Electrons and Atoms

An atom (Greek átomos meaning "the smallest indivisible particle of matter, i.e., something that cannot be divided") is the smallest particle characterizing a chemical element. An atom consists of a dense nucleus of positively charged protons and electrically neutral neutrons, surrounded by a much larger electron cloud consisting of negatively charged electrons. When the number of protons matches the number of electrons, an atom is electrically neutral; otherwise it is an ion and has a net charge. An atom is classified according to its protons and neutrons: the number of protons determines the chemical element and the number of neutrons determines the isotope of that element.

The concept of the atom as an indivisible component of matter was first proposed by early Indian and Greek philosophers. In the 17th and 18th centuries, chemists provided a physical basis for this idea by showing that certain substances could not be further broken down by chemical methods. During the late 19th and the early 20th centuries, physicists discovered "subatomic" components and structure inside the "atom", but the name remained unchanged even through it was understood to no longer be indivisible. The principles of quantum mechanics, including the wave–particle duality of matter, were used to successfully model the atom.

Relative to everyday experience, atoms are minuscule objects with proportionately tiny masses that can only be observed individually using special instruments such as the scanning tunnelling microscope. More than 99.9% of an atom's mass is concentrated in the nucleus, with protons and neutrons having about equal mass. Depending on the number of protons and neutrons, the nucleus may be unstable and subject to radioactive decay. The electrons surrounding the nucleus occupy a set of stable energy levels, or orbitals, and they can transition between these states by the absorption or emission of photons that match the energy differences between the levels. The electrons determine the chemical properties of an element, and strongly influence an atom's magnetic properties.

Electrons

The electron is a fundamental subatomic particle that carries anegative electric charge. It is a spin-½ lepton that participates in electromagnetic interactions, its mass is approximately 1 / 1836 of the proton. Together with atomic nuclei (protons and neutrons), electrons make up atoms. Their interaction with adjacent nuclei is the main cause of chemical bonding.

With thanks to the "Wikipedia"