Though the word atom originally denoted a particle that cannot be cut into smaller particles, in modern scientific usage the atom is composed of various subatomic particles. The constituent particles of an atom are the electron , the proton and the neutron ; all three are fermions. However, the hydrogen-1 atom has no neutrons and the hydron ion has no electrons. The electron is by far the least massive of these particles at 9. Under ordinary conditions, electrons are bound to the positively charged nucleus by the attraction created from opposite electric charges.
If an atom has more or fewer electrons than its atomic number, then it becomes respectively negatively or positively charged as a whole; a charged atom is called an ion.
Electrons have been known since the late 19th century, mostly thanks to J. Thomson ; see history of subatomic physics for details. Protons have a positive charge and a mass 1, times that of the electron, at 1. The number of protons in an atom is called its atomic number.
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Ernest Rutherford observed that nitrogen under alpha-particle bombardment ejects what appeared to be hydrogen nuclei. By he had accepted that the hydrogen nucleus is a distinct particle within the atom and named it proton. Neutrons have no electrical charge and have a free mass of 1, times the mass of the electron, or 1. Neutrons and protons collectively known as nucleons have comparable dimensions—on the order of 2. In the Standard Model of physics, electrons are truly elementary particles with no internal structure.
However, both protons and neutrons are composite particles composed of elementary particles called quarks. There are two types of quarks in atoms, each having a fractional electric charge. Neutrons consist of one up quark and two down quarks. This distinction accounts for the difference in mass and charge between the two particles. The quarks are held together by the strong interaction or strong force , which is mediated by gluons.
The protons and neutrons, in turn, are held to each other in the nucleus by the nuclear force , which is a residuum of the strong force that has somewhat different range-properties see the article on the nuclear force for more.
The gluon is a member of the family of gauge bosons , which are elementary particles that mediate physical forces. All the bound protons and neutrons in an atom make up a tiny atomic nucleus , and are collectively called nucleons. The radius of a nucleus is approximately equal to 1. The nucleons are bound together by a short-ranged attractive potential called the residual strong force.
At distances smaller than 2.
Atoms of the same element have the same number of protons, called the atomic number. Within a single element, the number of neutrons may vary, determining the isotope of that element. The total number of protons and neutrons determine the nuclide. The number of neutrons relative to the protons determines the stability of the nucleus, with certain isotopes undergoing radioactive decay. The proton, the electron, and the neutron are classified as fermions. Fermions obey the Pauli exclusion principle which prohibits identical fermions, such as multiple protons, from occupying the same quantum state at the same time.
Thus, every proton in the nucleus must occupy a quantum state different from all other protons, and the same applies to all neutrons of the nucleus and to all electrons of the electron cloud. A nucleus that has a different number of protons than neutrons can potentially drop to a lower energy state through a radioactive decay that causes the number of protons and neutrons to more closely match.
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As a result, atoms with matching numbers of protons and neutrons are more stable against decay. However, with increasing atomic number, the mutual repulsion of the protons requires an increasing proportion of neutrons to maintain the stability of the nucleus, which slightly modifies this trend of equal numbers of protons to neutrons. The number of protons and neutrons in the atomic nucleus can be modified, although this can require very high energies because of the strong force. Nuclear fusion occurs when multiple atomic particles join to form a heavier nucleus, such as through the energetic collision of two nuclei.
For example, at the core of the Sun protons require energies of 3—10 keV to overcome their mutual repulsion—the coulomb barrier —and fuse together into a single nucleus. The nucleus can also be modified through bombardment by high energy subatomic particles or photons. If this modifies the number of protons in a nucleus, the atom changes to a different chemical element. This deficit is part of the binding energy of the new nucleus, and it is the non-recoverable loss of the energy that causes the fused particles to remain together in a state that requires this energy to separate.
The fusion of two nuclei that create larger nuclei with lower atomic numbers than iron and nickel —a total nucleon number of about 60—is usually an exothermic process that releases more energy than is required to bring them together. For heavier nuclei, the binding energy per nucleon in the nucleus begins to decrease. That means fusion processes producing nuclei that have atomic numbers higher than about 26, and atomic masses higher than about 60, is an endothermic process.
These more massive nuclei can not undergo an energy-producing fusion reaction that can sustain the hydrostatic equilibrium of a star.
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The electrons in an atom are attracted to the protons in the nucleus by the electromagnetic force. This force binds the electrons inside an electrostatic potential well surrounding the smaller nucleus, which means that an external source of energy is needed for the electron to escape. The closer an electron is to the nucleus, the greater the attractive force. Hence electrons bound near the center of the potential well require more energy to escape than those at greater separations. Electrons, like other particles, have properties of both a particle and a wave.
The electron cloud is a region inside the potential well where each electron forms a type of three-dimensional standing wave —a wave form that does not move relative to the nucleus. This behavior is defined by an atomic orbital , a mathematical function that characterises the probability that an electron appears to be at a particular location when its position is measured. Each atomic orbital corresponds to a particular energy level of the electron. The electron can change its state to a higher energy level by absorbing a photon with sufficient energy to boost it into the new quantum state.
Likewise, through spontaneous emission , an electron in a higher energy state can drop to a lower energy state while radiating the excess energy as a photon. These characteristic energy values, defined by the differences in the energies of the quantum states, are responsible for atomic spectral lines. The amount of energy needed to remove or add an electron—the electron binding energy —is far less than the binding energy of nucleons.
For example, it requires only Atoms that have either a deficit or a surplus of electrons are called ions. Electrons that are farthest from the nucleus may be transferred to other nearby atoms or shared between atoms. By this mechanism, atoms are able to bond into molecules and other types of chemical compounds like ionic and covalent network crystals. By definition, any two atoms with an identical number of protons in their nuclei belong to the same chemical element.
Atoms with equal numbers of protons but a different number of neutrons are different isotopes of the same element. For example, all hydrogen atoms admit exactly one proton, but isotopes exist with no neutrons hydrogen-1 , by far the most common form,  also called protium , one neutron deuterium , two neutrons tritium and more than two neutrons.
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The known elements form a set of atomic numbers, from the single proton element hydrogen up to the proton element oganesson. However, only 90 of these nuclides are stable to all decay, even in theory. Another bringing the total to have not been observed to decay, even though in theory it is energetically possible. These are also formally classified as "stable". An additional 34 radioactive nuclides have half-lives longer than 80 million years, and are long-lived enough to be present from the birth of the solar system.
This collection of nuclides are known as primordial nuclides.
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Finally, an additional 51 short-lived nuclides are known to occur naturally, as daughter products of primordial nuclide decay such as radium from uranium , or else as products of natural energetic processes on Earth, such as cosmic ray bombardment for example, carbon For 80 of the chemical elements, at least one stable isotope exists. As a rule, there is only a handful of stable isotopes for each of these elements, the average being 3.
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Twenty-six elements have only a single stable isotope, while the largest number of stable isotopes observed for any element is ten, for the element tin. Elements 43 , 61 , and all elements numbered 83 or higher have no stable isotopes. Stability of isotopes is affected by the ratio of protons to neutrons, and also by the presence of certain "magic numbers" of neutrons or protons that represent closed and filled quantum shells.
These quantum shells correspond to a set of energy levels within the shell model of the nucleus; filled shells, such as the filled shell of 50 protons for tin, confers unusual stability on the nuclide. Of the known stable nuclides, only four have both an odd number of protons and odd number of neutrons: hydrogen-2 deuterium , lithium-6 , boron and nitrogen Also, only four naturally occurring, radioactive odd—odd nuclides have a half-life over a billion years: potassium , vanadium , lanthanum and tantalumm.
Most odd—odd nuclei are highly unstable with respect to beta decay , because the decay products are even—even, and are therefore more strongly bound, due to nuclear pairing effects. The large majority of an atom's mass comes from the protons and neutrons that make it up. The total number of these particles called "nucleons" in a given atom is called the mass number.
It is a positive integer and dimensionless instead of having dimension of mass , because it expresses a count. An example of use of a mass number is "carbon," which has 12 nucleons six protons and six neutrons. The actual mass of an atom at rest is often expressed using the unified atomic mass unit u , also called dalton Da.
This unit is defined as a twelfth of the mass of a free neutral atom of carbon , which is approximately 1. However, this number will not be exactly an integer except in the case of carbon see below. As even the most massive atoms are far too light to work with directly, chemists instead use the unit of moles.
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One mole of atoms of any element always has the same number of atoms about 6. Atoms lack a well-defined outer boundary, so their dimensions are usually described in terms of an atomic radius. This is a measure of the distance out to which the electron cloud extends from the nucleus. Atomic radii may be derived from the distances between two nuclei when the two atoms are joined in a chemical bond.