Atom

An atom is the smallest constituent unit of ordinary matter that has the properties of a chemical element.

A chemical element is a species of atom having the same number of protons in their atomic nuclei (that is, the same atomic number, or Z). For example, the atomic number of oxygen is 8, so the element oxygen consists of all atoms which have exactly 8 protons.

An atom is the smallest constituent unit of ordinary matter that has the properties of a chemical element.

All everyday objects that can be touched are ultimately composed of atoms, which are made up of interacting subatomic particles, and in everyday as well as scientific usage, "matter" generally includes atoms and anything made up of them, and any particles (or combination of particles) that act as if they have both rest mass and volume.

Every solid, liquid, gas, and plasma is composed of neutral or ionized atoms.

A pure gas may be made up of individual atoms (e.g. a noble gas like neon), elemental molecules made from one type of atom (e.g. oxygen), or compound molecules made from a variety of atoms (e.g. carbon dioxide).

Atoms are small enough that attempting to predict their behavior using classical physics – as if they were billiard balls, for example – gives noticeably incorrect predictions due to quantum effects.

Quantum mechanics (QM; also known as quantum physics, quantum theory, the wave mechanical model, or matrix mechanics), including quantum field theory, is a fundamental theory in physics which describes nature at the smallest scales of energy levels of atoms and subatomic particles.

The nucleus is made of one or more protons and typically a similar number of neutrons.

Protons and neutrons constitute the nuclei of atoms.

Every atom is composed of a nucleus and one or more electrons bound to the nucleus.

The atomic nucleus is the small, dense region consisting of protons and neutrons at the center of an atom, discovered in 1911 by Ernest Rutherford based on the 1909 Geiger–Marsden gold foil experiment.

If an atom has more or fewer electrons than protons, then it has an overall negative or positive charge, respectively, and it is called an ion.

An ion is an atom or molecule that has a non-zero net electrical charge.

The nucleus is made of one or more protons and typically a similar number of neutrons.

One or more protons are present in the nucleus of every atom; they are a necessary part of the nucleus.

The protons and neutrons in the nucleus are attracted to each other by a different force, the nuclear force, which is usually stronger than the electromagnetic force repelling the positively charged protons from one another.

The nuclear force (or nucleon–nucleon interaction or residual strong force) is a force that acts between the protons and neutrons of atoms.

Atoms can attach to one or more other atoms by chemical bonds to form chemical compounds such as molecules.

A chemical bond is a lasting attraction between atoms, ions or molecules that enables the formation of chemical compounds.

Under certain circumstances, the repelling electromagnetic force becomes stronger than the nuclear force, and nucleons can be ejected from the nucleus, leaving behind a different element: nuclear decay resulting in nuclear transmutation.

Because any element (or isotope of one) is defined by its number of protons (and neutrons) in its atoms, i.e. in the atomic nucleus, nuclear transmutation occurs in any process where the number of protons or neutrons in the nucleus is changed.

The ability of atoms to associate and dissociate is responsible for most of the physical changes observed in nature and is the subject of the discipline of chemistry.

Chemistry is the scientific discipline involved with elements and compounds composed of atoms, molecules and ions: their composition, structure, properties, behavior and the changes they undergo during a reaction with other substances.

Atoms can attach to one or more other atoms by chemical bonds to form chemical compounds such as molecules.

A molecule is an electrically neutral group of two or more atoms held together by chemical bonds.

The number of neutrons defines the isotope of the element.

All isotopes of a given element have the same number of protons but different numbers of neutrons in each atom.

The protons have a positive electric charge, the electrons have a negative electric charge, and the neutrons have no electric charge.

In ordinary matter, negative charge is carried by electrons, and positive charge is carried by the protons in the nuclei of atoms.

Every atom is composed of a nucleus and one or more electrons bound to the nucleus.

The Coulomb force interaction between the positive protons within atomic nuclei and the negative electrons without, allows the composition of the two known as atoms.

The word "atom" (atomos), meaning "uncuttable", was coined by the ancient Greek philosophers Leucippus and his pupil Democritus (c.

BCE) is reported in some ancient sources to have been a philosopher who was the earliest Greek to develop the theory of atomism—the idea that everything is composed entirely of various imperishable, indivisible elements called atoms.

Atoms are small enough that attempting to predict their behavior using classical physics – as if they were billiard balls, for example – gives noticeably incorrect predictions due to quantum effects.

Wave-like behavior of matter was first experimentally demonstrated by George Paget Thomson's thin metal diffraction experiment, and independently in the Davisson–Germer experiment both using electrons, and it has also been confirmed for other elementary particles, neutral atoms and even molecules.

Atoms can attach to one or more other atoms by chemical bonds to form chemical compounds such as molecules.

A chemical compound is a chemical substance composed of many identical molecules (or molecular entities) composed of atoms from more than one element held together by chemical bonds.

For example, he found that water absorbs carbon dioxide far better than it absorbs nitrogen.

Carbon dioxide consists of a carbon atom covalently double bonded to two oxygen atoms.

Therefore, they were not atoms, but a new particle, the first subatomic particle to be discovered, which he originally called "corpuscle" but was later named electron, after particles postulated by George Johnstone Stoney in 1874.

In the physical sciences, subatomic particles are particles much smaller than atoms.

In the fourteenth century, the rediscovery of major works describing atomist teachings, including Lucretius's De rerum natura and Diogenes Laërtius's Lives and Opinions of Eminent Philosophers, led to increased scholarly attention on the subject.

These phenomena are the result of regular, but purposeless motions and interactions of tiny atoms in empty space.

In 1913 the physicist Niels Bohr proposed a model in which the electrons of an atom were assumed to orbit the nucleus but could only do so in a finite set of orbits, and could jump between these orbits only in discrete changes of energy corresponding to absorption or radiation of a photon.

Niels Henrik David Bohr (7 October 1885 – 18 November 1962) was a Danish physicist who made foundational contributions to understanding atomic structure and quantum theory, for which he received the Nobel Prize in Physics in 1922.

This became known as the plum pudding model.

The plum pudding model is one of several scientific models of the atom.

In 1909, Hans Geiger and Ernest Marsden, under the direction of Ernest Rutherford, bombarded a metal foil with alpha particles to observe how they scattered.

and thereby pioneered the Rutherford model of the atom, through his discovery and interpretation of Rutherford scattering by the gold foil experiment of Hans Geiger and Ernest Marsden.