Valency can be defined as the combining capacity of an element. The electrons present in the outermostmost shell of an atom are known as valence electrons and they determine the valency.
The valence electrons take part in chemical reaction and they determine the chemical properties of the elements. Let us take an example, Theatomic number of carbon is 6 Its confguration is =2,4 It means valency of carbon is 4. In chemistry, valence, also known as valency or valence number, is the number of valence bonds[l] a given atom has formed, or can form, with one or more other atoms.
For most elements the number of bonds can vary. The IUPAC definition limits valence to the maximum number of univalent atoms that may combine with the atom, that is the maximum number of valence bonds that is possible for the given element.
 The valence of an element depends on the number of valence electrons that may be involved in the forming of valence bonds. A univalent (monovalent) atom, ion or group has a valence of one and thus can form one covalent bond. A divalent molecular entity has a valence of two and can form two sigma bonds to two different atoms or one sigma bond plus one pi bond to a single atom. ] Alkyl groups and hydroxyl ions are univalent examples; oxo ligands are divalent.
Over the last century, the concept of valence evolved into a range of approaches for describing the chemical bond, including Lewis structures (1916), valence bond theory (1927), molecular orbitals (1928), valence shell electron pair repulsion theory (1958) and all the advanced methods of quantum chemistry. The etymology of the word “valence” traces back to 1425, meaning “extract, preparation,” from Latin valentia “strength, capacity,” and the chemical meaning referring to the “combining power of an lement” is recorded from 1884, from German Valenz.
4] In 1789, William Higgins published views on what he called combinations of “ultimate” particles, which foreshadowed the concept of valency bonds.  If, for example, according to Higgins, the force between the ultimate particle of oxygen and the ultimate particle of nitrogen were 6, then the strength of the force would be divided accordingly, and likewise for the other combinations of ultimate particles (see illustration).
The exact inception, however, of the theory of chemical valencies can be traced to an 1852 aper by Edward Frankland, in which he combined the older theories of free radicals and “type theory’ with thoughts on chemical affinity to show that certain elements have the tendency to combine with other elements to form compounds containing 3, i. e. in the three atom groups (e. g.
, N03, NH3, N13, etc. ) or 5, i. e. in the five atom groups (e. g.
, N05, NH40, P05, etc. ), equivalents of the attached elements. It is in this manner, according to Frankland, that their affinities are best satisfied.
Following these examples and postulates, Frankland declares how obvious it is that: ” A tendency or law prevails (here), and that, no matter what the characters of the uniting atoms may be, the combining power of the attracting element, if I may be allowed the term, is always satisfied by the same number of these atoms. ” This “combining power” was afterwards called quantivalence or valency For elements in the main groups ot the periodic table, the valence can vary between one to seven, but usually these elements form a number of valence bonds between one and four.
The number of bonds formed by a given element was originally thought to be a fixed chemical property. In fact, in most cases this is not true. For example, phosphorus often has a valence of three, but can also have other valences. valence, also spelled valency, in chemistry, the property of an element that determines the number of other atoms with which an atom of the element can combine. Introduced in 1868, the term is used to express both the power of combination of an element in general and the numerical value of the power of combination.
A brief treatment of valence follows.
For full treatment, see chemical bonding: Valence bond theory. The explanation and the systematization of valence was a major challenge to 19th- century chemists. In the absence of any satisfactory theory of its cause, most of the effort centred on devising empirical rules for determining the valences of the elements. Characteristic valences for the elements were measured in terms of the number of atoms of hydrogen with which an atom of the element can combine or that it can replace in a compound.
It became evident, however, that the valences of many elements vary in different compounds. The first great step in the development of a satisfactory explanation of valence and chemical combination was made by the American chemist G.
N. Lewis (1916) with the identification of the chemical bond of rganic compounds with a pair of electrons held Jointly by two atoms and serving to hold them together. In the same year, the nature of the chemical bond between electrically charged atoms (ions) was discussed by German physicist W. Kossel.
After the development of the detailed electronic theory of the periodic system of the elements, the theory of valence was reformulated in terms of electronic structures and interatomic forces.
This situation led to the introduction of several new concepts “ionic valence, covalence, oxidation number, coordination number, metallic valence “corresponding to different modes of interaction of atoms. MOLECULE Nature of Molecules Molecules are made up of two or more atoms, either of the same element or of two or more different elements, Joined by one or more covalent chemical bonds.
According to the kinetic-molecular theory, the molecules of a substance are in constant motion. The state (solid, liquid, or gaseous) in which matter appears depends on the speed and separation of the molecules in the matter. Substances differ according to the structure and composition of their molecules. A molecular compound is represented by its molecular formula; for example, water is represented by the formula H20.
A ore complex structural formula is sometimes used to show the arrangement of atoms in the molecule. Evolution of Molecular Theory The terms atom and molecule were used interchangeably until the early 19th cent.
Initial experimental work with gases led to what is essentially the modern distinction. J. A.
C. Charles and R. Boyle had shown that all gases exhibit the same relationship between a change in temperature or pressure and the corresponding change in volume. J. L.
Gay-Lussac had shown that gases always combine in simple whole- number volume proportions and had rediscovered the earlier findings of Charles, hich had not been published DEFINITION molecule, the smallest identifiable unit into which a pure substance can be divided and still retain the composition and chemical properties of that substance.
The division of a sample of a substance into progressively smaller parts produces no change in either its composition or its chemical properties until parts consisting of single molecules are reached. Further subdivision of the substance leads to still smaller parts that usually differ from the original substance in composition and always differ from it in chemical properties. In this latter stage of fragmentation the hemical bonds that hold the atoms together in the molecule are broken.
Atoms consist of a single nucleus with a positive charge surrounded by a cloud of negatively charged electrons. When atoms approach one another closely, the electron clouds interact with each other and with the nuclei.
If this interaction is such that the total energy of the system is lowered, then the atoms bond together to form a molecule. Thus, from a structural point of view, a molecule may consist of a single atom, as in a molecule of a noble gas such as helium (He), or it may consist of an aggregation of atoms held together by valence forces.
Diatomic molecules contain two atoms that are chemically bonded. If the two atoms are identical, as in, for example, the oxygen molecule (02), they compose a homonuclear diatomic molecule, while if the atoms are different, as in the carbon monoxide molecule (CO), they make up a heteronuclear diatomic molecule. Molecules containing more than two atoms are termed polyatomic molecules, e.
g. , carbon dioxide (C02) and water (H20). Polymer molecules may contain many thousands of component atoms.
The ratio of the numbers of atoms that can be bonded together to form molecules is fixed; for example, every ater molecule contains two atoms of hydrogen and one atom of oxygen. It is this feature that distinguishes chemical compounds from solutions and other mechanical mixtures. Thus hydrogen and oxygen may be present in any arbitrary proportions in mechanical mixtures but when sparked will combine only in definite proportions to form the chemical compound water (H20).
It is possible for the same atoms to combine in different but definite proportions to form different molecules; for example, two atoms of hydrogen will chemically bond with one atom of oxygen to yield a water molecule, whereas two atoms of hydrogen can chemically bond with two toms of oxygen to form a molecule of hydrogen peroxide (H202). Furthermore, it is possible for atoms to bond together in identical proportions to form different molecules. Such molecules are called isomers and differ only in the arrangement of the atoms within the molecules.
For example, ethyl alcohol (CH3CH20H) and methyl ether (CH30CH3) both contain one, two, and six atoms of oxygen, carbon, and hydrogen, respectively, but these atoms are bonded in different ways. Not all substances are made up of distinct molecular units.
Sodium chloride (common table salt), for example, consists of sodium ions and chloride ions arranged in a lattice so hat each sodium ion is surrounded by six equidistant chloride ions and each chloride ion is surrounded by six equidistant sodium ions. The forces acting between any sodium and any adjacent chloride ion are equal.
Hence, no distinct aggregate identifiable as a molecule of sodium chloride exists. Consequently, in sodium chloride and in all solids of similar type”in general, all salts”the concept of the chemical molecule has no significance. The formula for such a compound, however, is given as the simplest ratio of the atoms”in the case of sodium chloride, NaCl. Molecules are held together by snared electron pairs, or covalent bonds.
Such bonds are directional, meaning that the atoms adopt specific positions relative to one another so as to maximize the bond strengths.
As a result, each molecule has a definite, fairly rigid structure, or spatial distribution of its atoms. Structural chemistry is concerned with valence, which determines how atoms combine in definite ratios and how this is related to the bond directions and bond lengths. The properties of molecules correlate with their structures; for example, the water molecule is bent tructurally and therefore has a dipole moment, whereas the carbon dioxide molecule is linear and has no dipole moment. The elucidation of the manner in which atoms are reorganized in the course of chemical reactions is important.
In some molecules the structure may not be rigid; for example, in ethane (H3CCH3) there is virtually free rotation about the carbon-carbon single bond.
The nuclear positions in a molecule are determined either from microwave vibration-rotation spectra or by neutron diffraction. The electron cloud surrounding the nuclei in a molecule can be studied by X-ray diffraction experiments. Further information can be obtained by electron spin resonance or nuclear magnetic resonance techniques. Advances in electron microscopy have enabled visual images of individual molecules and atoms to be produced.
Theoretically the molecular structure is determined by solving the quantum mechanical equation for the motion of the electrons in the field of the nuclei (called the Schr¶dinger equation). In a molecular structure the bond lengths and bond angles are those for which the molecular energy is the least.
The determination of structures by numerical solution of the Schr¶dinger equation has ecome a highly developed process entailing use of computers and supercomputers. The molecular weight of a molecule is the sum of the atomic weights of its component atoms.
If a substance has molecular weight M, then M grams of the substance is termed one mole. The number of molecules in one mole is the same for all substances; this number is known as Avogadrds number (6. 02214179 x 1023). Molecular weights can be determined by mass spectrometry and by techniques based on thermodynamics or kinetic transport phenomena.
CHEMICAL PROPORTIONS Law of definite proportions From Wikipedia, the free encyclopedia Jump to: navigation, search In chemistry, the law of definite proportions, sometimes called Proust’s Law, states that a chemical compound always contains exactly the same proportion of elements by mass.
An equivalent statement is the law of constant composition, which states that all samples of a given chemical compound have the same elemental composition by mass. For example, oxygen makes up about 8/9 of the mass of any sample of pure water, while hydrogen makes up the remaining 1/9 of the mass. Along with the law of multiple proportions, the law of definite proportions forms the basis of stoichiometry.  History This observation was first made by the French chemist Joseph Proust, based on several experiments conducted between 1798 and 1804. 2] Based on such observations, Proust made statements like this one, in 1806: I shall conclude by deducing from these experiments the principle I have established at the commencement ot this memoir, viz.
that iron like many other metals is subject to the law of nature which presides at every true combination, that is to say, that it unites with two constant proportions of oxygen. In this respect it does not differ from tin, mercury, and lead, and, in a word, almost every known combustible. The law of efinite proportions might seem obvious to the modern chemist, inherent in the very definition of a chemical compound.
At the end of the 18th century, however, when the concept of a chemical compound had not yet been fully developed, the law was novel. In fact, when first proposed, it was a controversial statement and was opposed by other chemists, most notably Proust’s fellow Frenchman Claude Louis Berthollet, who argued that the elements could combine in any proportion.
 The existence of this debate demonstrates that, at the time, the distinction between pure chemical compounds and mixtures had not yet been fully developed. ] The law of definite proportions contributed to, and was placed on a firm theoretical basis by, the atomic theory that John Dalton promoted beginning in 1803, which explained matter as consisting of discrete atoms, that there was one type of atom for each element, and that the compounds were made of combinations of different types of atoms in fixed proportions. [ law of multiple proportions law of multiple proportions, statement that when two elements combine with each other to form more than one compound, the weights of one element that combine with a fixed weight of the other are in a ratio of small whole numbers.
For example, there are five distinct oxides of nitrogen, and the weights of oxygen in combination with 14 grams of nitrogen are, in increasing order, 8, 16, 24, 32, and 40 grams, or in a ratio of 1, 2, 3, 4, 5. The law was announced (1804) by the English chemist John Dalton, and its confirmation for a wide range of compounds served as the most powerful argument in support of Dalton’s theory that matter consists of indivisible atoms.
LAW OF CHEMICAL COMBINATION The basic laws of chemical combination are the law of conservation of mass, the law of constant composition, and the law of multiple proportions.
LoCM – The mass of a closed system will remain constant over time, regardless of the processes acting inside the system. A similar statement is that mass cannot be created/destroyed, although it may be rearranged in space, and changed into different types of particles. This implies that for any chemical process in a closed system, the mass of the reactants must equal the mass of the products.
LOCC – The law of constant composition states that the composition of a substance is always the same, regardless of how the substance was made or where the substance is found. So, whenever we talk about water, we know that there are 2 atoms of ydrogen and 1 atom of oxygen in a molecule of water.
As soon as the composition of a molecule changes, then you have a different substance with different properties. LOMP – This law states that when elements combine they do so in a ratio of small whole numbers.
For example, carbon and oxygen react to form CO or C02, but not COI . 3 for instance. Furthermore, it states that if two elements form more than one compound between them then the ratios ot the masses ot the second element combined with a fixed mass of the first element will also be in ratios of small whole numbers.
Compounds are formed by chemical combination of reactants (atoms or molecules) which may be solid, liquid or gaseous. Chemical combination occurs in definite proportion by weight or by volume.
Based on various experiments performed by different scientists, the laws of chemical combinations were formulated. These laws laid the foundation of stoichiometry, a branch of chemistry in which quantitative relationship between masses of reactants and products is established. The study of these laws led to the development of a theory concerning the nature of matter.
There are five laws of chemical combinations. The first four deal with combination of substances by weight and the fifth with combination of gases by volume.
The total mass of substances taking part in a chemical reaction remains the same throughout the change. ” Law of multiple proportion: “When two elements A and B combine to form two or more compounds, then different weights of B which combine with a fixed weight of A bears a simple numerical ratio to one another”. Law of conservation of mass: When two elements combine separately with a definite mass of a third element, then the atio of their masses in which they do so is either the same or some whole number multiple of the ratio in which they combine with each other.
Limitations of Law multiple proportion: The law is valid till an element is present in one particular isotopic form in all its compounds.
When an element exists in the form of different isotopes in its compounds, the law does not hold good. Stoichiometry a branch of chemistry in which quantitative relationship between masses of reactants and products are established. The study of these laws led to the development of a theory concerning the nature of matter.