chem 高手入黎..

(1)首先，給你一個relative atomic mass 的定義，relative atomic mass is the weighed mean of the isotopic mass of its naturally occurring isotope. 這句指出relative atomic mass 是所有該元素在自然出現的同位素的平均值．由於是平均值，所以relative atomic mass usually not whole number．
(2) electronic configuration after 20(calcium),仍然是用2n2這式,只是其electronic configuration 是2,8,18,32,18.即是第三層電殼最多只填入18粒電子,而第四層電殼最多只填入32粒電子,但第五層電殼和第三層電殼一樣,只可填入最多18粒電子.
(3)relative isotopic mass is the relative mass of the isotope. Hence relative isotopic mass is nearly equal the mass number.
I hope you will find them helpful.