Atomic and Mass Number of Elements

Atomic number formed the basis of the modern periodic classification and electronic configuration of elements. 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 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 is characterized by its atomic number and not by atomic weight.

The most reasonable interpretation of Moseley’s work, if the atomic number of an element equal to the charge of the atomic nucleus. Therefore, in order to maintain the electroneutrality of atoms, the atom must carry the extranuclear electrons whose number equal to the atomic number. The atomic number of the hydrogen atom is one means it 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 an element.

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

Atomic Mass or Weight (Chemical Physical Scale)

The scale of atomic masses with natural oxygen of relative weight 16 taken as the reference 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 and introduced a new physical scale of atomic weights. Therefore, the atomic weights or masses with O-16 isotope of relative weight 16 taken as the reference is called the physical scale of atomic weight.

Definition of Atomic and Molecular Mass

The atomic weight of an element is a number that says how much heavy an atom of an element compared to one-twelfth of an atom of the carbon-12 isotope. But the gram atomic weight of an element is the weight in gram that contains the same number of atoms as twelve gram of C-12 isotope. The atomic weight of an element is unitless quantity but the gram atomic weight has the unit of gram.

Atomic mass unit (amu) is 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

The molecular mass or weight of the compound is the number that indicates how heavy a molecule of the compound is 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 Atomic Weight

In order to determine the atomic weight of silver, use the chemical reaction between silver and chlorine by Stas’s. The known weight of pure silver crystal heated with pure chlorine gas in the glass tube and 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. Therefore, the atomic weight of chlorine = 35.45 (taken), the atomic mass, or weight of silver = 107.94.

Chemical Bond

Parts of an atom

Electronic Structure