Mineralogy

Ionic crystals’ structures are all based on a fundamental group of atoms (or, more accurately, ions) consisting of one silicon atom surrounded by four oxygens - the Si0₄-group, which is the “unit brick” of silicate structure. The oxygens are arranged at the four corners of a regular tetrahedron, with the silicon at its centre. Such groups can occur separately, or may be linked together in a number of ways by sharing oxygen atoms with adjacent groups, thus forming more complex structures. Metallic ions, such as Mg, Fe, Ca, etc., can be accommodated in the interstices of the structure; which of these will enter a given structure depends chiefly on their size and, to a lesser degree, on their ionic charge (or valency), matters which will now be discussed.

Ionic Radii

It is convenient to think of ions as spheres having a definite radius. When two ions are brought close together, a force of repulsion between them sets in abruptly when they are a certain distance apart and resists any closer approach. The distance between their centers is then taken as the sum of the radii of the two ions. In this way, the ions are treated as spheres in contact, and by various methods their radii can be measured. The following list of ionic radii is due largely to the work of V. M. Goldschmidt, J. A. Wasastjerna, and others; it is arranged so that the columns correspond to the groups of the Periodic Table, and ionic charge is indicated at the top of each column.

2-	1-	0	1+	2+	3+	4+
O	F	He	Li	Be	Â	Â
1.32	1.33	Â	0.78	0.34	Â	Â
S	Cl	Ne	Na	Mg	Al	Si
1.74	1.81	Â	0.98	0.78	0.57	0.39
Se	Br	A	K	Ca	Ga	Ge
1.91	1.95	Â	1.33	1.06	0.62	0.44
Te	I	Kr	Rb	Sr	In	Sn
2.11	2.20	Â	1.49	1.27	0.92	0.74
Â	Â	X	Cs	Ba	Tl	Pb
Â	Â	Â	1.65	1.43	1.05	0.84

Monovalent:	Cu, 0′96	Ag, 1.13	Au, 1.37
Divalent:	Fe, 0.83	Co, 0.82	Ni, 0.78	Mn, 0.91
	Zn, 0.83	Cd, 1.03	Hg, 1.12	Pb, 1.32
Trivalent:	Cr, 0.64	Fe, 0.67	Mn, 0.70

Notice particularly the large size of the oxygen ion (1.32) in comparison with relatively small positive ions (cations) such as Si, AI, Mg, and Fe. It is the spacing of the oxygens, which are closely packed together in a silicate crystal that largely controls the scale of the structure; the smaller cations are situated in the interstices between the oxygens. Other negative ions (anions), e.g. F-, Cl- -, S- -, are similarly large.

The hydrogen ion is exceptional in being extremely small; when bound to an atom of oxygen it becomes embedded, as it were, in the oxygen, and the resulting (OH)-ion has about the same radius as that of oxygen. We can think of the H-ion as a centre of positive charge without dimensions.

When one or more electrons are removed from a group of atoms bound together by homopolar bonds, a charged radicle is formed. Thus the carbonate ion, C0₃- -, has one carbon atom to which are attached three evenly spaced oxygen atoms, and it carries a double negative charge. The sulphate ion, 504- -, has four oxygens arranged at the corners of a tetrahedron around a central atom of sulphur. The shapes of these ions affect the type of crystal structure of their compounds. Thus in calcite the Ca-ions are situated at the corners of a rhombohedron and the CO₃-ions, represented by triangles in the figure, are arranged with their centres midway along the rhombohedral edges and lie in horizontal planes. Physical and optical properties parallel and perpendicular to these planes differ greatly.

Co-ordination

In ionic crystals each positive ion (cation) is surrounded by a number of negative ions, at a distance fixed by the sum of their radii. The number of negative ions around anyone cation is called the co-ordination number, and is determined by the ratio of the radii of the two kinds of ion. Thus in sodium chloride, the relative sizes of the sodium and chlorine ions are such that every sodium is surrounded by six chlorines arranged at the corners of a regular octahedron, a grouping known as 6-fold co-ordination. The metals aluminium, iron, magnesium and titanium, among others, are also found in 6-fold co-ordination.In silicate minerals, silicon always has four oxygen atoms arranged around it at the corners of a tetrahedron; the space left between the four oxygens packed together thus is just sufficient to accommodate the small silicon ion. Other cations which are found in 4-fold co-ordination are beryllium and zinc. An important feature of many silicate structures is that aluminium, already mentioned as having 6-fold coordination, can also play the role of silicon (because its ionic radius is only a little larger) and thus occur in 4-fold co-ordination. Aluminium is, because of its size, a borderline case and can occur in either 4- or 6- co-ordination.

Groups with co-ordination numbers from 7 to 12 are formed by large cations such as Na, Ca, K, Sr, Ba, and Zr, but these larger groups tend to be less regular than the smaller groups. The tetrahedral and octahedral groups (4 and 6- co-ordination) are very regular in their form, and in building crystals they are put together so that neighboring groups share corners, edges, or faces, and are thus linked together so as to build up the pattern.

Previously, crystals were defined as bodies bounded by usually flat surfaces, arranged in a regular manner expressing the internal arrangement of the atoms. The study of the arrangement of atoms within a crystal, that is, of atomic structure, has been made possible in recent years by new methods of analysis in which X-rays are employed. This advance dates from the discovery by Laué and others, in 1912, of the diffraction of X-rays by crystals. The first analysis was made in 1913 by W. L. Bragg, on crystals of sodium chloride (common salt).

X-rays are somewhat like light waves but have a much shorter wave-length (see p. 149), this being comparable to the distances between atoms in a crystalline solid. When a beam of X-rays falls on a crystal, it is scattered or diffracted by the layers of atoms within the crystal, in the same way that light waves are diffracted by an optical grating. In making an analysis of a crystal structure, the diffracted X-rays are allowed to fall on a photographic plate, and the resulting photograph shows a series of spots or lines which form a more or less symmetrical pattern. From measurements made on the photograph, the arrangement of the atoms in the crystal can be deduced and also the distances between them. Distances are expressed in Angstrom units; one Angstrom (Ã…)=10-8 em. The several methods of taking X-ray photographs of crystals or of powdered minerals cannot be discussed here, but the principle is broadly that outlined above; details can be found in books on the subject. We are more concerned with the results of X-ray analysis, as these have thrown a flood of light on the structure of crystals, and in particular of minerals, and have confirmed the classes of symmetry worked out in the past by crystallographers from a study of external form, as previously. X-ray analysis has been especially helpful in connection with the big group of mineral silicates, and in this field the work of many investigators has given us a considerable, though not yet complete, knowledge of their structures.

The Unit Cell

Every crystal consists of certain atoms or groups of atoms arranged in a three-dimensional pattern, which is repeated throughout the crystal. The smallest complete unit of pattern is called the unit cell, and the whole pattern is formed by stacking unit cells together. To take a simple example, in crystals of sodium chloride (NaCI) the atoms of N a and CI are arranged at the corners of a series of cubes, as shown in Fig. 68. The unit cell of sodium chloride contains four atoms of Na and four of CI, whose arrangement is exactly similar to that in every other unit cell of the substance. It is to be noted that the number of atoms in the unit cell of a particular mineral is not necessarily the same as in its formula, but is usually some simple multiple; for NaCI this multiple is four. Verify by counting the atoms in Fig. 68, allowing for some atoms belonging half to the unit cell shown there and half to adjacent cells. The array of points in space at which the pattern repeats is called the lattice.

Atomic Bonds

There are four main kinds of bond which hold together the atoms in different crystal structures; they are known respectively as the ionic or polar bond, the homopolar or co-valent bond, the metallic bond, and the residual or van de Waals bond. Correspondingly, crystals may be divided into four classes, each characterized mainly by one of the above four types of bond. The van der Waals bond is very weak in character and is present in all crystals. Nearly all the common minerals have ionic bonding.

Minerals possess certain physical properties that are considered in this chapter in the following order:

1. Certain characters depending upon light, such as color, luster, transparency, translucency, phosphorescence and fluorescence. Other optical properties especially valuable in the recognition of minerals in thin section under the microscope are dealt with in a later chapter.
2. Characters depending upon certain senses, such as those of taste, odor and feel.
3. Characters depending upon the state of aggregation, such as form, pseudomorphism, polymorphism, hardness, tenacity, fracture, cleavage, and surface tension effects. Crystallography, - the study of crystals, is considered in the next chapter.
4. The specific gravity of minerals.
5. Characters depending upon heat, such as fusibility.
6. Characters depending upon magnetism, electricity and radioactivity.
7. Color, Luster, Transparency, Etc.

Color

Color depends upon the absorption of some and the reflection of others of the colored rays or vibrations which compose ordinary white light. When a body reflects light to so small an extent as not to affect the eye, it appears black, but when it reflects all the vibrations of the different colors which compose white light, it appears white. Again, if it reflects the red vibrations of ordinary light and absorbs all the other vibrations, it appears red.

A blue mineral, such as sapphire, absorbs all the vibrations of white light with the exception of those that give the sensation of blueness to the eye.

The color of a mineral is often its most striking property. Unfortunately for purposes of identification, however, the colors of minerals vary very greatly. Even in the same species specimens are found having very different colors. The mineral quartz, composed of silicon dioxide, is commonly colorless or white, but it is also found with pinkish-yellow, green, brown, amethystine and even black colors. Corundum, composed of alumina, varies in color from pale brown to deep red and dark blue, the two latter varieties being the gemstones ruby and sapphire. The same crystal of a mineral may exhibit different colors, sometimes arranged in a regular fashion as in some crystals of tourmaline, at other times in patches as in certain specimens of fluor-spar, calcium fluoride.

The streak of a mineral is the color of its powder and may be quite different from that of the mineral in mass. For instance, black hematite gives a red powder. Streak is observed by producing a small quantity of the powdered mineral by scratching with a knife or file or by rubbing the mineral on a piece of unglazed porcelain or roughened glass called a streak-plate.

Some minerals, when turned about or looked at in different directions, display a changing series of prismatic colors, such as are seen in the rainbow or on looking through a glass prism. This is called a play of colors. It is shown by the diamond and is produced by the splitting-up of a ray of white light into its colored constituents as it enters and emerges from the mineral. Change ‘of color is a somewhat similar phenomenon extending over broader surfaces, the succession of colors being produced as the mineral is turned. This phenomenon is excellently displayed by certain varieties of the mineral feldspar, the colors shown including blues, greens, yellows and reds. Such a feldspar is an abundant constituent of a rock from southern Norway, and polished slabs of this rock in which the feldspar crystals lie in various directions are used for ornamental purposes. The change of color is caused by the interference of light reflected from thin plates of other minerals enclosed in parallel planes within the feldspar. Schiller, a nearly metallic luster shown by certain surfaces of the minerals hypersthene, schiller-spar, etc., is due to a somewhat similar cause. Reflection takes place either from minute plates arranged on parallel planes, or from cavities due to chemical action along certain parallel planes within the mineral.

Opalescence is a somewhat pearly or milky appearance shown by opal and moonstone. Iridescence is a display of prismatic colors due to the interference of rays of light in minute fissures which wall in thin films of air or liquid. These fissures are often the result of incipient fracture. Iridescence may sometimes be seen in quartz, calcite and mica. The brilliant display of colors given by the precious opal is due to the presence of very thin curved or distorted layers with slightly different optical properties.

Some minerals tarnish on the surface when exposed to the air and sometimes exhibit iridescent colors. This tarnish may result either from oxidation, or from the chemical action of sulphur and other elements which are generally present in the atmosphere in minute quantities. Tarnish may be distinguished from the true color by chipping or scratching the mineral, when the superficial nature of the tarnish is revealed. Copper pyrites often tarnishes to an iridescent mixture of colors. The mineral erubescite tarnishes readily on exposure to the air, and some varieties are called peacock ore.

Some crystals display different colors when viewed in different directions by transmitted light. This property, called pleochroism, ‘is considered with the special optical properties on a later page.

Luster

The luster of minerals differs both in intensity and kind, depending upon the amount and type of reflection of light that takes place at their surfaces.

There are six kinds of luster:

1. M Dallic - The ordinary luster of metals. When feebly displayed this luster is termed sub-metallic, Gold, iron pyrites and galena have a metallic luster; chromite and cuprite have a submetallic luster.
2. Vitreous - The luster of broken glass. When less well developed, it is called subvitreous luster. Quartz and rock-salt afford examples of vitreous luster, calcite of subvitreous.
3. Resinous - The luster of resin. Opal, amber and some kinds of zinc blended have a resinous luster.
4. Pearly - The luster of a pearl. It is shown by surfaces parallel to which the mineral is more or less separated into thin plates, reproducing to some extent the conditions of a pile of thin glass sheets, such as cover-glasses. Talc, brucite and selenite show pearly luster.
5. Silky - The luster of silk. This luster is peculiar to minerals having a fibrous structure. The fibrous form of gypsum known as satin-spar, and the variety of asbestos called amianthus are good examples of minerals having a silky luster.
6. Adamantine - The luster of a diamond.

The luster of minerals may be of different degrees of intensity, according to the amount of light reflected from their surfaces. Thus, when the surface of a mineral is sufficiently brilliant to reflect objects distinctly, as a mirror would do, it is said to be splendent. Certain varieties of hematite have a splendent luster. When the surface is less brilliant and objects are reflected indistinctly, it is described as shining. When the surface is still less brilliant and is incapable of giving any image, it is termed glistening, and glimmering denotes a still feebler luster. Minerals with no luster are described as dull.

As shown later, the various surfaces of a crystal may show different kinds and degrees of luster.

Transparency and Translucency

A mineral IS transparent when the outlines of objects seen through it appear sharp and distinct. Rock crystal-a variety of quartz-and selenite are good examples. Minerals are said to be sub transparent or semitransparent when objects seen through them appear indistinct. A mineral which, though capable of transmitting light, cannot be seen through is translucent. This condition is very common among minerals. When no light is transmitted the mineral is opaque, but it must be noted that this refers only to the appearance as usually seen. A large number of apparently opaque minerals become translucent when cut into very thin sections, and this property is of great importance, as shown in a later chapter, in the identification of minerals in rocks.

Many minerals which are opaque in the mass are translucent on the sharply broken edges and in splinters, as in the case of the common black flint from the Chalk of the south of England.

Phosphorescence and Fluorescence

Phosphorescence is the property possessed by some substances of emitting light after having been subjected to certain conditions such as heating, rubbing, or exposure to electric radiation or to ultra-violet light. Some varieties of fluorspar, when powdered and heated on an iron plate, display bright phosphorescence. Pieces of quartz when rubbed together in a dark room emit a phosphorescent light. Exposure to sunlight or even ordinary diffused light elicits phosphorescence from many minerals, as may be observed by transferring them rapidly to a dark room. Diamond, ruby and certain other minerals show brilliant phosphorescence after exposure to X-rays. Willemite, zinc orthosilicate, phosphoresces when exposed to X-rays, a fact employed to make certain that this mineral has been completely extracted from its ore.

Some minerals emit light whilst exposed to certain electrical radiations. This phenomenon is best exhibited by fluor-spar and for this reason is called fluorescence.

Taste, Odor and Feel

Taste

The characters of minerals dependent upon taste are only perceptible when the minerals are soluble in water. The following are terms used in this connexion: saline, the taste of common salt; alkaline, that of potash and soda; cooling, that of nitre or potassium chlorate; astringent, that of green vitriol; sweetish astringent, that of alum; bitter that of Epsom salts, and sour, that of sulphuric acid.

Odor

Some minerals have characteristic odors when struck, rubbed, breathed upon or heated. Terms used are:

• Alliaceous - the odor of garlic, given when arsenic compounds are heated.
• Horse-radish - the odor of decaying horse-radish, given when selenium compounds are heated.
• Sulphurous - the odor of burning sulphur, given off by pyrites when struck, or by many sulphides when heated.
• Fcetid - the odor of rotten eggs, given by heating or rubbing certain varieties of quartz or limestone.
• Argillaceous or Clayey - the odor of clay when breathed upon.

Feel

Smooth, greasy or unctuous, harsh, or meager or rough, are kinds of feel of minerals that may aid in their identification. Certain minerals adhere to the tongue.

Solids, Liquids and Gases

Matter may exist in three states, the solid, the liquid, and the gaseous. Most minerals are solid, but some materials considered here, such as petroleum and natural gas, are fluids. Liquids and gases are “fluids,” i.e. unlike solids they flow under the action of gravity: a gas entirely fills the space containing it, whereas a liquid may not, but may be bounded by an upper horizontal surface. Most pure substances can exist in all three states, and may be caused to pass from one to another by heating or cooling. At sufficiently high temperatures many minerals are melted to liquids, although some are chemically decomposed by heat before they reach their melting point.

Elements, Compounds and Mixtures

A pure substance is one that possesses characteristic and invariable properties; matter can thus be divided into mixtures and single (or pure) substances. Pure substances may be of two kinds, viz., elements and compounds.

Elements are substances which have not so far been split up into simpler substances by any chemical means. About ninety elements are at present known, but many are extremely rare and of little importance to the mineralogist. I t has been estimated that the crust of the earth is composed of 46.5% oxygen, 27.6% silicon, 8.1% aluminum, 5.1% iron, 3.6% calcium, 2.6% potassium, 2.8% sodium, and 2.1% magnesium. Thus, over 98% of the earth’s crust is composed of but eight elements, and most of the elements of economic value are absent from this list.

Compounds are pure substances made up of two or more elements. They are formed as a result of chemical change and are different from mere mixtures in the following ways:

1. The elements constituting a compound are combined in definite proportions by weight.
2. A compound cannot easily be split up, whereas the components of a mixture can usually be separated by mechanical means. These components may themselves be either elements or compounds.
3. The properties of a compound are often very different from those of the elements it contains, whereas a mixture usually possesses the properties of its constituents.
4. Heat is either given out or absorbed when a compound is formed; this does not in general occur when substances are merely mixed.

Minerals are compounds of their constituent elements, while rocks are mixtures of their component minerals. Thus, the mineral quartz is a compound (silica) of the elements silicon and oxygen, whereas the rock granite, as we have seen, is a mixture of several minerals, one of which is quartz.

Atoms

The chemical and physical behavior of substances is best explained in terms of an Atomic Theory of Matter. It is possible to break down the matter of an element into smaller and smaller particles, and at one stage of this process the particle is called an atom. The atoms of one element are all alike and differ from those of other elements. Chemical combination is the binding together of atoms, and hence a useful definition is:

An atom is the smallest part of an element that can enter into chemical combination with another element.

Atoms unite with one another in definite proportions, though an atom of one element may unite with different numbers of atoms of another element in two or more different compounds. For example, the carbon atom combines with the oxygen atom to form two different compounds: carbon monoxide, in which one atom of oxygen is joined to one atom of carbon, and carbon dioxide, which has two atoms of oxygen combined with each carbon atom. Again, iron combines with oxygen in the proportions of 1:1 (ferrous oxide) and 2:3 (ferric oxide).

Molecules

The particles of a substance in the gaseous condition are widely separated from each other and in a state of rapid, random motion. These freely moving particles are called molecules, and they may consist of single atoms, as in the gas helium, or of two or more atoms of the same element, as in hydrogen or oxygen or, in the case of compounds, of two or more atoms of different elements, e.g. steam, carbon dioxide.

When a gas condenses to a liquid the molecules are no longer separated in space but come together and, to a certain extent, lose their identity. When the liquid is frozen to a solid, the atoms arrange themselves in a fairly rigid pattern, and it is no longer possible to segregate anyone group of atoms from the rest. The term “molecule” is thus not really applicable to the solid state.

Symbols and Formulae

For convenience, an atom of every element is represented by an abbreviation called a symbol which is usually the first letter, or the first and second letters, of the English or Latin name of the element. The molecule of a substance is represented by a formula: thus, ° is the symbol of an atom of oxygen, and C of an atom of carbon, and O₂ is the formula of a molecule of oxygen, and CO₂ the formula for a molecule of carbon dioxide. The proportions of the constituent elements of a solid or liquid compound are also represented by a formula; thus, calcite is CaCO₃. It should be clearly understood that this formula merely means that calcite is composed of calcium, carbon, and oxygen in the proportions of one atom of calcium, one atom of carbon, and three atoms of oxygen; it does not stand for a “molecule” of calcite” (see previous paragraph).

Atomic and Molecular Weights

The atomic weight of an element is the weight of an atom of the element compared with the weight of an atom of oxygen taken as 16. A table of atomic weights is given below.

The molecular weight of a substance is the sum of the atomic weights of the atoms composing a molecule of the substance. In the case of a solid, the formula weight is a convenient quantity, and is the sum of the weights of the atoms making up the formula of the compound. Thus, the atomic weight of calcium is 40, of carbon is 12, and of oxygen is 16; the formula weight of calcite (CaCO₃) is therefore (40+12+3×16) = 100.

Valency

The valency of an element is measured by the number of its atoms which will combine with or replace one atom of hydrogen. For example, chlorine combines with one atom of hydrogen and is therefore univalent; calcium replaces two atoms of hydrogen and is therefore divalent, and so on. . Several of the elements have different valencies in different compounds; thus iron is divalent in the compound FeO, or trivalent in the compound Fe₂0₃. The usual valencies of the commoner elements are given below:
Univalent: H, Cl, Br, I, F, Li, Na, K, Ag, Cu, Au. Divalent: 0, S, Se, Te, Be, Mg, Ca, Sr, Ba, Pb, Hg, Cu, Zn, Co, Ni, Fe, Mn, Cr, Sn.
Trivalent: B, Au, AI, Fe, Mn, Cr, Co, Ni, N, P, As, Sb, Bi.
Quadrivalent: C, S, Si, Ti, Zr, Sr, Mn, Pb.
Quinquavalent: P, As, Sb, Bi, Ta.
Hexavalent: S, Cr, Mo, W, U.
Heptavalent: Mn.
Note that some elements show variable valency, e.g. Fe, S, Mn.

The Structure of the Atom

According to views developed early in this century, the atoms themselves may be regarded as built up of still smaller units, called electrons and protons. The electron has a unit negative electric charge, and a mass about 1/1860 of that of the lightest atom, hydrogen; the proton has a mass about equal to that of’ the hydrogen atom and carries a unit positive charge. Although other similar small units exist, it is convenient to regard the electron and the proton as the bricks from which the atoms of the elements are built. In the Rutherford-Bohr theory, the atom consists of a central nucleus surrounded by electrons moving in orbits, rather like the planets round the sun. Most of the mass of the atom is concentrated in the nucleus, which is small compared with the diameter of the whole atom as defined by the outermost electrons. The nucleus carries a positive charge equal in magnitude to the total charge of the orbital electrons, so that the whole atom is electrically neutral.

Chemistry of Minerals					Â
	Atomic		Weights
		Atomic			Atomic
Element	Symbol	Weight	Element		Symbol	Weight
Aluminium	Al	26·97	Neodymium		Nd	144·27
Antimony	Sb	121·76	Neon		Ne	20·183
Argon	A	39·944	Nickel		Ni	58·69
Arsenic	As	74·91	Niobium		Kb
Barium	Ba	137·36	(Columbium)		(Cb)	92·91
Beryllium	Be	9·02	Nitrogen		N	14·008
Bismuth	Bi	209·00	Osmium		Os	190·2
Boron	B	10·82	Oxygen		0	16·0000
Bromine	Br	79·916	Palladium		Pd	106·7
Cadmium	Cd	112·41	Phosphorus		P	30·98
Ceesium	Cs	132·91	Platinum		Pt	195·23
Calcium	Ca	40·08	Potassium		K	39·096
Carbon	C	12·01	Praseodymium		Pr	140·92
Cerium	Ce	140·13	Radium		Ra	226·05
Chlorine	CI	35·457	Radon		Ra	222
Chromium	Cr	52·01	Rhenium		Re	186·31
Cobalt	Co	58·94	Rhodium		Rh	102·91
Copper	Cu	63·57	Rubidium		Rb	85·48
Dysprosium	Dy	162·46	Ruthenium		Ru	101·7
Erbium	Er	167·2	Samarium		Sm	150·43
Europium	Eu	152·0	Scandium		Sc	45·10
Fluorine	F	19·00	Selenium		Se	78·96
Gadolinium	Gd	156·9	Silicon		Si	28·06
Gallium	Ga	69·72	Silver		Ag	107·880
Germanium	Ge	72·60	Sodium		Na	22·997
Gold	Au	197·2	Strontium		Sr	87·63
Hafnium	Hf	178·6	Sulphur		S	32·06
Helium	He	4·003	Tantalum		Ta	180·88
Holmium	Ho	164·94	Tellurium		Te	127·61
Hydrogen	H	1·0080	Terbium		Tb	159·2
Indium	In	114.·76	Thallium		TI	204·39
Iodine	r	126·92	Thorium		Th	232·12
Iridium	Ir	193·1	Thulium		Tm	169·4
Iron	Fe	55·85	Tin		Sn	118·70
Krypton	Kr	83·7	Titanium		Ti	47·90
Lanthanum	La	138·92	Tungsten		W	183·92
Lead	Pb	207·21	Uranium		U	238·07
Lithium	Li	6·940	Vanadium		V	50·95
Lutecium	Lu	174·99	Xenon		Xe	131·3
Magnesium	Mg	24·32	Ytterbium		Yb	173·04
Manganese	Mn	54·93	Yttrium		Y	88·92
Mercury	Hg	200·61	Zinc		Zn	65·38
Molybdenum	Mo	95·95	Zirconium		Zr	91·22

Thus the lightest atom, hydrogen, consists of a single orbital electron and a nucleus of unit mass carrying unit positive charge (i.e. one proton). The next atom in order of weight, helium, consists of two orbital electrons and a nucleus of 4 units of mass with two positive charges, and so on with successively heavier atoms. Each atom differs from its next lighter neighbor in having one more orbital electron and, on the average, two more units of mass in the nucleus.

Ions

An atom which has lost or gained one or more electrons, and is thus no longer electrically neutral, is called an ion. There is plenty of evidence to show that a compound such as common salt (NaCl) is not an aggregate of chlorine and sodium atoms, but of sodium and chlorine ions, the sodium ion having a unit positive charge and the chlorine ion a unit negative charge (written Na⁺, Cl^- ‘}. The electrostatic attraction between these oppositely charged ions constitutes the binding or valency force in the compound sodium chloride, and is known as an electrovalency. If they can be melted or dissolved in water, such compounds conduct electricity and are called “ionic” or “polar” compounds j other examples are calcium sulphate (the mineral anhydrite) and iron sulphide (pyrites). But not all valency links are of this kind: in many cases the link is in the nature of a sharing of one or more electrons between atoms, and is known as the “co-valent” or “homopolar” bond.