Stylized lithium-7 atom: 3 protons, 4 neutrons & 3 electrons (~1800 times smaller than protons/neutrons). Lithium (ˈlɪθiəm is a Chemical element with the symbol Li and Atomic number 3 Rare Lithium-6 has only 3 neutrons, reducing the atomic weight (average) to 6. 941.

The atomic mass (m_a) is the mass of an atom, most often expressed in unified atomic mass units. Mass is a fundamental concept in Physics, roughly corresponding to the Intuitive idea of how much Matter there is in an object The unified atomic mass unit ( u) or Dalton ( Da) or sometimes universal mass unit, is an unit of Mass used to express ^[1] The atomic mass may be considered to be the total mass of protons, neutrons and electrons in a single atom (when the atom is motionless). The proton ( Greek πρῶτον / proton "first" is a Subatomic particle with an Electric charge of one positive This article is a discussion of neutrons in general For the specific case of a neutron found outside the nucleus see Free neutron. The electron is a fundamental Subatomic particle that was identified and assigned the negative charge in 1897 by J History See also Atomic theory, Atomism The concept that matter is composed of discrete units and cannot be divided into arbitrarily tiny The atomic mass is sometimes incorrectly used as a synonym of relative atomic mass, average atomic mass and atomic weight; however, these differ subtly from the atomic mass. The atomic mass is defined as the mass of an atom, which can only be one isotope at a time and is not an abundance-weighted average. Isotopes (Greek isos = "equal" tópos = "site place" are any of the different types of atoms ( Nuclides In the case of many elements that have one dominant isotope the actual numerical difference between the atomic mass of the most common isotope and the relative atomic mass or standard atomic weights can be very small such that it does not affect most bulk calculations but such an error can be critical when considering individual atoms. For elements with more than one common isotope the difference even to the most common atomic mass can be half a mass unit or more (e. g. chlorine). Chlorine (ˈklɔriːn from the Greek word 'χλωρóς' ( khlôros, meaning 'pale green' is the Chemical element with Atomic number 17 and The atomic mass of an uncommon isotope can differ from the relative atomic mass or standard atomic weight by several mass units.

The relative atomic mass (A_r) (also known as atomic weight and average atomic mass) is the average of the atomic masses of all the chemical element's isotopes as found in a particular environment, weighted by isotopic abundance. Isotopes (Greek isos = "equal" tópos = "site place" are any of the different types of atoms ( Nuclides ^[2] This is frequently used as a synonym for the standard atomic weight and it is not incorrect to do so since the standard atomic weights are relative atomic masses, although it is less specific to do so. Relative atomic mass also refers to non-terrestrial environments and highly specific terrestrial environments that deviate from the average or have different certainties (number of significant figures) than the standard atomic weights.

The standard atomic weight refers to the mean relative atomic mass of an element in the local environment of the Earth's crust and atmosphere as determined by the IUPAC Commission on Atomic Weights and Isotopic Abundances. The interior of Earth, similar to the other Terrestrial planets, is Chemically divided into layers Temperature and layers The temperature of the Earth's atmosphere varies with altitude the mathematical relationship between temperature and altitude varies among five The International Union of Pure and Applied Chemistry ( IUPAC) (aɪjuːpæk or ay-yoo-pec) is an international Non-governmental organization ^[3] These are what are included in a standard periodic table and is what is used in most bulk calculations. The periodic table of the chemical elements is a tabular method of displaying the Chemical elements Although precursors to this table exist its invention is An uncertainty in brackets is included which often reflects natural variability in isotopic distribution rather than uncertainty in measurement. ^[4] For synthetic elements the isotope formed depends on the means of synthesis, so the concept of natural isotope abundance has no meaning. In chemistry the Chemical elements labeled as synthetic are too unstable to be found naturally on Earth. Therefore, for synthetic elements the total nucleon count of the most stable isotope (ie, the isotope with the longest half-life) is listed in brackets in place of the standard atomic weight. Lithium represents a unique case where the natural abundances of the isotopes have been perturbed by human activities to the point of affecting the uncertainty in its standard atomic weight, even in samples obtained from natural sources such as rivers. Lithium (ˈlɪθiəm is a Chemical element with the symbol Li and Atomic number 3

The relative isotopic mass is the relative mass of the isotope, scaled with carbon-12 as exactly 12. Carbon-12 is the most abundant of the two stable Isotopes of the element Carbon, accounting for 98 No other isotopes have whole number masses due to the different mass of neutrons and protons, as well as loss/gain of mass to binding energy. Binding energy is the Mechanical energy required to disassemble a whole into separate parts However, since mass defect due to binding energy is minimal compared to the mass of a nucleon, rounding the atomic mass of an isotope tells you the total nucleon count. Binding energy is the Mechanical energy required to disassemble a whole into separate parts Neutron count can then be derived by subtracting the atomic number. See also List of elements by atomic number In Chemistry and Physics, the atomic number (also known as the proton

Mass defects in atomic masses

The pattern in the amounts the atomic masses deviate from their mass numbers is as follows: the deviation starts positive at hydrogen-1, becomes negative until a minimum is reached at iron-56, iron-58 and nickel-62, then increases to positive values in the heavy isotopes, with increasing atomic number. Hydrogen (ˈhaɪdrədʒən is the Chemical element with Atomic number 1 Iron (ˈаɪɚn is a Chemical element with the symbol Fe (ferrum and Atomic number 26 Nickel (ˈnɪkəl is a metallic Chemical element with the symbol Ni and Atomic number 28 This equals to the following: nuclear fission in an element heavier than iron produces energy, and fission in any element lighter than iron requires energy. Nuclear fission is the splitting of the nucleus of an atom into parts (lighter nuclei) often producing Free neutrons and other smaller nuclei which may Iron (ˈаɪɚn is a Chemical element with the symbol Fe (ferrum and Atomic number 26 The opposite is true of nuclear fusion reactions: fusion in elements lighter than iron produces energy, and fusion in elements heavier than iron requires energy. In Physics and Nuclear chemistry, nuclear fusion is the process by which multiple- like charged atomic nuclei join together to form a heavier nucleus

Measurement of atomic masses

Direct comparison and measurement of the masses of atoms is achieved with mass spectrometry. Mass spectrometry is an analytical technique that identifies the chemical composition of a compound or sample based on the Mass-to-charge ratio of charged particles

Conversion factor between atomic mass units and grams

The standard scientific unit for dealing with atoms in macroscopic quantities is the mole (mol), which is defined arbitrarily as the amount of a substance with as many atoms or other units as there are in 12 grams of the carbon isotope C-12. The mole (symbol mol) is a unit of Amount of substance: it is an SI base unit, and almost the only unit to be used to measure this The number of atoms in a mole is called Avogadro's number, the value of which is approximately 6. The Avogadro constant (symbols L, N A also called Avogadro's number, is the number of "elementary entities" (usually Atoms 022 × 10²³ mol^-1. One mole of a substance always contains almost exactly the relative atomic mass or molar mass of that substance (which is the concept of molar mass), expressed in grams; however, this is almost never true for the atomic mass. The atomic mass (ma is the Mass of an atom most often expressed in unified atomic mass units The atomic mass may be considered to be the total mass Molar mass, symbol M, is the Mass of one mole of a substance ( Chemical element or Chemical compound) Molar mass, symbol M, is the Mass of one mole of a substance ( Chemical element or Chemical compound) For example, the standard atomic weight of iron is 55. The atomic mass (ma is the Mass of an atom most often expressed in unified atomic mass units The atomic mass may be considered to be the total mass Iron (ˈаɪɚn is a Chemical element with the symbol Fe (ferrum and Atomic number 26 847 g/mol, and therefore one mole of iron as commonly found on earth has a mass of 55. 847 grams. The atomic mass of an ⁵⁶Fe isotope is 55. 935 u and one mole of ⁵⁶Fe will in theory weigh 55. 935g, but such amounts of pure ⁵⁶Fe have never existed.

The formulaic conversion between atomic mass and SI mass in grams for a single atom is:

where u is the atomic mass unit and N_A is Avogadro's number. The unified atomic mass unit ( u) or Dalton ( Da) or sometimes universal mass unit, is an unit of Mass used to express The Avogadro constant (symbols L, N A also called Avogadro's number, is the number of "elementary entities" (usually Atoms

Relationship between atomic and molecular masses

Similar definitions apply to molecules. In Chemistry, a molecule is defined as a sufficiently stable electrically neutral group of at least two Atoms in a definite arrangement held together by One can compute the molecular mass of a compound by adding the atomic masses of its constituent atoms (nuclides). The molecular mass (abbreviated m of a substance, more commonly referred to as molecular weight and abbreviated as MW, is the Mass of one One can compute the molar mass of a compound by adding the relative atomic masses of the elements given in the chemical formula. Molar mass, symbol M, is the Mass of one mole of a substance ( Chemical element or Chemical compound) A chemical formula is a way of expressing information about the Atoms that constitute a particular Chemical compound, and how the relationship between those atoms changes In both cases the multiplicity of the atoms (the number of times it occurs) must be taken into account, usually by multiplication of each unique mass by its multiplicity.

History

In the history of chemistry the first scientists to determine atomic weights were John Dalton between 1803 and 1805 and Jöns Jakob Berzelius between 1808 and 1826. The history of Chemistry begins with the discovery of Fire, then Metallurgy which allowed purification of metals and the making of alloys as well as the exploitation John Dalton FRS (6 September 1766 &ndash 27 July 1844 was an English Chemist, Meteorologist and Physicist. Friherre Jöns Jacob Berzelius (20 August 1779 &ndash 7 August 1848 was a Swedish chemist Atomic weight was originally defined relative to that of the lightest element hydrogen taken as 1. 00. In the 1860's Stanislao Cannizzaro refined atomic weights by applying Avogadro's law (notably at the Karlsruhe Congress of 1860). Avogadro's law ( Avogadro's Hypothesis, or Avogadro's Principle) is a Gas law named after Amedeo Avogadro, who in 1811 hypothesized The Karlsruhe Congress was an international meeting of chemists held in Karlsruhe, Germany from September 3 1860 to September 5 1860 He formulated a law to determine atomic weights of elements: the different quantities of the same element contained in different molecules are all whole multiples of the atomic weight and determined atomic weights and molecular weights by comparing the vapor density of a collection of gases with molecules containing one or more of the chemical element in question ^[5]. Vapour density is the Density of a Vapour in relation to that of Hydrogen.

In the early twentieth century, up until the 1960's chemists and physicists used two different atomic mass scales. Chemistry (from Egyptian kēme (chem meaning "earth") is the Science concerned with the composition structure and properties Physics (Greek Physis - φύσις in everyday terms is the Science of Matter and its motion. The chemists used a scale such that the natural mixture of oxygen isotopes had an atomic mass 16, while the physicists assigned the same number 16 to the atomic mass of the most common oxygen isotope (containing eight protons and eight neutrons). Oxygen (from the Greek roots ὀξύς (oxys (acid literally "sharp" from the taste of acids and -γενής (-genēs (producer literally begetteris the However, because oxygen-17 and oxygen-18 are also present in natural oxygen this led to two different tables of atomic mass. There are three stable isotopes of oxygen that lead to Oxygen ( O) having a standard atomic mass of 15 Oxygen-18 ( 18O) is a natural stable Isotope of Oxygen and one of the Environmental isotopes. Oxygen (from the Greek roots ὀξύς (oxys (acid literally "sharp" from the taste of acids and -γενής (-genēs (producer literally begetteris the The unified scale based on carbon-12, ¹²C, met the physicists' need to base the scale on a pure isotope, while being numerically close to the old chemists' scale.

The term atomic weight is being phased out slowly and being replaced by relative atomic mass, in most current usage. The history of this shift in nomenclature reaches back to the 1960's and has been the source of much debate in the scientific community. The debate was largely created by the adoption of the unified atomic mass unit and the realization that weight was in some ways an inappropriate term. The unified atomic mass unit ( u) or Dalton ( Da) or sometimes universal mass unit, is an unit of Mass used to express The argument for keeping the term "atomic weight" was primarily that it was a well understood term to those in the field, that the term "atomic mass" was already in use (as it is currently defined) and that the term "relative atomic mass" was in some ways redundant. In 1979, in a compromise move, the definition was refined and the term "relative atomic mass" was introduced as a secondary synonym. Twenty years later the primacy of these synonyms was reversed and the term "relative atomic mass" is now the preferred term; however the "standard atomic weights" have maintained the same name. ^[6]

Table of standard atomic weights

See also

• atomic number
• Atomic mass unit
• Isotope
• Molecular mass
• Jean Stas

External links

• NIST relative atomic masses of all isotopes and the standard atomic weights of the elements

• Tutorial on the concept and measurement of atomic mass

• Atomic Weights and the International Committee — A Historical Review

References