Definition: Avogadro's number is the number of particles found in one mole of a substance. It is the number of atoms in exactly 12 grams of carbon-12.

AVOGADRO’S NUMBER

Definition: Avogadro's number is the number of particles found in one mole of a substance. It is the number of atoms in exactly 12 grams of carbon-12. This experimentally determined value is approximately 6.022 x 1023 particles per mole.

AMEDEO AVOGADRO

Avogadro's number is named in honor of Amedeo Avogadro.

AVOGADRO'S LAW

Avogadro's law states that equal volumes of gases, at the same temperature and pressure, contain the same number of molecules. Avogadro's hypothesis wasn't generally accepted until after 1858 (after Avogadro's death), when the Italian chemist Stanislao Cannizzaro was able to explain why there were some organic chemical exceptions to Avogadro's hypothesis.

RESOLUTION OF THE CONFUSION

One of the most important contributions of Avogadro's work was his resolution of the confusion surrounding atoms and molecules (although he didn't use the term 'atom'). Avogadro believed that particles could be composed of molecules and that molecules could be composed of still simpler units,.atoms.

HONOR OF AVOGADRO'S THEORIES

The number of molecules in a mole (one gram molecular weight) was termed Avogadro's number (sometimes called Avogadro's constant) in honor of Avogadro's theories. Avogadro's number has been experimentally determined to be 6.023x1023 molecules per gram-mole

GENERAL ROLE IN SCIENCE

Avogadro's constant is a scaling factor between macroscopic and microscopic (atomic scale) observations of nature. As such, it provides the relation between other physical constants and properties. For example, it establishes a relationship between the gas constant R and the Boltzmann constant kB,

COULOMETRY

The earliest accurate method to measure the value of the Avogadro constant was based on coulometry. The principle is to measure the Faraday constant, F, which is the electric charge carried by one mole of electrons, and to divide by theelementary charge, e, to obtain the Avogadro constant.

HOW TO DETERMINE AVOGADRO’S CONSTANT

Ball-and-stick model of the unit cell of silicon. X-ray diffraction measures the cell parameter, a, which is used to calculate a value for Avogadro's constant.

A modern method to determine the Avogadro constant is the use of X-ray 

AVOGADRO'S NUMBER IN PRACTICE

How on Earth did chemists settle on such a seemingly arbitrary figure for Avogadro's number? To understand how it was derived, we have to first tackle the concept of the atomic mass unit (amu).

ATOMIC MASS UNIT

The atomic mass unit is defined as 1/12 of the mass of one atom of carbon-12 (the most common isotope of carbon). Here's why that's neat: Carbon-12 has six protons, six electrons and six neutrons, and because electrons have very little mass, 1/12 of the mass of one carbon-12 atom is very close to the mass of a single proton or a single neutron.

FIND A WAY TO BRIDGE THE GAP

Unfortunately, chemists don’t have a scale that can measure atomic mass units, and they certainly don’t have the ability to measure a single atom or molecule at a time to carry out a reaction. Since different atoms weigh different amounts, chemists had to find a way to bridge the gap between the invisible world of atoms and molecules and the practical world of chemistry laboratories filled with scales that measure in grams.

A RELATIONSHIP BETWEEN THE ATOMIC MASS UNIT AND THE GRAM

they created a relationship between the atomic mass unit and the gram, and that relationship looks like this:

1 amu = 1/6.0221415 x 1023 grams

CONVERSION

This relationship means that if we had Avogadro's number, or one mole, of carbon-12 atoms (which has an atomic weight of 12 amu by definition), that sample of carbon-12 would weigh exactly 12 grams. Chemists use this relationship to easily convert between the measurable unit of a gram and the invisible unit of moles, of atoms or molecules.

USE OF X-RAY

. Over time, scientists have found new and more accurate ways of estimating Avogadro's number, most recently using advanced techniques like using X-rays to examine the geometry of a 1 kilogram sphere of silicon and extrapolating the number of atoms it contained from that data. And while the kilogram is the basis for all units of mass, some scientists want to begin using Avogadro's number instead, much the way we now define the length of a meter based on the speed of light instead of the other way around.

THE COUNTING UNIT

You know that eggs , oranges and bananas are are counted in dozens, but paper by ream.

Thus, the counting unit depends on what you are counting.

REPRESENTATIVE PARTICLES

Just as a dozen eggs represent twelve eggs ,

a ream represent 500 papers , a mole of a substance represents 6.022 x 1023

representative particles of a substance.

MOLE 

Mole is a unit of measurement used in chemistry to express amounts of a chemical substance, defined as the amount of any substance that contains as many elementary entities (e.g., atoms, molecules, ions,electrons) as there are atoms in 12 grams of pure carbon-12 (12C), theisotope of carbon with relative atomic mass 12. 

IN OTHER WORDS

The mole may also be used to express the number of atoms, ions, or other elementary entities in a given sample of any substance. The concentration of a solution is commonly expressed by its molarity, defined as the number of moles of the dissolved substance per litre of solution.

MOLECULAR MASS.

The number of molecules in a mole (known as Avogadro's constant) is defined such that the mass of one mole of a substance, expressed in grams, is exactly equal to the substance's mean molecular mass.

EXAMPLE

For example, the mean molecular mass of natural water is about 18.015, so one mole of water is about 18.015 grams. Making use of this equation considerably simplifies many chemical and physical computations.

 GRAM-ATOM

The term gram-molecule was formerly used for essentially the same concept.[1] The term gram-atom (abbreviated gat.) has been used for a related but distinct concept, namely a quantity of a substance that contains Avogadro's number ofatoms, whether isolated or combined in molecules. Thus, for example, 1 mole of MgB2 is 1 gram-molecule of MgB2 but 3 gram-atoms of MgB2

MOLAR MASS

The mass per mole of a substance is called its molar mass. Since the standard unit for expressing the mass of molecules or atoms (atomic mass unit or the dalton) is defined as 1/12 of the mass of a 12C atom, it follows that the molar mass of a substance, measured in grams per mole, is exactly equal to its mean molecular or atomic mass, measured in unified atomic mass units or daltons; which is to say, to the substance's mean molecular or relative atomic mass.

RELATIVE ATOMIC MASS

Relative atomic mass (symbol: Ar) is a dimensionless physical quantity, the ratio of the average mass of atoms of anelement (from a single given sample or source) to 1/12 of the mass of an atom of carbon-12 (known as the unified atomic mass unit).[1][2