Atom | Definition, Structure, History, Examples, Diagram, & Facts (2024)

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Written by

James Trefil Clarence J. Robinson Professor of Physics, George Mason University, Fairfax, Virginia. Author of Science Matters: Achieving Scientific Literacy; Other Worlds: The Solar System and Beyond; and Encyclopedia...

James Trefil,

Sharon Bertsch McGrayne Science writer. Author of Nobel Prize Women in Science, Prometheans in the Lab, The Theory That Would Not Die, and others.

Sharon Bertsch McGrayneAll

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Key People:
Ernest Rutherford
Niels Bohr
Lev Davidovich Landau
Steven Chu
William D. Phillips
Related Topics:
subatomic particle
radioactivity
isotope
atomism
periodic table

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Top Questions

What is an atom?

An atom is the basic building block of chemistry. It is the smallest unit into which matter can be divided without the release of electrically charged particles. It also is the smallest unit of matter that has the characteristic properties of a chemical element.

Are all atoms the same size?

All atoms are roughly the same size, whether they have 3 or 90 electrons. Approximately 50 million atoms of solid matter lined up in a row would measure 1 cm (0.4 inches). A convenient unit of length for measuring atomic sizes is the angstrom, defined as 10−10 meters.

What does the mass of an atom consist of?

The mass of an atom consists of the mass of the nucleus plus that of the electrons. That means the atomic mass unit is not exactly the same as the mass of the proton or neutron.

How is the atomic number of an atom defined?

The single most important characteristic of an atom is its atomic number (usually denoted by the letter Z), which is defined as the number of units of positive charge (protons) in the nucleus. For example, if an atom has a Z of 6, it is carbon, while a Z of 92 corresponds to uranium.

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atom, the basic building block of all matter and chemistry. Atoms can combine with other atoms to form molecules but cannot be divided into smaller parts by ordinary chemical processes.

Most of the atom is empty space. The rest consists of three basic types of subatomic particles: protons, neutrons, and electrons. The protons and neutrons form the atom’s central nucleus. (The ordinary hydrogen atom is an exception; it contains one proton but no neutrons.) As their names suggest, protons have a positive electrical charge, while neutrons are electrically neutral—they carry no charge; overall, then, the nucleus has a positive charge. Circling the nucleus is a cloud of electrons, which are negatively charged. Like opposite ends of a magnet that attract one another, the negative electrons are attracted to a positive force, which binds them to the nucleus. The nucleus is small and dense compared with the electrons, which are the lightest charged particles in nature. The electrons circle the nucleus in orbital paths called shells, each of which holds only a certain number of electrons.

An ordinary, neutral atom has an equal number of protons (in the nucleus) and electrons (surrounding the nucleus). Thus the positive and negative charges are balanced. Some atoms, however, lose or gain electrons in chemical reactions or in collisions with other particles. Ordinary atoms that either gain or lose electrons are called ions. If a neutral atom loses an electron, it becomes a positive ion. If it gains an electron, it becomes a negative ion. These basic subatomic particles—protons, neutrons, and electrons—are themselves made up of smaller substances, such as quarks and leptons.

More than 90 types of atoms exist in nature, and each kind of atom forms a different chemical element. Chemical elements are made up of only one type of atom—gold contains only gold atoms, and neon contains only neon atoms--and they are ranked in order of their atomic number (the total number of protons in its nucleus) in a chart called the periodic table. Accordingly, because an atom of iron has 26 protons in its nucleus, its atomic number is 26 and its ranking on the periodic table of chemical elements is 26. Because an ordinary atom has the same number of electrons as protons, an element’s atomic number also tells how many electrons its atoms have, and it is the number and arrangement of the electrons in their orbiting shells that determines how one atom interacts with another. The key shell is the outermost one, called the valence shell. If this outermost shell is complete, or filled with the maximum number of electrons for that shell, the atom is stable, with little or no tendency to interact with other atoms. But atoms with incomplete outer shells seek to fill or to empty such shells by gaining or losing electrons or by sharing electrons with other atoms. This is the basis of an atom’s chemical activity. Atoms that have the same number of electrons in the outer shell have similar chemical properties.

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This article opens with a broad overview of the fundamental properties of the atom and its constituent particles and forces. Following this overview is a historical survey of the most influential concepts about the atom that have been formulated through the centuries.

Atomic model

Most matter consists of an agglomeration of molecules, which can be separated relatively easily. Molecules, in turn, are composed of atoms joined by chemical bonds that are more difficult to break. Each individual atom consists of smaller particles—namely, electrons and nuclei. These particles are electrically charged, and the electric forces on the charge are responsible for holding the atom together. Attempts to separate these smaller constituent particles require ever-increasing amounts of energy and result in the creation of new subatomic particles, many of which are charged.

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As noted in the introduction to this article, an atom consists largely of empty space. The nucleus is the positively charged centre of an atom and contains most of its mass. It is composed of protons, which have a positive charge, and neutrons, which have no charge. Protons, neutrons, and the electrons surrounding them are long-lived particles present in all ordinary, naturally occurring atoms. Other subatomic particles may be found in association with these three types of particles. They can be created only with the addition of enormous amounts of energy, however, and are very short-lived.

All atoms are roughly the same size, whether they have 3 or 90 electrons. Approximately 50 million atoms of solid matter lined up in a row would measure 1 cm (0.4 inch). A convenient unit of length for measuring atomic sizes is the angstrom (Å), defined as 10−10 metre. The radius of an atom measures 1–2 Å. Compared with the overall size of the atom, the nucleus is even more minute. It is in the same proportion to the atom as a marble is to a football field. In volume the nucleus takes up only 10−14 metres of the space in the atom—i.e., 1 part in 100,000. A convenient unit of length for measuring nuclear sizes is the femtometre (fm), which equals 10−15 metre. The diameter of a nucleus depends on the number of particles it contains and ranges from about 4 fm for a light nucleus such as carbon to 15 fm for a heavy nucleus such as lead. In spite of the small size of the nucleus, virtually all the mass of the atom is concentrated there. The protons are massive, positively charged particles, whereas the neutrons have no charge and are slightly more massive than the protons. The fact that nuclei can have anywhere from 1 to nearly 300 protons and neutrons accounts for their wide variation in mass. The lightest nucleus, that of hydrogen, is 1,836 times more massive than an electron, while heavy nuclei are nearly 500,000 times more massive.

Basic properties

Atomic number

The single most important characteristic of an atom is its atomic number (usually denoted by the letter Z), which is defined as the number of units of positive charge (protons) in the nucleus. For example, if an atom has a Z of 6, it is carbon, while a Z of 92 corresponds to uranium. A neutral atom has an equal number of protons and electrons so that the positive and negative charges exactly balance. Since it is the electrons that determine how one atom interacts with another, in the end it is the number of protons in the nucleus that determines the chemical properties of an atom.

Atom | Definition, Structure, History, Examples, Diagram, & Facts (2024)

FAQs

What is the history of the structure of the atom? ›

Early atomic models

Leucippus of Miletus (5th century bce) is thought to have originated the atomic philosophy of the early Greeks. His famous disciple, Democritus of Abdera, named the building blocks of matter atomos, meaning literally “indivisible,” about 430 bce.

What are the facts about the structure of an atom? ›

Atoms consist of an extremely small, positively charged nucleus surrounded by a cloud of negatively charged electrons. Although typically the nucleus is less than one ten-thousandth the size of the atom, the nucleus contains more that 99.9% of the mass of the atom.

What is the atom answer? ›

An atom is a particle of matter that uniquely defines a chemical element. An atom consists of a central nucleus that is surrounded by one or more negatively charged electrons. The nucleus is positively charged and contains one or more relatively heavy particles known as protons and neutrons.

Who invented atoms? ›

Thus, John Dalton known as father of atoms and atomic theory.

What are the 5 models of atom? ›

The better known atomic models are Dalton's atomic model, Thomson's atomic model, Rutherford's atomic model, Bohr's atomic model, and the quantum atomic model.

What is an atom with a diagram? ›

An atom consists of three main parts: protons, neutrons, and electrons. Protons have a positive electrical charge. Neutrons have no electrical charge. Electrons have a negative electrical charge. Protons and neutrons are found together in what is called the nucleus of the atom.

Are atoms a theory or fact? ›

The existence of atoms is an observable fact. The basic structure of each type of atom, including the number of protons, neutrons and electrons, is a demonstrable fact, as is the way in which atoms combine to produce compounds, crystals and other structure is also demonstrable fact.

What holds an atom together? ›

In an atom there are three fundamental forces that keep atoms together. electromagnetic force, strong nuclear force, and weak nuclear force. The electromagnetic force keeps the electrons attached to the atom. The strong nuclear force keeps the protons and neutrons together in the nucleus.

What atom is made of? ›

Atoms are composed of particles called protons, electrons and neutrons. Protons carry a positive electrical charge, electrons carry a negative electrical charge and neutrons carry no electrical charge at all.

Are humans made of atoms? ›

Atoms range in size from a few tenths of a nanometer to several nanometers across. More than 10 million hydrogen atoms would fit across the head of a pin. Atoms are the basic constituents of molecules, cells, humans, and planets. The human body contains about a billion billion billion (10^27) atoms.

What are the five parts of an atom? ›

Atomic Particles

Atoms consist of three basic particles: protons, electrons, and neutrons. The nucleus (center) of the atom contains the protons (positively charged) and the neutrons (no charge). The outermost regions of the atom are called electron shells and contain the electrons (negatively charged).

How long do atoms last? ›

Ultimately, even these stable atoms have a limit imposed by the lifetime of proton (>1025 years). Remember, though, that the best estimate of the present age of the universe is the much smaller number of 1010 years, so for all practical purposes, atoms are forever.

How many atoms are in the human body? ›

In summary, for a typical human of 70 kg, there are almost 7*1027 atoms (that's a 7 followed by 27 zeros!) Another way of saying this is "seven billion billion billion." Of this, almost 2/3 is hydrogen, 1/4 is oxygen, and about 1/10 is carbon. These three atoms add up to 99% of the total!

What are three examples of atoms? ›

Examples of atoms include single particles of the elements of the periodic table, such as sodium, uranium, argon, and chlorine.

What is the history of the understanding of the atom? ›

Around 450 B.C., the Greek philosopher Democritus introduced the idea of the atom. However, the idea was essentially forgotten for more than 2000 years. In 1800, John Dalton re-introduced the atom. He provided evidence for atoms and developed atomic theory.

How has the structure of the atom been discovered? ›

Rutherford's gold foil experiment showed that the atom is mostly empty space with a tiny, dense, positively-charged nucleus. Based on these results, Rutherford proposed the nuclear model of the atom.

Who introduced the first theory about the structure of an atom? ›

The 1st theory of an atom was given by sir John dalton that's why that theory was known as Dalton's atomic theory.

What is the theory of the atom structure? ›

atomic theory, ancient philosophical speculation that all things can be accounted for by innumerable combinations of hard, small, indivisible particles (called atoms) of various sizes but of the same basic material; or the modern scientific theory of matter according to which the chemical elements that combine to form ...

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