete its outer shell. Atoms with more than one shell require 8 electrons to complete their outer shells. 

Periodic Table of the Elements

Letters are used to represent atoms. The letters are called atomic symbols. For example, the symbol for carbon is C. The symbol for sodium is Na. 

The periodic table (below) is a table showing the atomic symbol, atomic mass, and atomic number of all of the elements. The elements are arranged from left to right according to their atomic number. Elements in the first seven rows are also arranged by the number of electron shells. Elements in the first row have one shell, those in the second row have two shells etc.

1 H 1.008

1 H 1.008

2 He 4.003

3 Li 6.939

4 Be 9.012

5 B 10.81

6 C 12.01

7 N 14.01

8 O 16.00

9 F 19.00

10 Ne 20.18

11 Na 22.99

12 Mg 24.31

13 Al 26.98

14 Si 28.09

15 P 30.97

16 S 32.06

17 Cl 35.45

18 Ar 39.95

19 K 39.10

20 Ca 40.08

21 Sc 44.96

22 Ti 47.90

23 V 50.94

24 Cr 52.00

25 Mn 54.94

26 Fe 55.85

27 Co 58.93

28 Ni 58.71

29 Cu 63.54

30 Zn 65.37

31 Ga 69.72

32 Ge 72.59

33 As 74.92

34 Se 78.96

35 Br 79.91

36 Kr 83.80

37 Rb 85.47

38 Sr 87.62

39 Y 88.91

40 Zr 91.22

41 Nb 92.91

42 Mo 95.94

43 Tc (99)

44 Ru 101.1

45 Rh 102.9

46 Pd 106.4

47 Ag 107.9

48 Cd 112.4

49 In 114.8

50 Sn 118.7

51 Sb 121.8

52 Te 127.6

53 I 126.9

54 Xe 131.3

55 Cs 132.9

56 Ba 137.3

57 La 138.9

72 Hf 178.5

73 Ta 180.9

74 W 183.9

75 Re 186.2

76 Os 190.2

77 Ir 192.2

78 Pt 195.1

79 Au 197.0

80 Hg 200.6

81 Ti 204.4

82 Pb 207.2

83 Bi 209.0

84 Po (209)

85 At (210)

86 Rn (222)

87 Fr (223)

88 Ra (226)

89 Ac (227)

104 Rf (261)

105 Ha (262)

106 Sg (263)

107 Ns (261)

108 Hs (265)

109 Mt (266)

110 Uun (269)

111 Uuu (272)

112 Uub (277)

58 Ce 140.1

59 Pr 140.1

60 Nd 144.2

61 Pm 144.9

62 Sm 150.4

63 Eu 152.0

64 Gd 157.3

65 Tb 158.9

66 Dy 162.5

67 Ho 164.9

68 Er 167.3

69 Tm 168.9

70 Yb 173.0

71 Lu 175.0

90 Th 232.0

91 Pa (231)

92 U 238.0

93 Np (237)

94 Pu 244.1

95 Am (243)

96 Cm (247)

97 Bk (247)

98 Cf 242.1

99 Es (254)

100 Fm 257.1

101 Md 258.1

102 No 259.1

103 Lw 260.1

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

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

Bonding and the Outer Shell

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.

Atoms with a complete outer shell do not react with other atoms. 

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.

Ionic Bonding

Ionic bonding occurs when electrons are transferred from one atom to another. The transfer enables the atoms to have complete outer shells.

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 see that the atomic number (number of protons) of sodium is 11 because the number of protons is the same as the number of electrons. Refer to the periodic table to verify that the atomic number of sodium is 11.

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

Atoms are neutral. When an atom transfers electrons to another, the atom that loses one or more electrons becomes positively charged and the atom that gains one or more electrons becomes negatively charged. These charged particles are called ions. Positively charged ions are cations. Negatively charged ions are anions.

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 attraction is called an ionic bond.

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

A large number of sodium and chloride ions form a crystal as seen below in the photograph of table salt (sodium chloride). 

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. Click here to view the answer.

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 are held together by covalent bonds.  They may be composed of atoms of the same element or of different elements.

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

The outer shells of oxygen atoms have six electrons. They need two additional electrons to become stable. This can be accomplished by sharing two pairs of electrons.

In a double bond, two atoms share two 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 Molecules

Atoms vary in their attraction for electrons. The strength of this attraction is an atom's electronegativity.

Two covalently-bonded atoms that differ in their electronegativity will not share electrons equally and the molecule will be polar. The atom that is more electronegative will exert a stronger attraction for the electrons and will therefore have a partial negative charge. The atom that is less electronegative will have a partial positive charge.

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 more electronegative; it has a stronger attraction for electrons and thus a partial negative charge. The hydrogen has a partial positive charge because it has less access to the shared electrons.

The diagram below shows that water is composed of two hydrogen atoms bonded to an oxygen atom by two covalent bonds. Each hydrogen atom shares one pair of electrons with the oxygen atom.

Oxygen is much more electronegative than hydrogen and the shared electrons spend more time closer to the oxygen part of the molecule than to the hydrogen part. Unequal sharing of electrons results in the oxygen having a partial negative charge and the hydrogen atoms having a partial positive charge.

Hydrogen Bonds

When hydrogen is bonded to a more electronegative atom such as oxygen or nitrogen, a partial positive charge develops on the hydrogen due to unequal sharing of electrons. The partial positive charge on the hydrogen atom will be attracted to atoms with a negative charge. The attraction between the hydrogen and a negatively charged atom is called a hydrogen bond.

Hydrogen bonds are weak.

The drawing below shows hydrogen bonds between water molecules. The hydrogen bond forms between the hydrogen of one molecule (partial positive charge) and the oxygen of another molecule (partial negative charge).

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.

Energy and Covalent Bonds

Energy is often defined as the ability to cause change. For example, energy is needed to move matter.

Energy is required to form a covalent bond and energy is released when a Covalent bond is broken.  Covalent bonds can therefore be used by organisms to store energy. The energy that is stored can be used to perform work.

Water

Water covers approximately 71% of Earth's surface. Life evolved in water. Living things are 70-90% water. In nature, water is a solvent for many kinds of chemical reactions.

The angle between the two hydrogen atoms in a water molecule is 104.5 degrees. Unequal sharing of electrons results in one side of the molecule having a partial positive charge and one side having a partial negative charge.

The hydrogen bonds between adjacent water molecules are very weak. As a result, they form and break rapidly, often lasting only a few trillionths of a second. At any instant in time, a large proportion of water molecules are bonded to nearby water molecules, giving water a cohesive property.

Water molecules are also attracted to other polar substances causing them to adhere to many kinds of materials. The meniscus shown below forms when water adheres to the sides of the glass container.

Ions and Polar Molecules Dissolve in Water

The partial positive and negative charges on a water molecule produce attractions with ions and other 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.

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

Ionization of Water

We learned earlier that atoms that gain or lose electrons become ions. Hydrogen is the smallest atom, composed of one electron and one proton. It can lose its electron to become an ion. A hydrogen ion, however, is a proton because there are no remaining electrons. The words "proton" and "hydrogen ion" are often used interchangeably in discussion of biological topics.

Within water, some of the molecules will temporarily lose a proton and leave the electron behind. The equation below describes this process.

H₂O «  H⁺ + OH^-

When water ionizes, equal numbers of hydrogen ions and hydroxide ions ( OH^-) are formed.

The process is reversible; hydrogen and hydroxide 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 (anion). HCl is therefore an acid.

Bases are substances that combine with hydrogen ions, thus lowering the concentration of hydrogen ions. Substances that produce hydroxide ions ( OH^-) are bases because hydroxide ions are capable of combining with hydrogen ions to form water, thus lowering the concentration of hydrogen ions. Bases are therefore proton acceptors.

OH^-  +  H⁺  ®  H₂O

NaOH is an example of a base. When it is dissolved in water, an electron from the sodium atom remains with the OH. This produces a sodium ion (Na⁺) and a hydroxide ion (OH^-). NaOH is therefore a base.

NaOH  ®  Na⁺ + OH^-

Most bases produce hydroxide ions and a cation (positive ion) when dissolved in water.

Water molecules have a slight tendency to dissociate, forming both H⁺ and OH^- as discussed previously. 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 scale 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.