MOLECULE

A molecule is defined as an electrically neutral group of at least two

atoms in a definite arrangement held together by very strong (covalent)

chemical bonds.[1][2] Molecules are distinguished from polyatomic ions in this

strict sense. In organic chemistry and biochemistry, the term molecule is used

less strictly and also is applied to charged organic molecules and

biomolecules.

A molecule may consist of atoms of a single chemical element, as with

oxygen (O2), or of different elements, as with water (H2O). Atoms and

complexes connected by non-covalent bonds such as hydrogen bonds or ionic

bonds are generally not considered single molecules.

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, which had not been published.

Evolution of Molecular Theory

One early theorist was John Dalton, best known for his atomic

theory. Dalton believed that gases were made up of tiny

particles, which he thought were atoms. He thought that these

atoms were stationary and in contact with one another and that heat

was a material substance, called caloric, that was contained in

shells around the atom (these shells of caloric were actually what

was in contact). When a gas was heated, the amount of caloric was

increased, the shells became larger, and the gas expanded. Dalton

did not accept Gay-Lussac's findings about combining volumes of

gases, perhaps because it could not be explained by his theory.

Dalton's Theory

A different theory from Dalton's that could explain the

combining volumes of gases was proposed by the Italian physicist

Amadeo Avogadro in 1811. According to his theory, under given

conditions of temperature and pressure, a given volume of any

gas contains a definite number of particles. From the earlier

observation that one volume of hydrogen gas and one volume of

chlorine gas react to form two volumes of hydrogen chloride gas

he deduced that the particles in gaseous hydrogen or chlorine

could not be single atoms, but must be some combination of

atoms. He called this combination a molecule.

Many shortcomings of Dalton's theory were uncovered, and

although a number of modifications were suggested, none were

very successful. It was not until 1858 that the Italian chemist

Stanislao Cannizaro suggested a merging of Avogadro's and

Dalton's theories. The acceptance of this revised theory was

assisted by the acceptance by physicists at about the same time

of the kinetic-molecular theory of gases that was first proposed in

1738 by Daniel Bernoulli.

Avogadro's Hypothesis

Cannizaro's Compromise

Prior to the nineteenth century, chemists pursued science simply by taking

measurements, before and after a chemical reaction, of the substances

involved. This was an external approach, rather like a person reaching into a

box and feeling of the contents without actually being able to see them. With

the evolution of atomic theory, chemistry took on much greater definition: for

the first time, chemists understood that the materials with which they worked

were interacting on a level much too small to see. The effects, of course, could

be witnessed, but the activities themselves involved the interactions of atoms in

molecules. Just as an atom is the most basic particle of an element, a molecule

is the basic particle of a compound. Whereas there are only about 90 elements

that occur in nature, many millions of compounds are formed naturally or

artificially. Hence the study of the molecule is at least as important to the

pursuit of modern chemistry as the study of the atom. Among the most

important subjects in chemistry are the ways in which atoms join to form

molecules—not just the numbers and types of atoms involved, but the shape

that they form together in the molecular structure.

CONCEPT

Sucrose or common table sugar, of course, is grainy

and sweet, yet it is made of three elements that share

none of those characteristics. The formula for sugar is

C12H22O11, meaning that each molecule is formed by the

joining of 12 carbon atoms, 22 hydrogens, and 11 atoms

of oxygen. Coal is nothing like sugar—for one thing, it is

as black as sugar is white, yet it is almost pure carbon.

Carbon, at least, is a solid at room temperature, like

sugar. The other two components of sugar, on the other

hand, are gases, and highly flammable ones at that.

HOW IT WORKS?

The question of how elements react to one another, producing

compounds that are altogether unlike the constituent parts, is one of the most

fascinating aspects of chemistry and, indeed, of science in general. Combined

in other ways and in other proportions, the elements in sugar could become

water (H2O), carbon dioxide (CO2), or even petroleum, which is formed by the

joining of carbon and hydrogen.

It is not enough, however, to know that a certain combination of atoms

forms a certain molecule, because molecules may have identical formulas and

yet be quite different substances. In English, for instance, there is the word

"rose." Simply seeing the word, however, does not tell us whether it is a

noun, referring to a flower, or a verb, as in "she rose through the ranks."

Similarly, the formula of a compound does not necessarily tell what it is, and

this can be crucial.

MOLECULAR STRUCTURE

For instance, the formula C2H6O identifies two very

different substances. One of these is ethyl alcohol, the type of

alcohol found in beer and wine. Note that the elements

involved are the same as those in sugar, though the

proportions are different: in fact, some aspects of the body's

reaction to ethyl alcohol are not so different from its response

to sugar, since both lead to unhealthy weight gain. In

reasonable small quantities, of course, ethyl alcohol is not

toxic, or at least only mildly so; yet methyl ether—which has an

identical formula—is a toxin.

A molecule can be most properly defined as a group of atoms joined in a specific

structure. A compound, on the other hand, is a substance made up of more than one type

of atom—in other words, more than one type of element. Not all compounds are

composed of discrete molecules, however. For instance, table salt (NaCl) is an ionic

compound formed by endlessly repeating clusters of sodium and chlorine that are not, in

the strictest sense of the word, molecules.

MOLECULES AND COMPOUND

Salt is an example of a crystalline solid, or a solid in which the

constituent parts are arranged in a simple, definite geometric pattern

repeated in all directions. There are three kinds of crystalline solids, only

one of which has a truly molecular structure. In an ionic solid such as

table salt, ions (atoms, or groups of atoms, with an electric charge) bond a

metal to a nonmetal—in this case, the metal sodium and the nonmetal

chlorine. Another type of crystalline solid, an atomic solid, is formed by

atoms of one element bonding to one another. A diamond, made of pure

carbon, is an example. Only the third type of crystalline solid is truly

molecular in structure: a molecular solid—sugar, for example—is one in

which the molecules have a neutral electric charge.

Just as the atoms of elements have a definite mass, so do molecules—a

mass equal to that of the combined atoms in the molecule. The figures for the

atomic mass of all elements are established, and can be found on the periodic

table; therefore, when one knows the mass of a hydrogen atom and an oxygen

atom, as well as the fact that there are two hydrogens and one oxygen in a

molecule of water, it is easy to calculate the mass of a water molecule.

MOLECULAR MASS

Note that the mass of an atom in a molecule does not change;

nor, indeed, do the identities of the individual atoms. An oxygen atom in water

is the same oxygen atom in sugar, or in any number of other compounds. With

regard to compounds, it should be noted that these are not the same thing as a

mixture, or a solution. Sugar or salt can be dissolved in water at the appropriate

temperatures, but the resulting solution is not a compound; the substances are

joined physically, but they are not chemically bonded.

BONDING BETWEEN MOLECULES

Chemical bonding is the joining, through electromagnetic force, of atoms

representing different elements. Each atom possesses a certain valency, which

determines its ability to bond with atoms of other elements. Valency, in turn, is

governed by the configuration of valence electrons at the highest energy level

(the shell) of the atom.

Chemical Bonding

The energy required to pull apart a molecule is known as bond energy.

Covalent bonds that involve hydrogen are among the weakest bonds between

atoms, and hence it is relatively easy to separate water into its constituent

parts, hydrogen and oxygen. (This is sometimes done by electrolysis, which

involves the use of an electric current to disperse atoms.) Double and triple

covalent bonds are stronger, but strongest of all is an ionic bond. The strength

of the bond energy in salt, for instance, is reflected by its melting point of

1,472°F (800°C), much higher than that of water, at 32°F (0°C).

Attractions Between Molecules

Not all elements bond covalently in the same way. Each has a certain

value of electronegativity—the relative ability of an atom to attract valence

electrons. Elements capable of bonding are assigned an electronegativity value

ranging from a minimum of 0.7 for cesium to a maximum of 4.0 for fluorine. The

greater the electronegativity value, the greater the tendency of an element to

attract valence electrons.

Electronegativity

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 H2O. A more complex structural

formula is sometimes used to show the arrangement of atoms in the molecule.

Nature of Molecules

Molecules differ in size and molecular weight as well as in structure. In a

chemical reaction between molecular substances, the molecules are often

broken apart into atoms or radicals that recombine to form other

molecules, i.e., other substances. In other cases two or more molecules will

combine to form a single larger molecule, or a large molecule will be broken up

into several smaller molecules.

Colloids were originally defined by Thomas

Graham in 1861 as substances, such as starch or

gelatin, which will not diffuse through a membrane. He

distinguished them from crystalloids (e.g. inorganic

salts), which would pass through membranes. Later it

was recognized that colloids were distinguished from

true solutions by the presence of particles that were too

small to be observed with a normal microscope yet

were much larger than normal molecules. Colloids are

now regarded as systems in which there are two or

more phases, with one (the dispersed phase)

distributed in the other (the continuous phase).

Moreover, at least one of the phases has small

dimensions (in the range 10−9–10−6 m).

COLLOIDS

CLASSIFICATION OF COLLOIDS

Sols are dispersions of small solid particles in a liquid. The particles may be

macromolecules or may be clusters of small molecules. Lyophobic sols are

those in which there is no affinity between the dispersed phase and the liquid.

An example is silver chloride dispersed in water. In such colloids the solid

particles have a surface charge, which tends to stop them coming together.

Lyophobic sols are inherently unstable and in time the particles aggregate and

form a precipitate. Lyophilic sols, on the other hand, are more like true

solutions in which the solute molecules are large and have an affinity for the

solvent. Starch in water is an example of such a system. Association

colloids are systems in which the dispersed phase consists of clusters of

molecules that have lyophobic and lyophilic parts. Soap in water is an

association colloid .

Emulsions are colloidal systems in which the dispersed and continuous phases are

both liquids, e.g. oil-in-water or water-in-oil. Such systems require an

emulsifying agent to stabilize the dispersed particles.

Gels are colloids in which both dispersed and continuous phases have a three-

dimensional network throughout the material, so that it forms a jelly-like mass.

Gelatin is a common example. One component may sometimes be removed

(e.g. by heating) to leave a rigid gel (e.g. silica gel).

Other type of colloid

Aerosols (dispersions of liquid or solid particles in a gas, as in a mist or smoke) and

foams (dispersions of gases in liquids or solids).

Colloids are mixtures whose particles are larger than the size of a molecule

but smaller than particles that can be seen with the naked eye. Colloids are one of

three major types of mixtures, the other two being solutions and suspensions. The

three kinds of mixtures are distinguished by the size of the particles that make

them up. The particles in a solution are about the size of molecules, approximately

1 nanometer (1 billionth of a meter) in diameter. Those that make up suspensions

are larger than 1,000 nanometers. Finally, colloidal particles range in size between

1 and 1,000 nanometers. Colloids are also called colloidal dispersions because

the particles of which they are made are dispersed, or spread out, through the

mixture.

Types of colloids

Colloids are common in everyday life. Some examples include whipped

cream, mayonnaise, milk, butter, gelatin, jelly, muddy water, plaster, colored

glass, and paper.

Parts of Colloids

Colloidal particle is a small amount of matter having size typical for

colloids and with a clear phase boundary (phase colloids). A group of such

particles (aggregate, agglomerate) or being a macromolecule (eg. solution

of polymer molecules is a molecular colloid) or a molecular aggregate (e.g.

micelle).

Dispersing medium is the substance in which the colloidal particles

are distributed.

In muddy water, for example, the colloidal particles are tiny grains of

sand, silt, and clay. The dispersing medium is the water in which these

particles are suspended.

Dispersed

Material Dispersed in Gas

Dispersed in

Liquid

Dispersed in

Solid

Gas (bubbles) Not possible

Foams: soda pop;

whipped cream;

beaten egg whites

Solid foams:

plaster; pumice

Liquid (droplets) Fogs: mist; clouds;

hair sprays

Emulsions: milk;

blood; mayonnaise butter; cheese

Solid (grains) Smokes: dust;

industrial smoke

Sols and gels:

gelatin; muddy

water; starch

solution

Solid sol: pearl;

colored glass;

porcelain; paper

Each type of mixture has special properties by which it can be identified.

For example, a suspension always settles out after a certain period of time.

That is, the particles that make up the suspension separate from the medium

in which they are suspended and fall to the bottom of a container. In contrast,

colloidal particles typically do not settle out. Like the particles in a solution,

they remain in suspension within the medium that contains them.

Properties of Colloids

Colloids also exhibit Brownian movement. Brownian movement is the

random zigzag motion of particles that can be seen under a microscope. The

motion is caused by the collision of molecules with colloid particles in the

dispersing medium. In addition, colloids display the Tyndall effect.

When a strong light is shone through a colloidal dispersion, the light

beam becomes visible, like a column of light. A common example of this

effect can be seen when a spotlight is turned on during a foggy night. You

can see the spotlight beam because of the fuzzy trace it makes in the fog (a

colloid).

Light shining through a solution of sodium hydroxide (left) and a colloidal

mixture. The size of colloidal particles makes the mixture,

which is neither a solution nor a suspension, appear cloudy.

Interaction between colloid particles

The following forces play an important role in the interaction of colloid

particles:

Excluded volume repulsion: This refers to the impossibility of any overlap

between hard particles.

Electrostatic interaction: Colloidal particles often carry an electrical charge

and therefore attract or repel each other. The charge of both the continuous

and the dispersed phase, as well as the mobility of the phases are factors

affecting this interaction.

van der Waals forces: This is due to interaction between two dipoles that

are either permanent or induced. Even if the particles do not have a

permanent dipole, fluctuations of the electron density gives rise to a

temporary dipole in a particle. This temporary dipole induces a dipole in

particles nearby. The temporary dipole and the induced dipoles are then

attracted to each other. This is known as van der Waals force, and is always

present (unless the refractive indexes of the dispersed and continuous

phases are matched), is short-range, and is attractive.

Entropic forces: According to the second law of thermodynamics, a system

progresses to a state in which entropy is maximized. This can result in

effective forces even between hard spheres.

Steric forces between polymer-covered surfaces or in solutions containing

non-adsorbing polymer can modulate interparticle forces, producing an

additional steric repulsive force (which is predominantly entropic in origin) or

an attractive depletion force between them. Such an effect is specifically

searched for with tailor-made superplasticizers developed to increase the

workability of concrete and to reduce its water content.

Gas is one of three classical states of matter. Near absolute zero, a

substance exists as a solid. As heat is added to this substance it melts into a

liquid at its melting point (see phase change), boils into a gas at its boiling point,

and if heated high enough would enter a plasma state in which the electrons are

so energized that they leave their parent atoms from within the gas. A pure gas

may be made up of individual atoms (e.g. a noble gas or atomic gas like neon),

elemental molecules made from one type of atom (e.g. oxygen), or compound

molecules made from a variety of atoms (e.g. carbon dioxide). A gas mixture

would contain a variety of pure gases much like the air. What distinguishes a gas

from liquids and solids is the vast separation of the individual gas particles. This

separation usually makes a colorless gas invisible to the human observer. The

interaction of gas particles in the presence of electric and gravitational fields are

considered negligible.

GAS

The gaseous state of matter is found between the liquid and plasma

states[2], the latter of which provides the upper temperature boundary for gases.

Bounding the lower end of the temperature scale lie degenerative quantum

gases[3] which are gaining increased attention these days.[4] High-density atomic

gases super cooled to incredibly low temperatures are classified by their

statistical behavior as either a Bose gas or a Fermi gas.

Physical characteristics

Drifting smoke particles provide clues to the movement of the

surrounding gas.

As most gases are difficult to observe directly with our senses, they are

described through the use of four physical properties or macroscopic

characteristics: the gas’s pressure, volume, number of particles (chemists

group them by moles), and temperature. These four characteristics were

repeatedly observed by men such as Robert Boyle, Jacques Charles, John

Dalton, Joseph Gay-Lussac and Amedeo Avogadro for a variety of gases in a

great many settings. Their detailed studies ultimately led to a mathematical

relationship among these properties expressed by the ideal gas law.

Gas particles are widely separated from one another, and as such do not

influence adjacent particles to the same degree as liquids or solids. This

influence (intermolecular forces) results from the magnetic charges that

these gas particles carry. Like charges repel, while oppositely charged

particles attract one another. Gases made from ions carry permanent

charges, as do compounds with their polar covalent bonds. These polar

covalent bonds produce permanent charge concentrations within the

molecule while the compound's net charge remains neutral.

Compared to the other states of matter, gases have an incredibly low

density and viscosity. Pressure and temperature influence the particles within

a certain volume. This variation in particle separation and speed is referred to

as compressibility. This particle separation and size influences optical

properties of gases as can be found in the following list of refractive indices.

Finally, gas particles spread apart or diffuse in order to homogeneously

distribute themselves throughout any container.

Drifting smoke particles provide clues to the

movement of the surrounding gas.

When observing a gas, it is typical to specify a frame of reference or

length scale. A larger length scale corresponds to a macroscopic or global

point of view of the gas. This region (referred to as a volume) must be

sufficient in size to contain a large sampling of gas particles. The resulting

statistical analysis of this sample size produces the "average" behavior (i.e.

velocity, temperature or pressure) of all the gas particles within the region. By

way of contrast, a smaller length scale corresponds to a microscopic or

particle point of view.

MACROSCOPIC

Pressure

The symbol used to represent pressure in equations is "p" or "P" with SI

units of pascals.

When describing a container of gas, the term pressure (or absolute

pressure) refers to the average force the gas exerts on the surface area of the

container. Within this volume, it is sometimes easier to visualize the gas

particles moving in straight lines until they collide with the container (see

diagram at top of the article). The force imparted by a gas particle into the

container during this collision is the change in momentum of the particle.

Temperature

The symbol used to represent temperature in equations is T with SI units

of kelvins.

The speed of a gas particle is proportional to its absolute temperature. The

volume of the balloon in the image to the right shrinks when the trapped gas

particles slow down with the addition of extremely cold nitrogen. The

temperature of any physical system is related to the motions of the particles

(molecules and atoms) which make up the [gas] system.[

Specific Volume

The symbol used to represent specific volume in equations is "v" with SI

units of cubic meters per kilogram.

The symbol used to represent volume in equations is "V" with SI units of

cubic meters.

When performing a thermodynamic analysis, it is typical to speak of

intensive and extensive properties. Properties which depend on the amount of

gas (either by mass or volume) are called extensive properties, while

properties that do not depend on the amount of gas are called intensive

properties.

Specific volume is an example of an intensive property because it is the

ratio of volume occupied by a unit of mass of a gas that is identical throughout

a system at equilibrium.[11] 1000 atoms of protactinium as a gas occupy the

same space as any other 1000 atoms for any given temperature and pressure.

This concept is easier to visualize for solids such as iron which are

incompressible compared to gases. When the seat ejection is initiated in the

image above the specific volume increases with the expanding gases, while

mass is conserved. Since a gas fills any container in which it is

placed, volume is an extensive property.

The symbol used to represent density in equations is ρ (pronounced rho)

with SI units of kilograms per cubic meter. This term is the reciprocal of

specific volume. Since gas molecules can move freely within a container, their

mass is normally characterized by density. Density is the mass per volume of

a substance or simply, the inverse of specific volume. For gases, the density

can vary over a wide range because the particles are free to move closer

together when constrained by pressure or volume or both. This variation of

density is referred to as compressibility. Like pressure and

temperature, density is a state variable of a gas and the change in density

during any process is governed by the laws of thermodynamics

Density

English chemist Robert Boyle (1627-1691), who

made a number of important contributions to

chemistry—including his definition and identification

of elements—seems to have been influenced by

Torricelli. If so, this is an interesting example of

ideas passing from one great thinker to another:

Torricelli, a student of Galileo Galilei (1564-

1642), was no doubt influenced by Galileo's

thermoscope.

GAS LAW

The gas laws are not a set of government regulations concerning use of

heating fuel; rather, they are a series of statements concerning the behavior

of gases in response to changes in temperature, pressure, and volume.

These were derived, beginning with Boyle's law, during the seventeenth,

eighteenth, and nineteenth centuries by scientists whose work is

commemorated through the association of their names with the laws they

discovered. In addition to Boyle, these men include fellow English chemists

John Dalton (1766-1844) and William Henry (1774-1836); French physicists

and chemists J. A. C. Charles (1746-1823) and Joseph Gay-Lussac (1778-

1850); and Italian physicist Amedeo Avogadro (1776-1856).

Boyle's law holds that in isothermal conditions (that is, a situation in

which temperature is kept constant), an inverse relationship exists between

the volume and pressure of a gas. (An inverse relationship is a situation

involving two variables, in which one of the two increases in direct proportion

to the decrease in the other.) In this case, the greater the pressure, the less

the volume and vice versa. Therefore, the product of the volume multiplied

by the pressure remains constant in all circumstances.

BOYLE’S AND CHARLES LAW

Charles's law also yields a constant, but in this case the temperature

and volume are allowed to vary under isobarometric conditions—that is, a

situation in which the pressure remains the same. As gas heats up, its

volume increases, and when it cools down, its volume reduces accordingly.

Hence, Charles established that the ratio of temperature to volume is

constant.

From Boyle's and Charles's law, a pattern should be emerging: both treat

one parameter (temperature in Boyle's, pressure in Charles's) as

unvarying, while two other factors are treated as variables. Both, in turn, yield

relationships between the two variables: in Boyle's law, pressure and volume

are inversely related, whereas in Charles's law, temperature and volume are

directly related.

In Gay-Lussac's law, a third parameter, volume, is treated as a

constant, and the result is a constant ratio between the variables of pressure

and temperature. According to Gay-Lussac's law, the pressure of a gas is

directly related to its absolute temperature.

GAY LUSSAC’S LAW

In 1811, Amedeo Avogadro verified that equal volumes of pure gases

contain the same number of particles. His theory was not generally accepted

until 1858 when another Italian chemist Stanislao Cannizzaro was able to

explain non-ideal exceptions. For his work with gases a century prior, the

number that bears his name Avogadro's constant represents the number of

atoms found in 12 grams of elemental carbon-12 (6.022×1023 mol-1). This

specific number of gas particles, at standard temperature and pressure (ideal

gas law) occupies 22.40 liters and is referred to as the molar volume.

AVOGADRO’S LAW

In 1801, John Dalton published the Law of Partial Pressures from his

work with ideal gas law relationship: The pressure of a mixture of gases is

equal to the sum of the pressures of all of the constituent gases alone.

Mathematically, this can be represented for n species as:

Pressuretotal = Pressure1 + Pressure2 + ... + Pressuren

Among his key journal observations upon mixing unreactive "elastic fluids"

(gases) were the following.[20]:

Unlike liquids, heavier gases did not drift to the bottom upon mixing.

Gas particle identity played no role in determining final pressure (they

behaved as if their size was negligible).

DALTON’S LAW

From the preceding gas laws, a set of propositions known collectively as

the kinetic theory of gases has been derived. Collectively, these put forth the

proposition that a gas consists of numerous molecules, relatively far apart in

space, which interact by colliding. These collisions are responsible for the

production of thermal energy, because when the velocity of the molecules

increases—as it does after collision—the temperature increases as well.

KINETIC THEORY OF GASES

There are five basic postulates to the kinetic theory of gases:

1. Gases consist of tiny molecular or atomic particles.

2. The proportion between the size of these particles and the distances

between them is so small that the individual particles can be assumed to

have negligible volume.

3. These particles experience continual random motion. When placed in a

container, their collisions with the walls of the container constitute the

pressure exerted by the gas.

4. The particles neither attract nor repel one another.

5. The average kinetic energy of the particles in a gas is directly related to

absolute temperature.