Atomic mass in periodic table is equal to ‘atomic weight’ or ‘mass number’. Its unit is called atomic mass units (amu).
Atomic number in periodic table is equal to ‘electron number’ on the orbits or ‘proton number’.
For example:
Carbon twelve (中文称为碳12) has 12 amu, 6 atomic number or electron or proton.
Iron (铁原子) has about 56 amu (exactly 55.845 amu), 26 atomic number or electron or proton.
Average atomic mass = % of 1st atomic mass + % of 2nd atomic mass
For example:
When a sample of natural copper is vaporised and injected into a mass spectrometer, the results shown in Fig. 3.3 are obtained. Use these data to compute the average mass of natural copper. (The mass values for 63Cu and 65Cu are 62.93 amu and 64.93 amu, respectively.)
3.3图告诉我们every 100 atoms of natural copper里，69.09 are 63Cu and 30.91 are 65Cu:
(69.09 atoms)*(62.93 amu/atom) + (30.91 atoms)*(64.93 amu/atom) = 6355 amu
The average mass of a copper atom is:
6355 amu/100 atoms = 63.55 amu/atom
大家发现在运算中使用单位是非常便捷的方式。