Relative atomic mass is a term used to describe the average mass of an atom of an element, relative to the mass of an atom of carbon-12, which is used as the standard reference. It is represented by the symbol Ar and is given in atomic mass units (amu).
Average Mass of One Atom of the Element
\({A_r} = \frac{{\text{Average Mass of One Atom of the Element}}}{{\frac{1}{12} (\text{The Mass of One Atom of Carbon} – 12)}}
\)
1/12 x The Mass of One Atom of Carbon – 12
The unit of relative atomic mass is the atomic mass unit, with the symbol a.m.u.
1 a.m.u = 1.66 x 10-24 gram
Compounds are represented by chemical formulas, just as elements are represented by symbols that respect their valencies. There are two types of chemical formulas: empirical formulas and molecular formulas.
The formula that displays the minimum relative numbers of each type of atom in a molecule is known as the Empirical Formula. The Empirical Formula illustrates the simplest ratio of each atom that is present in a molecule, but it does not indicate the exact number of atoms in the molecule. Additionally, the Empirical Formula provides information on the types of elements present in the molecule.
The molecular formula displays the actual number of atoms of each element that are present in a molecule. This formula is obtained from the empirical formula. The molecular formula mass is calculated by adding the atomic weights of all the atoms in the molecule. Additionally, the molecular formula of a compound may be the same as its empirical formula, or it may be a multiple of it.
The formula used to calculate the molecular formula of a compound is:
Molecular formula = (Empirical formula)n
where “n” is the integer value that represents the number of repeating units in the molecule. The value of “n” is determined by dividing the molecular mass of the compound by the empirical formula mass and rounding to the nearest whole number.
The table below shows the differences between molecular formula and empirical formula:
| Compound | Molecular Formula | Empirical Formula |
|---|---|---|
| Glucose | C6H12O6 | CH2O |
| Ethanol | C2H6O | CH3O |
| Aspirin | C9H8O4 | C9H8O4 |
| Methane | CH4 | CH4 |
| Sucrose | C12H22O11 | C12H22O11 |
The atomic number is the number of protons in the nucleus of an atom. Every element has a unique atomic number that determines its place in the periodic table. For example, carbon has an atomic number of 6, which means it has six protons in its nucleus. The atomic number also tells you the number of electrons in a neutral atom since the number of protons and electrons is equal.
The atomic number of an element is represented by the symbol “Z”. So, the formula for the atomic number of an element is:
Z = number of protons in the nucleus of an atom
For example, the atomic number of carbon is 6, which means that it has 6 protons in its nucleus.
The atomic mass is the mass of an atom, usually measured in atomic mass units (amu). The atomic mass includes the mass of the protons, neutrons, and electrons in an atom. Protons and neutrons have approximately the same mass and contribute most of the atomic mass, while electrons contribute very little to the mass of an atom. The atomic mass is not the same as the atomic number, although they are both used to identify an element. For example, carbon has an atomic mass of about 12 amu, which means the total mass of its protons, neutrons, and electrons is about 12 times the mass of a hydrogen atom.
Molecular mass is the mass of one molecule of a substance. It is calculated by adding up the atomic masses of all the atoms in a molecule. The molecular mass is expressed in atomic mass units (amu).
The atomic number of an element is represented by the symbol “Z”. So, the formula for the atomic number of an element is:
Z = number of protons in the nucleus of an atom
For example, the atomic number of carbon is 6, which means that it has 6 protons in its nucleus.
The sum of all atomic masses present in a molecule is known as molecular mass.
Formula mass is the mass of one formula unit of an ionic compound. It is calculated by adding up the atomic masses of all the atoms in the compound. The formula mass is also expressed in atomic mass units (amu).
The mathematical formula to calculate the formula mass of an ionic compound is:
Formula mass = sum of the atomic masses of all the atoms in the formula unit
For example, the formula mass of sodium chloride (NaCl) can be calculated as follows:
Formula mass of NaCl = (1 x atomic mass of sodium) + (1 x atomic mass of chlorine)
= (1 x 22.990 amu) + (1 x 35.453 amu) = 58.443 amu
Although molecular mass and formula mass are two different terms neverthless, the method of calculation is same of both terms.
The atomic mass, molecular mass and formulas mass of any substance expressed in grams is known as the one mole of that substance.
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