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Matter, Changes in matter, Structure of an Atom, Molecule


Matter. Anything which occupies space and has weight. Examples: Solids

    such as wood, iron, copper, gold, and salt; Liquids such as water,

    alcohol, gasoline and turpentine; Gases such as oxygen or acetylene.

Composition of Matter. Chemists have found that all complex substances

    --- wood, steel, glass, plastics, even the waters of the ocean and the

    air we breathe --- are mixtures of chemical compounds. Nearly a million

    compounds have been identified, and these, in turn, are merely different

    combinations of only about a hundred chemical elements known to science.

    A compound is a substance consisting of a particular type of molecule.

    A molecule consists of two or more atoms that are chemically bonded

    together. Atoms are composed of neutrons, protons and electrons. There

    are around a hundred kinds of atoms corresponding to the different

    chemical elements.

Molecule. The smallest particle into which matter may be divided without

      destroying its characteristic properties. It is a particle of matter

      consisting of one or more atoms chemically bonded together.

Atom. The smallest particle of a chemical element.

Three Types of Changes in Matter.

    1. Physical

    2. Chemical

    3. Nuclear

Physical Change. In a physical change the composition of the molecules of

    the substance is not changed. Examples: water freezing, dissolving sugar

    in water.

Chemical Change. In a chemical change the composition of the molecules of

    the substance is changed, and new substances with new properties are

    produced. Example: Iron rusting to produce iron oxide.

Nuclear Change. In a nuclear change new materials are formed by changes

    in the identity of the atoms themselves. Example: gradual change of

    radium atoms into lead atoms.


Structure of an atom. An atom is generally viewed as containing a central core called


the nucleus about which particles called electrons revolve in a manner similar to the way a planet orbits the sun. The nucleus consists of several kinds of particles, the main ones being protons and neutrons. Protons carry a unit of positive charge, neutrons carry no charge, and electrons carry a unit of negative charge. All atoms have the same number of electrons as protons. Thus the atom as a whole is electrically neutral.


Let us now consider the structure of the simplest atom, the hydrogen atom. A hydrogen atom consists of a single electron orbiting a nucleus that consists of a single proton. See Fig. 1. The electron orbits the nucleus in what is called the K shell.

 There are a couple of variations of the hydrogen atom called isotopes. In one isotope called Deuterion the nucleus contains a proton and a neutron. In yet another isotope, Tritium, the nucleus contains a proton and two neutrons. Except for that the atoms are identical. In general, the isotopes of a particular element differ by the number of neutrons in the nucleus.                            

Let us now consider the next simplest element, helium. A helium atom consists two electrons orbiting a nucleus that consists of two protons and two neutrons. Both of these electrons orbit in the K shell. See Fig. 2.


The next simplest element is Lithium. It has 3 protons and 4 neutrons in its nucleus and 3 electrons. Two of the electrons orbit in the K shell and one in a shell further out called the L shell. See Fig. 3.

We can progress in this way to progressively more complicated atoms. The number of protons in the nucleus of an atom is called its atomic number. It is the atomic number that distinguishes between elements, that identifies an element. There are over100 elements, each with its own atomic number. The atomic numbers run progressively 1, 2, 3, ... , 100+.                                                                           

Almost the entire mass of an atom comes from its protons and neutrons. The electrons contribute almost nothing to the mass. An electron weighs about 1/1837 of a proton. Protons and neutrons weigh the same. There is a weight scale devised for atomic size weights. In this system the weight of a proton is one atomic weight unit (a.w.u.). Thus the weight of the hydrogen atom is 1 a.w.u.(we neglect the weight of the electron), the weight of a helium atom is 4 a.w.u., the weight of a lithium atom is 7 a.w.u., etc.               

Size of an atom. A helium atom has a diameter of about 1 Angstrom (10-10 meters) while its nucleus has a diameter of only 1 femtometer (10-15 meters). Thus the diameter of the atom is 100,000 times that of the nucleus. If you made a scale model of an aluminum atom with a nucleus the size of a marble, the outermost electrons would fly around the nucleus at a distance of 150 feet. An atom is thus mostly empty space.

Atoms are extremely small, so small it is hard to imagine a particle so small. For example, if a single drop of water were magnified to the size of the earth, the individual atoms would be about the size of tennis balls.

Orbiting of the electrons. To visualize the movement of electrons about the nucleus envision an imaginary hollow sphere on which electrons move at enormous speeds in all directions, their orbital planes cutting through the sphere’s center in random directions. Occasionally the electrons may come close to each other and because of their similar charges they may veer apart before striking and continue in new directions, in new orbits. On more complex atoms there are several spheres, one inside another, on which they move.

Distribution of electrons in shells. There are a total of seven shells, labeled with letters from the alphabet: K, L, M, N, O, P, and Q. Below we give the electron distribution in the various shells for the elements with atomic numbers ranging from 1 to 56 plus a few others.


Table 1


Element                     Atomic No.                K       L         M        N         O        P         Q

Hydrogen                     1                                 1 

Helium                         2                                 2

Lithium                        3                                 2          1

Beryllium                    4                                 2          2

Boron                           5                                 2          3

Carbon                         6                                 2          4

Nitrogen                      7                                 2          5

Oxygen                        8                                 2          6

Fluorine                       9                                 2          7

Neon                           10                                2          8

Sodium                       11                                2          8           1

Magnesium                 12                                2          8           2

Aluminum                  13                                2          8           3

Silicon                        14                                2          8           4

Phosphorus                 15                                2          8           5

Sulfur                          16                                2          8           6

Chlorine                      17                                2          8           7

Argon                         18                                2          8           8

Potassium                   19                                2          8           8          1

Calcium                      20                                2          8           8          2

Scandium                    21                                2          8           9          2

Titanium                     22                                2          8          10         2

Vanadium                   23                                2          8          11         2

Chromium                  24                                2          8          13         1

Manganese                 25                                2          8          13         2

Iron                             26                                2          8          14         2

Cobalt                         27                                2          8          15         2

Nickel                         28                                2          8          16         2

Copper                        29                                2          8          18         1

Zinc                            30                                2          8          18         2

Gallium                      31                                2          8          18         3

Germanium                32                                2          8          18         4

Arsenic                       33                                2          8          18         5

Selenium                    34                                2          8          18         6

Bromine                      35                                2          8          18         7

Krypton                      36                                2          8          18         8

Rubidium                    37                                2          8          18         8         1

Strontium                    38                                2          8          18         8         2

Yttrium                       39                                2          8          18         9         2

Zirconium                   40                                2          8          18        10        2

Niobium                     41                                2          8          18        12        1

Molybdenum              42                                2          8          18        13        1

Technetium                43                                2          8          18        14        1

Ruthenium                  44                                2          8          18        15        1

Rhodium                     45                                2          8          18        16        1

Palladium                   46                                2          8          18        18 

Silver                          47                                2          8          18        18        1

Cadmium                    48                                2          8          18        18        2

Indium                        49                                2          8          18        18        3

Tin                              50                                2          8          18        18        4

Antimony                   51                                2          8          18        18        5

Tellurium                    52                                2          8          18        18        6

Iodine                         53                                2          8          18        18        7

Xenon                         54                                2          8          18        18        8

Cesium                       55                                2          8          18        18        8          1

Barium                        56                                2          8          18        18        8          2



Platinum                     78                                2          8          18        32        17        1

Gold                            79                                2          8          18        32        18        1

Mercury                      80                                2          8          18        32        18        2

Thallium                     81                                2          8          18        32        18        3

Lead                            82                                2          8          18        32        18        4

Bismuth                      83                                2          8          18        32        18        5

Radon                         86                                2          8          18        32        18        8

Radium                       88                                2          8          18        32        18        8          2

Uranium                     92                                2          8          18        32        21        9          2


Examination of the above table will show how electrons are added to shells as one progresses to successively higher atomic numbers. First shell K fills to its maximum of two. Then shell L fills to its maximum of 8. Then shell M fills to 8 with Argon. Then shell N fills to 2 with Calcium. All goes regularly through Calcium. Then with Scandium, element 21, electrons start going into shell M again and it starts filling to a maximum of 18. Note the irregularity at Chromium where suddenly two electrons are added to the M shell and one is lost in the N shell. Note a similar phenomenon with Copper and others. Thus we see that the process is quite regular but with an occasional interesting irregularity.

The number of electrons in the outer shells is of special significance in determining how elements combine with each other in forming molecules. The capacity of an element for combining with another element or radical is given by a number called its valence. Following is a table of valences for various elements and radicals.


Table 2

Valences shown by common elements and radicals


Aluminum      Al                    +3

Ammonium    NH4                +1

Barium            Ba                   +2

Calcium          Ca                   +2

Chromium      Cr                    +3

Cobalt             Co                   +2

Copper            Cu                   +1, +2

Hydrogen        H                     +1

Iron                 Fe                    +2, +3

Lead                Pb                   +2

Magnesium     Mg                  +2

Mercury          Hg                   +1, +2

Nickel             Ni                    +2

Potassium       K                     +1

Silver              Ag                   +1

Sodium           Na                   +1

Zinc                Zn                   +2


Acetate           C2H3O2          -1

Bicarbonate    HCO3             -1

Bisulfate         HSO4              -1

Bromide          Br                    -1

Carbonate       CO4                -2

Chlorate          ClO3               -1

Chloride          Cl                    -1

Chromate        CrO4               -2

Ferricyanide   Fe(CN)6          -3

Ferrocyanide   Fe(CN)6          -4

Hydroxide      OH                  -1

Hypochlorite  ClO                 -1

Iodide             I                      -1

Nitrate            NO3                -1

Nitrite             NO2                -1

Oxide              O                     -2

Permanganate MnO4             -1

Phosphate       PO4                 -3

Sulfate            SO4                 -2

Sulfide            S                     -2

Sulfite             SO3                 -2

Tartrate           C4H4O6          -2


Let us consider some of the elements above with positive valence values. Examination will show that in almost all cases the valence corresponds to the number of electrons in the outer shell of the element. Consider aluminum. It has a valence of +3 and Table 1 shows that aluminum has 3 electrons in its outer shell. Barium has a valence of +2 and Table 1 shows that it has 2 electrons in its outer shell. Checking shows the rule also holds for cobalt, copper, hydrogen, iron, magnesium, mercury, nickel, potassium, silver, sodium and zinc. Chromium and lead are exceptions.

Elements combine in certain ways, according to certain rules. A compound consists of two parts. One part has a positive valence and the other part has a negative valence. The sum of the positive and negative valences of the two parts must be zero in any compound. The valence of an element or radical is thus a number associated with that element or radical that helps us in constructing valid compounds. That is, it helps us in determining what combinations of elements will combine to make valid compounds and which will not.


1. Consider the compound sodium chloride, NaCl. Here we have two atoms, one Na atom and one Cl atom. From Table 2 we note that Na has a valence of +1 and Cl has a valence of -1. The sum of the valences in the compound NaCl is thus +1 + (-1) = 0.

2. Consider the compound H2O. From Table 2 we see that H has a valence of +1 and O has a valence of -2. The compound H2O then consists of the portion H2 (two atoms of H) with a total valence of +2 and O with a valence of -2. The sum of the valences is +2 + (- 2) = 0.

3. Consider sulfuric acid, H2SO4. From Table 2 we see that H has a valence of +1 and the sulfate radical SO4 has a valence of -2. The portion of this compound with the positive valence is H2 with a valence of +2. The portion of the compound with the negative valence is the sulfate radical SO4 with a valence of -2. The sum of the valences is +2 + (- 2) = 0.

4. Consider the compound magnesium hydroxide, Mg(OH)2. From Table 2 we see that Mg has a valence of +2 and the hydroxide radical OH has a valence of -1. Magnesium hydroxide then consists of the Mg portion with the positive valence of +2 and two copies of the OH radical with a total negative valence of -2.

The above are examples of valid compounds. It is of course simple to demonstrate all kinds of combinations of elements that would not combine to produce compounds. Any two elements, both of which had positive valences, such as Al and Ca, would not combine to produce a compound. The parts of a compound thus come from two groups: 1) the group of elements or radicals with positive valences and 2) the group of elements or radicals with negative valences. The two parts of the compound must be such that the sum of the positive and negative valences is zero.

Chemical bonding. The mechanism by which two elements (or an element and radical) unite to form a compound is generally considered to be the following: the electrons in the outer shell of one atom are either transferred to another atom or shared with another atom so as to make the outer shells of both atoms as complete as possible. Thus in the case of the compound H2O two hydrogen atoms each share their single electron with an oxygen atom (whose outer shell contains 6 electrons). By this sharing, the outer shell of the oxygen atom gains two electrons and thus becomes completed with 8 electrons, providing chemical stability. With this interpretation on chemical bonding, the valence of an element then corresponds to the number of electrons gained, lost, or shared in forming a chemical bond.

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