|Isotope||Atomic mass (Da)||Isotopic abundance (amount fraction)|
|13C||13.003 354 835(2)||[0.0096, 0.0116]|
2020-11-21 by Nick Connor Atomic Mass of Carbon Atomic mass of Carbon is 12.0107 u. Therefore the C electron configuration will be 1s 2 2s 2 2p 2. Video: Boron Electron Configuration Notation The configuration notation provides an easy way for scientists to write and communicate how electrons are arranged around the nucleus of an atom.
The 12C isotope has served since 1960 as the scale-determining reference for the definition of theunified atomic mass unit and is the basis of all atomic weights. The zero value for the delta scale usedin relative isotope-ratio measurements of carbon since the 1950s was based on a sample of fossil marine carbonate(Belemnitella Americana, Peedee Formation, Cretaceous Period, South Carolina, also known as PDB).
- Atomic number of an element never changes: for example, the atomic number of oxygen is always 8, and the atomic number of Chlorine is always 18. The atomic number is marked with the symbol Z, taken from a German word zahl (or atomzahl, which is 'atomic number' in German).
- C 12 is most abundant on Earth, constituting about 98.89% of the atoms in one mole of carbon, C 13 is about 1.109% and C 14 is the rarest (1 part in trillion). The longest-lived among the radioactive isotopes is carbon-14, with a half-life of 5700 years.
In 1961, the Commission recommended Ar(C) = 12.011 15(5) and in 1969 it recommended Ar(C) = 12.011(1).The larger uncertainty was assigned to include all terrestrial sources of carbon whose isotopic compositions had been measured tothat time. After the supply of PDB was exhausted, a modified delta scale was recommended for relativecarbon isotope-ratio measurements (referred to as the Vienna PDB, or VPDB scale) that yields the samezero value as the PDB scale when based on measurements of a new carbonate reference material knownas NBS 19. In 1995, the Commission recommended Ar(C) = 12.0107(8) as a result of a re-evaluationof variations in normal terrestrial materials.
Variations in the n(13C)/n(12C) ratio of terrestrial sources of carbon are caused largely by biogeochemicalreactions and physical processes. Some of the largest effects are associated with oxidation-reductionreactions including photosynthesis, such that organic substances and reduced natural gases typicallyare depleted in 13C relative to carbonate materials and the atmosphere. Differences in the degreeof 13C depletion during photosynthesis are characteristic of some groups of plants and may be passedalong to plant consumers, such that carbon isotope studies can be used to identify features of animal dietsand paleoclimates. Variations in the relative rates of organic carbon production, burial, and oxidation throughgeologic time are recorded in the isotopic compositions of sedimentary rocks. The highest reported 13C abundance is from dissolved carbonate in reduced marine sediment pore water with x(13C) = 0.011 466 andAr(C) = 12.011 50. The lowest reported 13C abundance is from crocetane recovered from the ocean bottom at cold seeps in the northern Pacific Ocean with x(13C) = 0.009 629 and Ar(C) = 12.009 66.
The radioactive 14C isotope has a half-life of 5730 a. It is introduced continuously to the near-surfaceenvironment of the earth by cosmic-ray reactions, from cosmic dust, and by nuclear technology. Itis of great interest for prehistoric dating as well as archaeological, anthropological, paleotemperature,and zoological studies. Yet, this isotope never occurs in normal carbon sources in concentrations high enoughto affect significantly the Ar(C) value. Before nuclear weapons tests, the abundance of 14C in the atmospherehad an average value of only about 10−16. It should be noted that a half-life of 5568 a (theso-called 'Libby half-life'), has been adopted by convention for calculations in geochronology.
Atomic Number Of C-14
Atomic weights of the elements 2009 by M.E. Wieser and T.B. Coplen. Pure Appl. Chem. 2011 (83) 359-396
Ar(C) = [12.0096, 12.0116] since 2009
The name derives from the Latin carbo for 'charcoal'. It was known in prehistoric times in the form ofcharcoal and soot. In 1797, the English chemist Smithson Tennant proved that diamond is pure carbon.
Natural variations of carbon isotopic composition
Isotopic reference materials of carbon.
How big is an atom? A simple question maybe, but the answer is not at all straighforward. To a first approximation we can regard atoms as 'hard spheres', with an outer radius defined by the outer electron orbitals. However, even for atoms of the same type, atomic radii can differ, depending on the oxidation state, the type of bonding and - especially important in crystals - the local coordination environment.
Take the humble carbon atom as an example: in most organic molecules a covalently-bonded carbon atom is around 1.5 Ångstroms in diameter (1 Ångstrom unit = 0.1 nanometres = 10-10 metres); but the same atom in an ionic crystal appears much smaller: around 0.6 Ångstroms. In the following article we'll explore a number of different sets of distinct atomic radius sizes, and later we'll see how you can make use of these 'preset' values with CrystalMaker.
Atomic radii represent the sizes of isolated, electrically-neutral atoms, unaffected by bonding topologies. The general trend is that atomic sizes increase as one moves downwards in the Periodic Table of the Elements, as electrons fill outer electron shells. Atomic radii decrease, however, as one moves from left to right, across the Periodic Table. Although more electrons are being added to atoms, they are at similar distances to the nucleus; and the increasing nuclear charge 'pulls' the electron clouds inwards, making the atomic radii smaller.
Atomic radii are generally calculated, using self-consistent field functions. CrystalMaker uses Atomic radii data from two sources:
VFI Atomic Radii:
Vainshtein BK, Fridkin VM, Indenbom VL (1995) Structure of Crystals (3rd Edition). Springer Verlag, Berlin.
CPK Atomic Radii:
Clementi E, Raimondi DL, Reinhardt WP (1963). Journal of Chemical Physics 38:2686-
The covalent radius of an atom can be determined by measuring bond lengths between pairs of covalently-bonded atoms: if the two atoms are of the same kind, then the covalent radius is simply one half of the bond length.
Whilst this is straightforward for some molecules such as Cl2 and O2, in other cases one has to infer the covalent radius by measuring bond distances to atoms whose radii are already known (e.g., a C--X bond, in which the radius of C is known).
CrystalMaker uses covalent radii listed on CrystalMaker-user Mark Winter's excellent Web Elements website.
Van-der-Waals radii are determined from the contact distances between unbonded atoms in touching molecules or atoms. CrystalMaker uses Van-der-Waals Radii data from:
Bondi A (1964) Journal of Physical Chemistry 68:441-
These are the 'realistic' radii of atoms, measured from bond lengths in real crystals and molecules, and taking into account the fact that some atoms will be electrically charged. For example, the atomic-ionic radius of chlorine (Cl-) is larger than its atomic radius.
The bond length between atoms A and B is the sum of the atomic radii,
Atomic Number Of Cobalt
dAB = rA + rB
CrystalMaker uses Atomic-Ionic radii data from:
Slater JC (1964) Journal of Chemical Physics 39:3199-
Perhaps the most authoritative and highly-respected set of atomic radii are the 'Crystal' Radii published by Shannon and Prewitt (1969) - one of the most cited papers in all crystallography - with values later revised by Shannon (1976). These data, originally derived from studies of alkali halides, are appropriate for most inorganic structures, and provide the basis for CrystalMaker's default Element Table. The data are published in:
Shannon RD Prewitt CT (1969) Acta Crystallographica B25:925-946
Shannon RD (1976) Acta Crystallographica A23:751-761
The Colours of Atoms
Colour-coding atoms by element type is an important way of representing structural information. Of course, atoms don't have 'colour' in the conventional sense, but various conventions have been established in different disciplines.
Many organic chemists use the so-called CPK colour scheme These colours are derived from those of plastic spacefilling models developed by Corey, Pauling and (later improved on by) Kultun ('CPK').
Atomic Number Of Chlorine
Whilst the standard CPK colours are limited to the elements found in organic compounds, CrystalMaker's VFI Atomic Radii, CSD Default Radii and Shannon & Prewitt Crystal Radii Element Tables provide a more diverse range of contrasting colours.
Atomic Number Of Cu
Organic Structures Alert! CrystalMaker's default Element Table is the Shannon & Prewitt 'Crystal' radii, which is appropriate for most inorganic structures. When working with organic structures, one of the covalent or Van-der-Waals sets will be more appropriate.