Symbol: Sc Atomic number: 21 Atomic weight: 44.956 Rare soft silvery metallic element belonging to group 3 of the periodic table. There are ten isotopes, nine of which are radioactive and have short half lives. Predicted in 1869 by Mendeleev,….

1. Scandium-48 Please visit the Scandium element page for information specific to the chemical element of the periodic table.
2. Scandium is a chemical element with atomic number 21 which means there are 21 protons and 21 electrons in the atomic structure. The chemical symbol for Scandium is Sc. The atom consist of a small but massive nucleus surrounded by a cloud of rapidly moving electrons. The nucleus is composed of protons and neutrons.
3. Scandium - Atomic Mass - Atomic Weight - Sc. The Scandium atomic mass is the mass of an atom. The density of a substance strongly depends on its atomic mass and also on the atomic number density.
4. Atomic Packing Factor (APF) tells you what percent of an object is made of atoms vs empty space. You can think of this as a volume density, or as an indication of how tightly-packed the atoms are. Calculating the atomic packing factor for a crystal is simple: for some repeating volume, calculate the volume of the atoms inside and divide by the total volume.

Scandium

History

From the Latin word Scandia, Scandinavia. On the basis of the Periodic System, Mendeleev predicted the existence of ekaboron, which would have an atomic weight between 40 of calcium and 48 of titanium.The element was discovered by Nilson in 1878 in the minerals euxenite and gadolinite, which had not yet been found anywhere except in Scandinavia. By processing 10 kg of euxenite and other residues of rare-earth minerals, Nilson was able to prepare about 2g of highly pure scandium oxide. Later scientists pointed out that Nilson's scandium was identical with Mendeleev's ekaboron.

Sources

Scandium is apparently much more abundant (the 23rd most) in the sun and certain stars than on earth (the 50th most abundant). It is widely distributed on earth, occurring in very minute quantities in over 800 mineral species. The blue color of beryl (aquamarine variety) is said to be due to scandium. It occurs as a principal component in the rare mineral thortveitite, found in Scandinavia and Malagasy. It is also found in the residues remaining after the extraction of tungsten from Zinnwald wolframite, and in wiikite and bazzite.

Most scandium is presently being recovered from thortveitite or is extracted as a by-product from uranium mill tailings. Metallic scandium was first prepared in 1937 by Fischer, Brunger, and Grienelaus who electrolyzed a eutectic melt of potassium, lithium, and scandium chlorides at 700 to 800°C. Tungsten wire and a pool of molten zinc served as the electrodes in a graphite crucible. Pure scandium is now produced by reducing scandium fluoride with calcium metal.

The production of the first pound of 99% pure scandium metal was announced in 1960.

Properties

Scandium is a silver-white metal which develops a slightly yellowish or pinkish cast upon exposure to air. A relatively soft element, scandium resembles yttrium and the rare-earth metals more than it resembles aluminum or titanium.

It is a very light metal and has a much higher melting point than aluminum, making it of interest to designers of spacecraft. Scandium is not attacked by a 1:1 mixture of HNO₃ and 48% HF.

Chemically it is one of the alkaline earth elements; it readily forms a white coating of nitride in air, reacts with water, burns with a yellow-red flame.

Uses

About 20 kg of scandium (as Sc₂O₃) are used yearly in the U.S. to produce high-intensity lights. The radioactive isotope ⁴⁶Sc is used as a tracing agent in refinery crackers for crude oil, etc.

Scandium iodide added to mercury vapor lamps produces a highly efficient light source resembling sunlight, which is important for indoor or night-time color TV.

Handling

Little is yet known about the toxicity of scandium; therefore it should be handled with care.

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

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.

Proton Number Of Scandium

Atomic radii are generally calculated, using self-consistent field functions. CrystalMaker uses Atomic radii data from two sources:

1. VFI Atomic Radii:
   Vainshtein BK, Fridkin VM, Indenbom VL (1995) Structure of Crystals (3rd Edition). Springer Verlag, Berlin.

2. CPK Atomic Radii:
   Clementi E, Raimondi DL, Reinhardt WP (1963). Journal of Chemical Physics 38:2686-

Covalent Radii

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

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-

Sc Atomic Number

Atomic-Ionic Radii

Sc Atomic Number

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,

Sc Atomic Number 9

d_AB = r_A + r_B

CrystalMaker uses Atomic-Ionic radii data from:

Slater JC (1964) Journal of Chemical Physics 39:3199-

Crystal Radii

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

Sc Atomic Number Chart

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').

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.

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.