Glynda Lee Smith, age 12, of Ponca City, Oklahoma, for her question:

What is meant by the structure of the atom?

The ancient Greeks thought about atoms more than 2,000 years ago. They were puzzled by the changes in nature and tried to explain them with one of the logical theories. Everything, they reasoned, must be made of tiny atoms that form various materials as they arrange and rearrange themselves.

The logical thinkers of Greece reasoned that the atom is the smallest possible piece of matter and that the atom cannot be divided into anything smaller than itself. This idea prevailed for centuries, right up to the Age of Science. Then the atom was found to be a unit of still smaller particles of matter. What's more, as atomic particles were discovered one by one, the atom was found to be a very orderly unit of matter. When we refer to the structure of the atom, we mean the orderly fashion in which it is built, the neat way in which its assorted particles are arranged and held together.

Scientists have identified more than a hundred different atoms, each one a different size and having a different number of basic particles. But all atoms are constructed along the same lines. The basic structure of the atom can be depended upon in all cases. This structure has been compared to a miniature solar system. Its sun is a central nucleus, a tight fist of assorted particles. Its orbiting planets are speeding electrons. The various atomic particles are infinitesimal bundles of energy. They may be electrically neutral or bear charges of positive or negative electricity.

The major particles in the tight nucleus are positive protons and neutral neutrons. Also present in the tight core is an assortment of tiny meson particl & s and quotas of dynamic energies to bind the nucleus together. The number of protons gives the nucleus its quota of positive electricity. The structure of the normal atom calls for an equal and opposite quota of negative electricity. This is provided by the orbiting electrons. Each electron has a negative charge to matchithe positive charge of one proton. Every proton, however, is 1,836 times more massive than an electron. But the tiny electron may be 250 or even 1,000 times more massive than the nuclear mesons. The basic structure of the atom has a series of orderly paths for the busy electrons, even though they orbit the nucleus billions of times per second

There are definite details that make each of the known atoms different from all the rest. But in each basic structure is a miniature solar system, teeming with energy and obeying rigid rules. Each different atom has been given a number and each atom on the list has one more proton than the one before it. The atomic number of gold is 79 because its nucleus has 79 protons and hence its structure calls for 79 orbiting electrons. Mercury has 80 protons and naturally its atomic number is 80. No atom can change its basic number because if it gains or loses a proton, it immediately becomes at atom of something else.

Suppose the average atom were as big as a football field. The nucleus in the center would be a marble and the electrons mere specks orbiting as far away as the 50 yard lines. Some of an_atom's electrons tend to arrive and depart or team up with other atoms. But this does not change the nucleus at the heart of the atomic structure. This can, however, be split apart by radioactivity or atom smashers. These nuclear processes taught us much of what we know of the tiny atom, its structure and its assorted particles.