Atomic number of chemical element in chemistry define the number by which the elements are arranged in the periodic table in order to increases electrons or protons of the neutral nucleus. The modern periodic system is formed on the basis of atomic number and electronic configuration of the atom but Mendeleev classification is based on atomic weight or mass. For example, the atom of boron has atomic number 5 and electronic configuration 1s22s22p1 suggests the position of boron in the periodic table (Group-13, period-2). Rutherford model or 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 electromagnetic spectrum data, find from x-ray discovered by Moseley. The atomic number correlates with the x-ray spectrum, not atomic mass or weight. Therefore, the basic characteristics of chemical elements define by the atomic number and not by weight.
The most reasonable interpretation of Moseley’s work indicates that the atomic number of a chemical element equal to the charge of the nucleus. In order to maintain the neutrality of atoms, the atom must carry the extranuclear electron particles whose number refers to the atomic number. The atomic number of the hydrogen atom equal to one determines hydrogen carries a positive charge on the nucleus with one outer electron. The number of protons and the number of neutrons together refers to the mass number of a chemical element.
Atomic Mass or Weight Scale
In the physical scale of atomic mass or weight in chemistry, the natural oxygen atom (relative weight 16) is taken as the reference. But natural oxygen was found to be the mixture of three isotopes of mass number 16, 17, and 18 with the percentage of abundance 993575, 0.039, and 0.204 respectively. 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 to represents the new scale of atomic weights. In the chemical scale of the atomic weight or masses, Aston uses the exact number 16 taken as the reference.
Atomic Mass and Molecular Mass
The atomic weight of an element meaning the number that says how much heavy an atom of chemical elements compared to one-twelfth of the carbon-12 isotope. The gram atomic weight of an element determines the mass in gram that represents the same number of atoms as twelve grams of C-12 isotope. The atomic weight or mass of an element is a unitless quantity, But the gram atomic weight is calculated in the gram unit.
The atomic mass unit represents by the symbol amu, means exactly the one-twelfth of the mass of the carbon-12 atom. Therefore, one amu 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 is compared to one-twelfth of the carbon-12 isotope (atomic number = 12). When molecular mass expressed in gram is called gram molecular weight.
Determination of Atomic Mass or Weight
Scientist Stas in chemistry uses the chemical bonding between silver and chlorine for determines the atomic weight or mass of the chemical elements. He has taken 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) in chemical science equal to 35.45. The calculated atomic mass or weight of silver atom = 107.94.