Not to be confused with Atomic mass, Mass number, or Atomic weight.
An explanation of the superscripts and subscripts seen in atomic number notation. Atomic number is the number of protons, and therefore also the total positive charge, in the atomic nucleus.The Rutherford–Bohr model of the hydrogen atom (Z = 1) or a hydrogen-like ion (Z > 1). In this model, it is an essential feature that the photon energy (or frequency) of the electromagnetic radiation emitted (shown) when an electron jumps from one orbital to another be proportional to the mathematical square of atomic charge (Z2). Experimental measurements by Henry Moseley of this radiation for many elements (from Z = 13 to 92) showed the results as predicted by Bohr. Both the concept of atomic number and the Bohr model were thereby given scientific credence.
The atomic number or nuclear charge number (symbol Z) of a chemical element is the charge number of an atomic nucleus. For ordinary nuclei composed of protons and neutrons, this is equal to the proton number (np) or the number of protons found in the nucleus of every atom of that element. The atomic number can be used to uniquely identify ordinary chemical elements. In an ordinary uncharged atom, the atomic number is also equal to the number of electrons.
For an ordinary atom which contains protons, neutrons and electrons, the sum of the atomic number Z and the neutron number N gives the atom's atomic mass number A. Since protons and neutrons have approximately the same mass (and the mass of the electrons is negligible for many purposes) and the mass defect of the nucleon binding is always small compared to the nucleon mass, the atomic mass of any atom, when expressed in daltons (making a quantity called the "relative isotopic mass"), is within 1% of the whole number A.
Atoms with the same atomic number but different neutron numbers, and hence different mass numbers, are known as isotopes. A little more than three-quarters of naturally occurring elements exist as a mixture of isotopes (see monoisotopic elements), and the average isotopic mass of an isotopic mixture for an element (called the relative atomic mass) in a defined environment on Earth determines the element's standard atomic weight. Historically, it was these atomic weights of elements (in comparison to hydrogen) that were the quantities measurable by chemists in the 19th century.
The conventional symbol Z comes from the German word Zahl 'number', which, before the modern synthesis of ideas from chemistry and physics, merely denoted an element's numerical place in the periodic table, whose order was then approximately, but not completely, consistent with the order of the elements by atomic weights. Only after 1915, with the suggestion and evidence that this Z number was also the nuclear charge and a physical characteristic of atoms, did the word Atomzahl (and its English equivalent atomic number) come into common use in this context.
The rules above do not always apply to exotic atoms which contain short-lived elementary particles other than protons, neutrons and electrons.
The atomicnumber or nuclear charge number (symbol Z) of a chemical element is the charge number of an atomic nucleus. For ordinary nuclei composed of...
is a type of atom which has a specific number of protons in its atomic nucleus (i.e., a specific atomicnumber, or Z). The definitive visualisation of...
mass number (symbol A, from the German word: Atomgewicht, "atomic weight"), also called atomic mass number or nucleon number, is the total number of protons...
lowest mass) has an atomic weight of 1.007825 Da. The value of this number is called the atomic mass. A given atom has an atomic mass approximately equal...
atomicnumber, denoted by Zeff, may refer to: Effective nuclear charge of an individual atom, as felt by electrons within that atom Effective atomic number...
contributions. Thus, the numeric value of the atomic mass when expressed in daltons has nearly the same value as the mass number. Conversion between mass in kilograms...
Lead is a chemical element; it has symbol Pb (from Latin plumbum) and atomicnumber 82. It is a heavy metal that is denser than most common materials. Lead...
unique atomicnumber (Z) representing the number of protons in its nucleus. All elements have multiple isotopes, variants with the same number of protons...
identified by the number of protons in the nuclei of their atoms, known as the element's atomicnumber. For example, oxygen has an atomicnumber of 8, meaning...
chemical element with the symbol Au (from the Latin word aurum) and the atomicnumber 79. In its pure form, it is a bright, slightly orange-yellow, dense...
Hydrogen is a chemical element; it has symbol H and atomicnumber 1. It is the lightest element and, at standard conditions, is a gas of diatomic molecules...
The atomic radius of a chemical element is a measure of the size of its atom, usually the mean or typical distance from the center of the nucleus to the...
Oxygen is a chemical element; it has symbol O and atomicnumber 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal...
(from Latin carbo 'coal') is a chemical element; it has symbol C and atomicnumber 6. It is nonmetallic and tetravalent—meaning that its atoms are able...
Nitrogen is a chemical element; it has symbol N and atomicnumber 7. Nitrogen is a nonmetal and the lightest member of group 15 of the periodic table...
λίθος (líthos) 'stone') is a chemical element; it has symbol Li and atomicnumber 3. It is a soft, silvery-white alkali metal. Under standard conditions...
(or nuclides) of the same chemical element. They have the same atomicnumber (number of protons in their nuclei) and position in the periodic table (and...
each element has a unique number of protons, each element has its own unique atomicnumber, which determines the number of atomic electrons and consequently...
Boron is a chemical element; it has symbol B and atomicnumber 5. In its crystalline form it is a brittle, dark, lustrous metalloid; in its amorphous...
romanized: helios, lit. 'sun') is a chemical element; it has symbol He and atomicnumber 2. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic...
Neon is a chemical element; it has symbol Ne and atomicnumber 10. It is the second noble gas in the periodic table. Neon is a colorless, odorless, inert...
Beryllium is a chemical element; it has symbol Be and atomicnumber 4. It is a steel-gray, strong, lightweight and brittle alkaline earth metal. It is...
building blocks of reality Atomic City (disambiguation) Atomic formula, a formula without subformulas Atomicnumber, the number of protons found in the nucleus...
their atomicnumber, electron configuration and recurring chemical properties. In the basic form, elements are presented in order of increasing atomic number...
Fluorine is a chemical element; it has symbol F and atomicnumber 9. It is the lightest halogen and exists at standard conditions as a highly toxic, pale...
In quantum mechanics, an atomic orbital (/ˈɔːrbɪtəl/) is a function describing the location and wave-like behavior of an electron in an atom. This function...
which had the temporary name of unniloctium, based on the digits of its atomicnumber. There are also some historical symbols that are no longer officially...
In atomic physics and quantum chemistry, the Aufbau principle (/ˈaʊfbaʊ/, from German: Aufbauprinzip, lit. 'building-up principle'), also called the Aufbau...
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