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Atoms are the smallest neutral particles into which matter can be divided by chemical reactions. An atom consists of a small, heavy nucleus surrounded by a relatively large, light cloud of electrons. An atomic nucleus consists of 1 or more protons and 0 or more neutrons. Protons and neutrons are, in turn, made of quarks.
This is much smaller than the radius of the atom, which is on the order of 10 5 fm. The nucleons are bound together by a short-ranged attractive potential called the residual strong force. At distances smaller than 2.5 fm this force is much more powerful than the electrostatic force that causes positively charged protons to repel each other. [43]
Atoms of one element can be transformed into atoms of a different element in nuclear reactions, which change an atom's atomic number. Historically, the term "chemical element" meant a substance that cannot be broken down into constituent substances by chemical reactions, and for most practical purposes this definition still has validity.
Three divisions of matter are recognized in science: masses, molecules and atoms. A Mass of matter is any portion of matter appreciable by the senses. A Molecule is the smallest particle of matter into which a body can be divided without losing its identity. An Atom is a still smaller particle produced by division of a molecule.
Lavoisier defined an element as a substance whose smallest units cannot be broken down into a simpler substance. [9] Lavoisier's book contained a list of "simple substances" that Lavoisier believed could not be broken down further, which included oxygen , nitrogen , hydrogen , phosphorus , mercury , zinc and sulfur , which formed the basis for ...
The carbon dioxide "atom" is indivisible in the sense that it cannot be divided into smaller carbon dioxide particles. [4] [19] Dalton made the following assumptions on how "elementary atoms" combined to form "compound atoms" (what we today refer to as molecules). When two elements can only form one compound, he assumed it was one atom of each ...
An element is a chemical substance made up of a particular kind of atom and hence cannot be broken down or transformed by a chemical reaction into a different element, though it can be transmuted into another element through a nuclear reaction.
Many more types of subatomic particles have been found. Most such particles (but not electrons) were eventually found to be composed of even smaller particles such as quarks. Particle physics studies these smallest particles; nuclear physics studies atomic nuclei and their (immediate) constituents: protons and neutrons.