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What the atoms is like

Perhaps you have seen a picture of the solar system with the sun in the centre and several planets, one of which is our Earth, revolving around it. Except for the fact that the atom is so small, and the sun and the planets so huge, they are much alike.

Each atom has a centre, called its nucleus. In this centre are one or more particles called protons. A proton is a particle that has a positive electrical charge. Revolving around this centre, or nucleus, are particles of negative electricity called electrons.

An electron is very light. It takes 1,845 electrons to weigh as much as one proton. As you can read in the article on electricity , positive and negative electricity attract each other. You may have seen how this works with two magnets. Each magnet has a positive end and a negative end. Touch the positive end of one magnet to the negative end of another and they will cling together; but two negative ends, or two positive ends, will not stay together. The atom is held together because the protons arc positive and the electrons are negative. Every atom has exactly as many protons as electrons, so that the positive and the negative balance each other. The nucleus of the hydrogen atom has one proton, and therefore the hydrogen atom has one electron. All other atoms arc heavier than hydrogen, having two or more protons in their nuclei. The "atomic number" of any element is the number of protons and electrons in each of its atoms. For example, the atomic number of hydrogen is 1.

The atomic number of uranium is 92, because there are 92 protons and 92 electrons in the uranium atom. There is still another particle that goes to make up the atom. This is called a neutron. A neutron weighs about the same as a proton, but it does not have any electrical charge at all, either positive or negative. Neutrons are found only in the nucleus of the atom, as protons are. Neutrons add to the weight of the atom. Because of these neutrons, the same element may have atoms of different weights. Different atoms of the same element are called isotopes. Uranium, for example, has several isotopes. The uranium atom always has 92 protons is its nucleus, but one of its isotopes, called U-238, has 146 neutrons (146 neutrons + 92 protons = 238), while the isotope called U-235 has 143 neutrons in its nucleus ( 143 + 92 = 235).

The atomic weight is about the same as the neutrons plus the protons; for example, the atomic weight of U-238 is about 238. A stable element is composed of atoms that do not rapidly throw off their particlesó their protons, neutrons, and electronsó in the form of rays. Light elements are usually stable. The heavier elements are often unstable. They readily break down into different isotopes and even into different elements. This is explained in the articles on atomic energy and† radio activity. Hundreds of years ago there were men called alchemists who tried to change cheap metals, such as lead, into gold. They never succeeded, but today it is possible to change some elements into other elements by changing the number of protons, neutrons and electrons in them. To do this, we must knock out some of the particles, or add other particles. This is very hard to do, because the forces binding the particles together are very great. But as science progressesFeature Articles, it is being done more and more.

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