The mass of an atom is very small, compared to a kilogram; for example, the mass of a carbon atom, `text()^(12)C`, is `1.992647 xx 10^(-26) kg`. Kilogram is not a very convenient unit to measure such small quantities. Therefore, a different mass unit is used for expressing atomic masses. This unit is the atomic mass unit (amu), defined as `1//12^(th)` of the mass of the carbon (`text()^(12)C`) atom. According to this definition
`1text(amu)=(text(mass of one) text()^(12)C text(atom))/(12)`
`= 1.660539 xx 10^(−27) kg`
The atomic masses of various elements expressed in atomic mass unit (amu) are close to being integral multiples of the mass of a hydrogen atom. There are, however, many striking exceptions to this rule. For example, the atomic mass of chlorine atom is `35.46 text(amu)`.
Accurate measurement of atomic masses is carried out with a mass spectrometer, The measurement of atomic masses reveals the existence of different types of atoms of the same element, which exhibit the same chemical properties, but differ in mass. Such atomic species of the same element differing in mass are called isotopes. (In Greek, isotope means the same place, i.e. they occur in the same place in the periodic table of elements.) It was found that practically every element consists of a mixture of several isotopes. The relative abundance of different isotopes differs from element to element. Chlorine, for example, has two isotopes having masses `34.98text(amu)` and `36.98 text(amu)`, which are nearly integral multiples of the mass of a hydrogen atom. The relative abundances of these isotopes are `75.4` and `24.6` per cent, respectively. Thus, the average mass of a chlorine atom is obtained by the weighted average of the masses of the two isotopes, which works out to be
`=(75.4xx34.98 + 24.6xx36.98)/(100)`
`=35.47text(amu)`
which agrees with the atomic mass of chlorine.
The mass of an atom is very small, compared to a kilogram; for example, the mass of a carbon atom, `text()^(12)C`, is `1.992647 xx 10^(-26) kg`. Kilogram is not a very convenient unit to measure such small quantities. Therefore, a different mass unit is used for expressing atomic masses. This unit is the atomic mass unit (amu), defined as `1//12^(th)` of the mass of the carbon (`text()^(12)C`) atom. According to this definition
`1text(amu)=(text(mass of one) text()^(12)C text(atom))/(12)`
`= 1.660539 xx 10^(−27) kg`
The atomic masses of various elements expressed in atomic mass unit (amu) are close to being integral multiples of the mass of a hydrogen atom. There are, however, many striking exceptions to this rule. For example, the atomic mass of chlorine atom is `35.46 text(amu)`.
Accurate measurement of atomic masses is carried out with a mass spectrometer, The measurement of atomic masses reveals the existence of different types of atoms of the same element, which exhibit the same chemical properties, but differ in mass. Such atomic species of the same element differing in mass are called isotopes. (In Greek, isotope means the same place, i.e. they occur in the same place in the periodic table of elements.) It was found that practically every element consists of a mixture of several isotopes. The relative abundance of different isotopes differs from element to element. Chlorine, for example, has two isotopes having masses `34.98text(amu)` and `36.98 text(amu)`, which are nearly integral multiples of the mass of a hydrogen atom. The relative abundances of these isotopes are `75.4` and `24.6` per cent, respectively. Thus, the average mass of a chlorine atom is obtained by the weighted average of the masses of the two isotopes, which works out to be
`=(75.4xx34.98 + 24.6xx36.98)/(100)`
`=35.47text(amu)`
which agrees with the atomic mass of chlorine.