Atomic mass
Atomic mass is not the same as mass number. The mass number of an atom is a whole number, and is obtained by adding the number of protons and neutrons in the nucleus of the atom. Since protons and neutrons are the heaviest of the three types of subatomic particles, the mass number of an atom is very close to the mass of the atom.
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Atomic mass unit (amu)
The atomic mass of an atom is often expressed using atomic mass unit (amu).
Definition
One atomic mass unit is by definition one twelfth of the mass of a carbon-12 (12C) atom. It is also the approximate mass of a hydrogen atom, a proton or a neutron. In other words, a single atom of carbon-12 has a mass of 12 amu.
Since atomic mass unit is a mass unit, it can be converted to the more familiar SI unit of gram. Avogadro's number is the conversion factor between atomic mass unit (amu) and the gram (g).
- 6.022 x 1023 amu = 1 gram
- or, inversely,
- 1 amu = 1.661 x 10-24 gram
Atomic mass unit is used to describe the mass of very tiny objects like an atom.
- the atomic mass of ONE carbon atom = 12.011 amu
12.011 amu = 0.00000000000000000000001995 gram, or 1.995 x 10-23. It's not very heavy!
- the atomic mass of ONE magnesium atom = 24.3050 amu
24.3050 amu = 0.00000000000000000000004037 gram or 4.037 x 10-23. This is still not very heavy.
- the atomic mass of ONE copper atom= 63.546 amu ..... etc ...
63.546 amu = 0.0000000000000000000001055 g or 1.055 x 10-22. This is not an amount that can be weighed.
Atomic mass of elements 1 to 10
The periodic table of elements shows the atomic mass of each element. Note that atomic mass is NOT a whole number! Take a look at the periodic table closely to verify the atomic mass for the first 10 elements.
Each element's atomic mass is given in these units: amu/atom or gram/mole.
Atomic mass of hydrogen:
1.008 amu/atom or
1.008 gram/mole
Atomic mass of helium:
4.003 amu/atom or
4.003gram/mole
Atomic mass of lithium:
6.941 amu/atom or
6.941 gram/mole
Atomic mass of beryllium:
9.012 amu/atom or
9.012 gram/mole
Atomic mass of boron:
10.811 amu/atom or
10.811 gram/mole
Atomic mass of carbon:
12.011 amu/atom or
12.011 gram/mole
Atomic mass of nitrogen:
14.007 amu/atom or
14.007 gram/mole
Atomic mass of oxygen:
15.999 amu/atom or
15.999 gram/mole
Atomic mass of fluorine:
18.998 amu/atom or
18.998 gram/mole
Atomic mass of neon:
20.179 amu/atom or
20.179 gram/mole
Conversion between amu and gram
To express mass given in atomic mass unit (amu) in grams, take the mass in amu and divide by 6.022 x 1023 amu/gram.
It's pretty obvious why the unit of grams is a bit cumbersome to express the mass of a tiny object like an atom.
Content suitability
BCIT courses: CHEM 0011
