The Mole

Simply put, the mole represents a number. Just as the term dozen refers to the number twelve, the mole represents the number 6.02 x 10²³.

Now that's a big number! While a dozen eggs will make a nice omelet, a mole of eggs will fill all of the oceans on earth more than 30 million times over.

The mole is used when we're talking about numbers of atoms and molecules. Atoms and molecules are very tiny things. A drop of water the size of the period at the end of this sentence would contain 10 trillion water molecules. Instead of talking about trillions and quadrillions of molecules (and more), it's much simpler to use the mole.

History of the Mole

The number of objects in one mole, that is, 6.02 x 10²³, is commonly referred to as Avogadro´s number. Amadeo Avogadro was an Italian physics professor who proposed in 1811 that equal volumes of different gases at the same temperature contain equal numbers of molecules About fifty years later, an Italian scientist named Stanislao Cannizzaro used Avogadro's hypothesis to develop a set of atomic weights for the known elements by comparing the masses of equal volumes of gas. Building on this work, an Austrian high school teacher named
Johann Josef Loschmidt calculated the size of a molecule of air in 1865, and thus developed an estimate for the number of molecules in a given volume of air. While these early estimates have since been refined, they led to the concept of the mole - that is, the theory that in a defined mass of an element (its atomic weight) there is a precise number of atoms: Avogadro's number.

Molar Mass

A sample of any element with a mass equal to that element's atomic weight (in grams) will contain precisely one mole of atoms (6.02 x 10²³ atoms).  For example, helium has an atomic weight of 4.00.  Therefore, 4.00 grams of helium will contain one mole of helium atoms.  You can also work with fractions (or multiples) of moles:  

Mole/Weight Relationship Examples Using Helium
Moles Helium	Helium Atoms	Grams Helium
1/4	1.505 x 1023	1 g
1/2	3.01 x 1023	2 g
1	6.02 x 1023	4 g
2	1.204 x 1024	8 g
10	6.02 x 1024	40 g

Other atomic weights are listed on the periodic table. For each listed, measuring out a quantity of the element equal to its atomic weight in grams will yield 6.02 x 10²³ atoms of that element.

The atomic weight of an element identifies both the mass of one mole of that element and the total number of protons and neutrons in an atom of that element. How can that be? Let's look at hydrogen. One mole of hydrogen atoms will weigh 1.01 grams.

Each hydrogen atom consists of one proton surrounded by one electron. But remember, the electron weighs so little that it does not contribute much to an atom's weight. Ignoring the weight of hydrogen's electrons, we can say that one mole of protons (H nuclei) weighs approximately one gram. Since protons and neutrons have about the same mass, a mole of either of these particles will weigh about one gram. For example, in one mole of helium, there are two moles of protons and two moles of neutrons - four grams of particles.

Molecular Weight

If you stand on a scale with a friend, the scale will register the combined weight of both you and your friend.  When atoms form molecules, the atoms bond together, and the molecule's weight is the combined weight of all of its parts.

For example, every water molecule (H₂O) has two atoms of hydrogen and one atom of oxygen.  One mole of water molecules will contain two moles of hydrogen and one mole of oxygen.  

  

A bottle filled with exactly 18.02 g water will contain 6.02 x 10²³ water molecules. The concept of fractions and multiples described above also applies to molecules: 9.01 g of water would contain 1/2 mole, or 3.01 x 10²³ molecules.  You can calculate the molecular weight of any compound simply by summing the weights of atoms that make up that compound. 

Molecular Weight Calculator

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PERIODIC TABLE

History of the periodic table







Although elements such as gold, silver, tin, copper, lead and mercury have been known since antiquity, the first scientific discovery of an element occurred in 1649 when Hennig Brand discovered phosphorous. By 1869, a total of 63 elements had been discovered. As the number of known elements grew, scientists began to recognize patterns in properties and began to develop classification schemes. 

In 1817 Johann Dobereiner proposed that nature contained triads of elements the middle element had properties that were an average of the other two members when ordered by  the atomic weight (the Law of Triads).

In 1864 John Newlands published his version of the periodic table and proposed the Law of Octaves (by analogy with the seven intervals of the musical scale). This law stated that any given element will exhibit analogous behavior to the eighth element following it in the table.

In 1869 Russian chemist Dimitri Mendeleev started the development of the periodic table, arranging chemical elements by atomic mass. He predicted the discovery of other elements, and left spaces open in his periodic table for them. Mendeleev grouped the elements in a table that had both rows and columns. 



Unknown to Mendeleev, Lothar Meyer was also working on a periodic table. Although his work was published in 1864, and was done independently of Mendeleev, few historians regard him as an equal co-creator of the periodic table. For one thing, Meyer's table only included 28 elements. Furthermore, Meyer classified elements not by atomic weight, but by valence alone. Finally, Meyer never came to the idea of predicting new elements and correcting atomic weights. Only a few months after Mendeleev published his periodic table of all known elements (and predicted several new elements to complete the table, plus some corrected atomic weights), Meyer published a virtually identical table. While a few people consider Meyer and Mendeleev the co-creators of the periodic table, most agree that, by itself, Mendeleev's accurate prediction of the qualities of the undiscovered elements lands him the larger share of credit. In any case, at the time Mendeleev's predictions greatly impressed his contemporaries and were eventually found to be correct.

In 1914 Henry Moseley found a relationship between an element's X-ray wavelength and its atomic number (Z), and therefore resequenced the table by nuclear charge rather than atomic weight.



The modern periodic table of elements is based on Mendeleev's observations; however, instead of being organized by atomic weight, the modern table is arranged by atomic number (Z).  As one moves from left to right in a row of the periodic table, the properties of the elements gradually change.  At the end of each row, a drastic shift occurs in chemical properties. The next element in order of atomic number is more similar (chemically speaking) to the first element in the row above it; thus a new row begins on the table. 



 

Rows in the periodic table are called periods.  As one moves from left to right in a given period, the chemical properties of the elements slowly change.  Columns in the periodic table are called groups.  Elements in a given group in the periodic table share many similar chemical and physical properties.  The link below will open a copy of the periodic table of elements with  information about them. 



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