Atomic and Mass Number of Elements

Atomic number formed the basis of the modern periodic table classification and electronic configuration of elements. Modern periodic classification based on the atomic number is more fundamental for the understanding of chemical properties rather then Mendeleev classification based on atomic weight or mass of the chemical elements. Rutherford alpha ray scattering experiments allow for the determination of positive charge of the nucleus of an atom but the identity of the nuclear charge and the atomic number of an element established by x-ray electromagnetic spectrum data obtained by Moseley. Therefore, the correlation between the x-ray spectrum and atomic number, not atomic mass or weight indicated that an element characterized by its atomic number and not by weight.

The most reasonable interpretation of Moseley’s work, if the atomic number of a chemical element equal to the charge of the nucleus. Therefore, in order to maintain the electroneutrality of atoms, the atom must carry the extranuclear electron particles whose number equal to the atomic number. The atomic number of the hydrogen atom equal to one means, hydrogen carries one positive charge in the nucleus and one extranuclear electron. But the number of protons and the number of neutrons together called the mass number of a chemical element.

Definition and determination of atomic number and mass or weight of element

Atomic Number and Mass or Weight Scale

The scale of atomic masses with natural oxygen of relative weight 16 taken as the reference. This is called the physical scale of atomic weight. But natural oxygen found to be the mixture of three isotopes of muss numbers 16, 17, and 18 with the percentage of abundance 993575, 0.039, and 0.204 respectively. Therefore, the true mass of natural oxygen = (16 × 0.99757) + (17 × 0.00039) + (18 × 0.00204) = 16.00447. Aston use the exact number 16 as the relative mass of oxygen. Hence he introduced a new physical scale of atomic weights. In this scale of the atomic weight or masse, the oxygen-16 isotope with relative weight 16 taken as the reference. This is called the chemical scale of atomic weight.

Definition of Atomic Mass and Molecular Mass

The atomic weight of an element meaning the number that says how much heavy an atom of an element compared to one-twelfth of an atom like carbon-12 isotope. But the gram atomic weight of an element is the mass in gram that contains the same number of atoms as twelve grams of C-12 isotope. The atomic weight or mass of an element is unitless quantity, But the gram atomic weight calculated in the gram unit. Atomic mass unit (amu) means exactly the one-twelfth of the mass of a carbon-12 atom. Thus the mass of an atom expressed in amu which numerically equal to its atomic weight on the carbon-12 scale. 1 amu = (12/6.023) × (1/12) × 10-23 gm = 1.6603 × 10-24 gm. In learning chemistry, the molecular mass or weight of the compound meaning the number that indicates how heavy a molecule of the compound compared to one-twelfth of an atom of the carbon-12 isotope. When molecular mass expressed in gram is called gram molecular weight.

Determination of Weight from Atomic Number

In order to determine the atomic weight of silver in chemistry, use the chemical bonding between silver and chlorine by Stas’s. The known weight of pure silver crystal heated with pure chlorine gas in the glass tube. After the reaction the resulting silver chloride weighted. He also dissolved a known quantity of pure silver crystal in pure nitric acid and precipitated the silver as silver chloride and weighted. 35.45 gm of chlorine combines with 107.94 gm of silver to give 143.39 gm of AgCl. The atomic weight of chlorine (atomic number 17) equal to 35.45. Therefore the calculated atomic mass or weight of silver atom = 107.94.



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