In nature, there are many kinds of substances with different properties. However, the number of chemical elements, the basic components of these substances, is limited. By 2007, a total of 118 chemical elements have been discovered and synthesized all over the world, of which 94 exist on earth.
Since ancient times, scientists and philosophers have been exploring the composition of matter and looking for what chemical elements are. Ancient Greek philosophers believed that air, water, fire and earth were the four elements that made up all things in the world. In the late Middle Ages, when alchemists skillfully carried out some chemical experiments, they put forward the theory of sulfur, mercury and salt.
British chemist Boyle first put forward the scientific definition of chemical element: general chemical methods that are not composed of other substances can no longer be decomposed into simpler objects. In 1798, French chemist Lavoisier listed the first list of chemical elements, including some compounds and mixtures. By 1803, British chemist Dalton put forward the atomic theory, pointed out the atomic properties of chemical elements, and called the same kind of atoms elements.
At the beginning of the 20th century, scientists discovered that the nucleus of an atom is composed of protons and neutrons. They also found isotopes and realized that chemical elements are the general name of the same kind of atoms with the same nuclear charge number (proton number).
In 1969, the single atom of uranium and thorium was observed by electron microscope. The nucleus is very small. It is composed of protons and neutrons. Protons are positively charged, neutrons are not charged, and the number of protons is the number of nuclear charges. The diameter of the atom is 10-8 cm, and the nucleus is one ten thousandth of it; The mass of the atom is concentrated in the nucleus, and the electron is only one thousandth of that of the nucleus.
The chemical properties of an element are mainly related to the number and arrangement of electrons outside the nucleus, especially the number and energy of the outermost electrons, which basically determine the chemical properties of the element.
When various chemical elements are arranged in the order of increasing the number of nuclear charges, there will be the law of periodic changes in physical and chemical properties, which is the periodic law of elements. The table listed in this order and law is the periodic table of elements. Chemical elements can be divided into metallic elements and non-metallic elements according to their physicochemical properties. Among them, natural elements with atomic number greater than 83 are radioactive. These elements form millions of compounds that make up the whole world.