Gram Atomic Mass | Definition, History, Examples

Definition of Gram Atomic Mass

The quantity of an element whose mass expressed in grams is numerically equal to its atomic mass is known as gram atomic mass.

gram atomic mass

Gram atomic mass is the mass of one mole of an element. It is determined by taking the atomic weight for an element on the periodic table and expressing it in grams.

Other Definitions

  1. Gram atomic mass(weight) of an element is the mass of Avogadro number ( 6.023 x 1023) of atoms of that element in grams.”
  2. Grams molecular mass is the mass in grams of one mole of a molecular substance. Gram molecular mass is the same as molar mass. The only difference is that gram molecular mass specifies the mass unit to be used. Gram molecular mass may be reported in grams or grams per mole (g/mol).
  3. The amount of a substance whose mass in gram is numerically equal to its atomic mass is called the gram atomic of that substance. The gram atomic mass of a substance represents the mass of 1 mole of atoms (6.022 × 1023) of that substance. The molar mass of an element is the mass of 1 mole of its atoms. The molar mass of an element has (6.022 × 1023) atoms of the element in it.
  4. The grams atomic mass is another term for the mass, in grams, of one mole of atoms of that element. “Gram atom” is a former term for a mole. … This is a dimensionless quantity (i.e., a pure number, without units) equal to the molar mass divided by the molar mass constant.

History of Gram Atomic Mass

The first scientists to determine relative atomic masses were John Dalton and Thomas Thomson between 1803 and 1805 and Jon’s Jakob Berzelius between 1808 and 1826.

Relative atomic mass (Atomic weight) was initially defined relative to that of the lightest element, hydrogen, which was taken as 1.00, and in the 1820s, Prout’s hypothesis stated that atomic masses of all elements would prove to be exact multiples of that of hydrogen.

However, Berzelius soon proved that this was not even approximately true. And for some elements, such as chlorine, relative atomic mass, at about 35.5, falls almost halfway between two integral multiples of that of hydrogen.

Still later, this was shown to be largely due to a mix of isotopes and that the atomic masses of pure isotopes, or nuclides, are multiples of the hydrogen mass, to within about 1%. In the 1860s, Stanislao Cannizzaro refined relative atomic masses by applying Avogadro’s law (notably at the Karlsruhe Congress of 1860).

He formulated a law to determine relative atomic masses of elements: the different quantities of the same elements contained in different molecules are all whole multiples of the atomic weight and determined relative atomic masses and molecular masses by comparing the vapor density of a collection of gases with molecules containing one or more of the chemical element in question.

How to Find Gram Atomic Mass?

Use the molecular formula to calculate mass

  1. Look up the relative atomic mass of each element in the formula.
  2. Multiply the subscript after each element symbol (the number of atoms) by that element’s atomic mass. If there is no subscript, there is only one atom of that element in the molecule.
  3. Add all of the values together to find the gram molecular mass.


examples of gram atomic mass

Here are some examples of grams atomic mass

  • mass of 1 atom of oxygen = 16 amu = 16 x 1.66 x 10-24 gm
    Mass of 6.023 x 1023 atoms of oxygen= 16 x 6.023 x 1023 x 1.66 x 10-24 = 16 gm.
    Hence, Gram atomic weight of oxygen = 16 gm.
  • Sodium (Na) has an atomic weight of 22.99 u, so it has a gram atomic mass of 22.99 grams. So one mole of sodium atoms has a mass of 22.99 g.
  • The molecular mass of N2 is 28, so the gram molecular mass of N2 is 28 g.
  • Mass of CO2 is 44 amu. The mass of CO2 in grams is 44 g. It is also called the molar mass of molecule.
  • The gram atomic mass of a hydrogen atom is 1.008 g.

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