![]() Attempts to classify materials such as these resulted in the concepts of classical elements, alchemy, and various similar theories throughout human history. The history of the discovery and use of the elements began with primitive human societies that discovered native minerals like carbon, sulfur, copper and gold (though the concept of a chemical element was not yet understood). Air is primarily a mixture of molecular nitrogen and oxygen, though it does contain compounds including carbon dioxide and water, as well as atomic argon, a noble gas which is chemically inert and therefore does not undergo chemical reactions. Nearly all other naturally occurring elements occur in the Earth as compounds or mixtures. Only a few elements, such as silver and gold, are found uncombined as relatively pure native element minerals. When different elements undergo chemical reactions, atoms are rearranged into new compounds held together by chemical bonds. Atoms can be transformed into different elements in nuclear reactions, which changes the atom's atomic number.Īlmost all of the baryonic matter of the universe is composed of chemical elements (among rare exceptions are neutron stars). Two or more atoms of the same element can combine to form molecules, in contrast to chemical compounds or mixtures, which contain atoms of different elements. For example, oxygen has an atomic number of 8, meaning that each oxygen atom has 8 protons in its nucleus. Chemical elements are identified by the number of protons in the nuclei of their atoms, known as the element's atomic number. The basic particle that constitutes a chemical element is the atom. The valence electrons are held closer towards the nucleus of the atom.A chemical element is a chemical substance that cannot be broken down into other substances by chemical reactions. This means that the nucleus attracts the electrons more strongly, pulling the atom's shell closer to the nucleus. The effect of increasing proton number is greater than that of the increasing electron number therefore, there is a greater nuclear attraction. ![]() However, at the same time, protons are being added to the nucleus, making it more positively charged. This is because, within a period or family of elements, all electrons are added to the same shell. Atomic radius patterns are observed throughout the periodic table.Ītomic size gradually decreases from left to right across a period of elements. The covalent radii of these molecules are often referred to as atomic radii. Nevertheless, it is possible for a vast majority of elements to form covalent molecules in which two like atoms are held together by a single covalent bond. Some are bound by covalent bonds in molecules, some are attracted to each other in ionic crystals, and others are held in metallic crystals. However, this idea is complicated by the fact that not all atoms are normally bound together in the same way. The atomic radius is one-half the distance between the nuclei of two atoms (just like a radius is half the diameter of a circle). This is caused by the increase in atomic radius. Electron affinity decreases from top to bottom within a group.This is caused by the decrease in atomic radius. Electron affinity increases from left to right within a period.This causes the electron to move closer to the nucleus, thus increasing the electron affinity from left to right across a period. Moving from left to right across a period, atoms become smaller as the forces of attraction become stronger. With a larger distance between the negatively-charged electron and the positively-charged nucleus, the force of attraction is relatively weaker. ![]() This means that an added electron is further away from the atom's nucleus compared with its position in the smaller atom. \( \newcommand\): Periodic Table showing Electron Affinity TrendĮlectron affinity generally decreases down a group of elements because each atom is larger than the atom above it (this is the atomic radius trend, discussed below).
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