Atomic number of elements

Atomic number of chemical elements in chemistry is the number of protons of an atom by which the elements are arranged in the periodic table. The modern periodic system is formed on the basis of atomic number and electronic configuration but Mendeleev’s classification is based on the atomic weight or mass of an element.

Atomic number of all periodic table elements

In learning chemistry, a good correlation between electromagnetic spectrum (X-ray) and atomic number indicated that an element is characterized by its atomic number, not atomic weight. The trends and properties like ionization energy, electron affinity, shielding effect of an atom are better to describe by its atomic number.

Moseley atomic number experiment

Rutherford model or alpha ray scattering experiments allow for the determination of the positive charge of the nucleus of an atom. The identification of the nuclear charge and the atomic number of an element established by electromagnetic spectrum data find from the x-ray discovered by Moseley.

  • When a beam of high-energy electrons falls on the metal target, the atom of the metal excited and emitted radiant energy.
  • Moseley experiment with over thirty metals from aluminum to gold.
  • Mosely plotted the square root of wavelength for each element against atomic weight.
  • A straight line plot was obtained but few elements are out of line.
  • A better plot was obtained when atomic weight is replaced by the order number or the serial number of the elements in the periodic table.

The most reasonable interpretation of the Mosely experiment, the atomic number of elements is equal to the number of protons in the atomic nucleus. To maintain electroneutrality, the atom must carry an equal number of extranuclear electrons.

Mass number of elements

The mass number or atomic mass number (symbol A) is the total number of protons and neutrons in an atomic nucleus. It is the nearest integer of atomic weight. If an element has mass number A and atomic number Z. The shortfall of (A − Z) determines the number of neutrons in the nucleus of an atom.

For example, the atomic number and mass number of hydrogen are equal to one. Therefore, the hydrogen atom does not carry any neutron inside the nucleus.

The atomic number of boron = 5 and mass number = 11. Therefore, the number of neutrons = (11 − 5) = 6.

Atomic mass of elements

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

On the physical scale of atomic mass or weight, 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

The exact number 16 as the relative mass of oxygen represents the new scale of atomic weights. In the chemical scale of the atomic weight or masses, the exact number 16 is taken as the reference.

Carbon-12 scale of atomic mass

The atomic mass of an element meaning the number that says how much heavy an atom of chemical elements is 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.

What is atomic mass unit?

The atomic mass unit represents by the symbol amu, which means exactly one-twelfth of the mass of the carbon-12 atom. Therefore, one amu is numerically equal to its atomic weight on the carbon-12 scale,
1 amu = (12/6.023) × (1/12) × 10−23 g
= 1.6603 × 10−24 g

What is molecular mass?

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. When molecular mass expressed in gram is called gram molecular weight.

How to find atomic weight of an element?

Scientists Stas in chemistry uses silver and chlorine to find the atomic weight or mass of the chemical elements.

  • He has taken the known weight of pure silver heated with pure chlorine 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 AgCl and weighted.
  • He found that 35.45 g of chlorine combines with 107.94 g of silver to give 143.39 g of AgCl.
  • Therefore, the calculated atomic weight or mass of chlorine is equal to 35.45 and silver = 107.94.

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