Moles and Atoms

atom

Mole is a unit of measurement for the amount of substance in the SI standard unit. It’s defined as the amount of any chemical substance that contains atoms, molecules, ions, electrons, and protons. The mole is widely used in chemistry as a convenient way to express amounts of reactants and products of chemical reactions. The mole may also be used to express the number of atoms. The concentration of a solution in chemical reactions, is usually expressed by its molarity. Molarity is the number of moles of the dissolved substance per liter of solution. Meanwhile, the mass per mole of a substance is called the molar mass.

John Dalton has studied and explained in his previous work and developed the law of multiple proportions. If 2 elements can be combined to form a number of possible compounds, then the ratios of the masses of the second element which combine with a fixed mass of the first element will be ratios of the small whole numbers. The flaw in Dalton’s theory was corrected in principle in 1811 by Amedeo Avogadro. Avogadro had explained that equal volumes of any two gases, at equal temperature and pressure, contain equal numbers of molecules. Measurement of the weights of standard volumes or as known as the densities of different gasses permits direct comparison of the weights of individual gas molecules. The term atomic weight is frequently used, while atomic mass would perhaps be more accurate. Meanwhile, mass is a measure of the quantity of matter in a body, whereas weight is the force exerted on the body by the influence of gravity. Thus, atomic weight is measured in atomic mass units.

Far from being a solid bit of matter, atom was found to consist mostly of space. At the center of this space there is an infinitesimally small core called the nucleus. Rutherford explained that the mass of the atom is concentrated in its own nucleus. He also proposed that satellites called electrons travel in orbits around the nucleus . The nucleus has a positive charge of electricity, the electrons each have a negative charge. The charges carried by the electrons added up to the same amount of electricity as resides in the nucleus, and thus, the normal electrical state of the atom became neutral.

To explain the structure of the atom, Niels Bohr developed in 1913 a hypothesis known in general as the Bohr Theory of the atom or as known as the Quantum Theory. He assumed that electrons are arranged in definite shells, or quantum levels, at a considerable distance from the nucleus. The arrangement of these electrons is called the electron.

Atoms in different elements have different masses. When we perform chemical calculations, we should know how heave one atom is compared to another. The mass of a single atom is so small that it is impossible to weigh it directly, that’s why, we have to weigh a lot of atoms to know the weight of the atom itself. Scientists have chosen to use the isotope carbon-12 as the standard. This has been given a mass of exactly 12 units of atom. The mass of other atoms is found by comparing their mass with the mass of the carbon-12 atoms This is called the relative atomic mass or as known as Ar. We represent the nucleon number (the total number of neutrons plus protons in an atom) by the number written at the top left corner of the atom’s symbol in periodic table.

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