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The study of the nucleus (center) of an atom is known as nuclear physics. As a result of this study, scientists have discovered ways of splitting the nucleus to release huge amounts of energy. One nucleus is split, causing many others to split, in what is called a nuclear chain reaction. A controlled nuclear reaction inside a power station can be used to provide heat and light for our homes.

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We determine how hot or cold something is by a measure called temperature. This tells us how much energy the atoms (tiny particles) inside a body contain. The more energy the atoms contain, the faster they move, and the higher the temperature. For each element (simple type of substance), there are specific temperatures at which changes of state occur. These are known as melting and freezing points. Scientists believe that absolute zero is the lowest possible temperature, below which atoms would have no energy.

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Density is the amount of mass, or atoms, that an object contains in a given space. The metal object contains more tightly packed atoms than the wooden object; therefore it has a greater density. An object made of metal is usually much heavier than a wooden one of the same volume. They are both occupying the same amount of space, but the metal object has the greater density.

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Radioactive substances are composed of unstable atoms that break down and release high-level energy called radiation. These atoms eventually form new elements. There are three types of radiation, known as alpha rays, beta rays, and gamma rays. Alpha rays consist of four atomic particles: two protons and two neutrons. They are the weakest type of radiation and have a positive electrical charge. Beta rays are more powerful and are composed of atomic particles called electrons, which have a negative charge. Gamma rays are the most powerful and are a pure form of energy with no mass and no charge.

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Isotopes are different forms of the same element (simple type of substance). They have identical chemical properties and occupy the same place in the periodic table (the word isotope means «same place»). It is their physical properties that differ, because their atoms (tiny particles) have different masses. Each „isotopic form“ has a different number of neutrons, in the nucleus (center) of its atoms, which gives it a different mass. Hydrogen gas, for example, can exist naturally in three isotopic forms: common hydrogen, heavy hydrogen, and radioactive hydrogen.

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There are two types of nuclear reactions: nuclear fission and nuclear fusion. Nuclear power stations use fission to produce their energy. Fast-moving atomic particles called neutrons are fired at the nucleus (center) of an atom to split it. This splitting is called fission and it causes other atoms to split in a chain reaction. Some mass (the number of heavy particles inside an atom) is lost in the process. This is converted into large amounts of nuclear energy.

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Nuclear fusion is a type of nuclear reaction that creates huge amounts of energy. It takes place naturally inside the sun, creating the heat energy that we need to survive on Earth. At temperatures of about 25 million F dgr. (14 million C dgr.) the nuclei (centers) of two hydrogen atoms fuse, or join together. In the process, some mass (the number of heavy particles inside an atom) is lost and converted into energy. Scientists are trying to develop this form of nuclear reaction as a safer alternative to the nuclear fission that takes place in power stations place.