For example, the atmospheric pressure on Mars is too low to keep water in a liquid state. The state a substance exists in depends on the nature of the substance, its temperature, and pressure. The fourth state, plasma, consists of ionized gases, that is, gaseous atoms that have been stripped of their electrons. Three of those states are familiar to us: solid, liquid, and gas. Matter can exist in of four physical states. Https:/A water molecule is an example of a compound. Radioactive atoms emit three kinds of radiation: Alpha, Beta, and Gamma. Molecules made of two or more different elements, such as water (H 2O) or carbon dioxide (CO 2) are called compounds. Two or more atoms bound together form a molecule. Atoms can form bonds with each other by exchanging or sharing one or more orbital electrons. Isotopes of the same element are designated with a superscript of their respective atomic mass, for example, 12C and 14C, being two isotopes of carbon. Such atoms, called isotopes, have slightly different masses and some are unstable, undergoing radioactive decay into lighter, most stable atoms. Some atoms of the same element may have differing numbers of neutrons, and therefore, have different atomic masses. The total number of protons and neutrons in the atom is the atomic mass. Instead of just four elements, the modern periodic table contains ninety-two naturally occurring elements and over twenty-five elements that have been produced artificially in nuclear reactors and particle accelerators. Today, an element is defined as a substance that cannot be chemically broken down into any other substances. The heaviest naturally occurring element, uranium has ninety-two protons in its nucleus, giving it an atomic number of ninety-two. The lightest atom, hydrogen, has an atomic number of one and therefore, only has one proton in the nucleus. The number of protons in the nucleus, known as the atomic number, determines what element the atom is. In Rutherford’s model, the nucleus of an atom contains two particles: positively charge protons and neutrons, which lack any charge. Most of the atom, in fact, was empty space! Rutherford concluded that electrons must orbit the nucleus much like planets orbit the Sun. Ernest Rutherford’s experiments with alpha particles revealed that, instead a “plum pudding” model, most of the atom’s mass was concentrated in a tiny point in the center, a nucleus containing protons and neutrons. At first, physicists modeled atom as a mass of positive charges with negatively charged electrons embedded in it, like plums embedded in a pudding.Įventually, however, other particles were discovered, such as the alpha particle, which we now know is a helium nucleus, containing two protons and two neutrons. Suddenly, atoms no longer seemed so “inseparable” at all, as there appeared to be a particle smaller and more fundamental than the atom. Matter was indeed made of tiny, inseparable particles.īut then, in 1900, Joseph John Thompson discovered the electron, a particle with a negative charge and that was lighter than the lightest known atom. This fundamental particle was thus, inseparable.įor thousands of years, philosophers favored the idea of primal elements over Democritus’ atomic theory, but by the late nineteenth century, physicists had concluded from observation that Democritus had been right all along. He called these particles atoms, from the Greek word for “inseparable.” Democritus argued that any object could be cut into smaller parts but eventually, if we kept cutting, we would reach a tiny particle that could no longer split into smaller parts. In contrast, the Greek philosopher Democritus, held that matter was made up of tiny particles. Under this model, all matter was made up of different combinations of these four elements. Many held that all matter was made up of four primal substances or elements, such as air, earth, fire, and water. \)Īncient philosophers pondered the nature of matter for thousands of years.
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