Size and Speed
It is difficult to appreciate the world of atoms and bonds because
they are so tiny. To get an idea of just how small they really are,
imagine yourself in one of those science fiction films in which
everything but you has been enlarged. David Goodsell, in a
fascinating book ("The Machinery of Life", Springer-Verlag), has
illustrated what living things look like at various magnifications.
Four pictures taken from his book show familiar biological items at
100, 1,000, 100,000 and 1 million times magnification.
At one million times enlargement, a typical human cell would be about the size of your bedroom. If you filled the room from top to bottom with peas, you would get an idea of what a cell looks like, the peas representing polymers. At 30 million fold magnification, individual atoms are about the size of grain of rice, small molecules about the size of pea, and most polymers about the size of a baseball. In this same context, a typical bacterium would be about 50 yards long, and the average human cell about the size of a football stadium.
While these pictures give you some idea of the size of individual entities and their static relationship to one another, the most extraordinary aspect of this situation doesn't appear until you examine the movement of the molecules. If you speeded up everything in relationship to their size, at body temperature all these substances would be moving very rapidly. For example, the baseballs would be coursing around at the amazing speed of millions of miles per hour. The small molecules even faster. But, because there are so many polymers and small molecules -- especially water -- around, and because all would be bumping into each other so frequently, they wouldn't travel in a straight line for very long. In fact, the entire scene would be chaotic: one great jumble of rapidly moving objects, bouncing off each other at enormous speeds.
• Elements combine to form compounds. Atoms combine to form molecules. These two statements really say the same thing except that "elements" and "compounds" refer to the substances you can feel and put in a bottle, whereas "atom" and "molecule" refer to the ultimate unit of element and compound, respectively.
• Atoms form bonds with each other because they are composed of charged particles (negative electrons and positive protons). Two charged particles push away one another when their charges are of the same sign and pull toward ones another when they are different. A bond between two atoms is a balance between pulls and pushes.
• The electrons of an atom can be thought of as residing in shells of clouds which have a certain maximum occupancy. When that occupancy is attained, as in the case of the noble gases, the atom is stable and will not combine with other atoms. When occupancy of the outer shell is less than maximum, two atoms will share electrons so as to mimic the state of maximum occupancy. An atom is thereby endowed with a certain combining power which is equal to the number of electrons that must be gained or lost to attain maximum occupancy of the outer shell.
• A bond formed by equal sharing of an electron pair is called a covalent bond and is a strong bond. A bond formed between two charged atoms &emdash; charged because one lost one or more electrons and the other gained one or more &emdash; is called an ionic bond. A bond formed between a partially charged hydrogen atom and oxygen or nitrogen is called a hydrogen bond. Ionic bonds and hydrogen bonds are both weak bonds compared to covalent bonds.
• If an atom is bound to two or more atoms, the bonds will be inclined at a definite angle to one another giving the molecule a definite three-dimensional shape.
• Atoms and small molecules are incredibly small, but they
move around very quickly in relation to their size. In a typical
organism, the molecules are bumping into each other at very high
speed and with great frequency.
