An atom is the smallest building block of everything that exists. The smallest object you can see under a microscope has more than 10 billion atoms. There are over a hundred different types of atoms, that we call elements. They form the world we live in. Well known elements are hydrogen, oxygen, iron or lead. When two elements get together they form a compound. For example, water has two atoms of hydrogen and one atom of oxygen. Atoms all have about the same size, but they have different weights. Plutonium is one of the heaviest atoms, about 200 times heavier than hydrogen, which is the lightest element. Parts of the Atom An atom consists of three parts : protons, neutrons, electrons. Protons and neutrons are in the center of an atom, which is called the nucleus. It is very small if you compare it with the whole atom and it has almost all of an atom's mass. If an atom had a diameter of about 6 km the nucleus would only be as big as a tennis ball. The rest of the atom outside the nucleus is mostly empty. Electrons fly around in an atom very, very quickly. They have almost no mass and travel around the nucleus millions of times every second. The parts of an atom have electrical charges. Each proton carries a positive electrical charge and each electron has a negative electrical charge. Neutrons have no charge. In most cases an atom has the same number of protons and electrons. It is electrically neutral. The energy of the nucleus keeps the electron inside the atom—just like the Earth keeps the moon in its orbit. But electrons have energy themselves. They want to break away from the nucleus. If an electron has a lot of energy it moves around farther away from the nucleus. Electrons move around the nucleus in up to seven round paths, called shells. The first shell is closest to the nucleus. It can hold two electrons. The second shell can hold 8, the third 18 and the fourth 32 electrons. In most atoms, the outer shells are never completely filled with electrons. The inner electrons travel fastest, the outer ones are the slowest. The atomic number tells you how many protons an atom has. For example, every hydrogen atom has the atomic number 1 because it only has 1 proton. Elements that have atomic numbers of up to 92 can be found in nature; those over 92 are created by scientists in a laboratory. The atomic number tells us where we can find an element in the periodic table. This table shows all the atoms in groups. The atomic mass is the number of portions and neutrons in an atom. Although all atoms of the same element have the same number of protons, they sometimes have more neutrons. Such atoms are called isotopes. For example, hydrogen has three isotopes. Most of the time a hydrogen atom has one proton and one neutron. Sometimes you can find hydrogen isotopes that have two or three neutrons, but they too have only one proton. In most lighter elements the nucleus of each atom has the same number of protons and neutrons. but heavier elements have more neutrons than protons. Uranium, for example has 92 protons and 146 neutrons. It's atomic mass is 238. The atomic mass is never a whole number, because scientists do not just add protons and neutrons together. They use a complicated formula. Normally, an atom is electrically neutral. But it can gain or lose electrons when it crashes with other atoms. Atoms that gain or lose electrons are called ions. They have an electric charge. Atoms that lose electrons become positive ions; atoms that win electrons become negative ions. In some atoms the nucleus can change naturally. Such an atom is radioactive. When a nucleus changes it produces rays. In nature, there are some elements that are radioactive, like uranium or radium. In labs scientists can produce radioactivity by bombarding atoms with smaller particles. In the 1930s and 40s scientists found out that if they bombarded a uranium atom with a neutron the nucleus would split up into two parts. When this happens energy is set free. We call this nuclear fission. Fission was first used in atomic bombs that the Americans dropped over Japan to end the Second World War. The bombs released so much energy that they killed hundreds of thousands of people. Later on scientists found out how this energy could be used in a peaceful way. In the 1950s the first atomic reactors were built. They produced energy by splitting atoms. Compounds are groups of two or more elements that are tied together. They are created when two different atoms share the same electron or when electrons travel from one atom to another. SaltEvery compound has certain features and a chemical formula. Water, for example, is a compound that has two hydrogen atoms and one oxygen atom in it. Its chemical formula is H 2O. When sodium (Na) combines with chlorine (Cl) you get a compound called salt (NaCl). Carbon and hydrogen atoms form methane, or natural gas (CH 4). Compounds can be divided into two groups: organic compounds always have carbon atoms in them. You can find them in all living creatures, in plants and in a lot of our food, for example sugar and fat. All other compounds are called inorganic compounds. Compounds can be solids, liquids or gases. They may also turn into many different colours. Some compounds react very quickly, others don't react at all.