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All atoms contain a positively charged nucleus and negatively charged electrons. According to the planetary model of atom, the electrons orbit around the nucleus, similar to the planets orbiting around the star. The distance between the electrons and the nucleus is much (about 10,000 times) larger than the size of the nucleus itself. Such model of an atom is not actually correct (because, according to classical electromagnetism, an electron moving in circle would emit electromagnetic waves, lose energy, and ultimately fall into the nucleus), but it is useful in explaining some phenomena. The theory of quantum mechanics provides a very accurate description of the distribution of the electrons inside the atom.

The nucleus consists of positively charged protons and neutral (not carrying electric charge) particles called neutrons. The electric charge of a proton is equal in magnitude to the electric charge of an electron. The neutrons and protons are similar in mass and much (about 1000 times) heavier than electrons. Not all atoms have neutrons in their nuclei, for example, a nucleus of a Hydrogen atom has just one proton. The number of electrons in an atom always equal the number of protons in the nucleus, so the whole atom is electrically neutral.

Each proton and neutron consists of particles called quarks. There are different types of quarks, and they carry electric charge, either positive or negative, that is 1/3 or 2/3 of the charge of an electron. So far it is not known if the quarks could be further broken down into smaller elementary particles.


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