1. Isotopes:
* Atoms of the same element can have different numbers of neutrons. These variations are called isotopes. For example, Carbon-12 and Carbon-14 are both isotopes of carbon, with different numbers of neutrons.
* Each isotope has a specific mass.
2. Abundance:
* Isotopes occur in nature with different abundances. For example, Carbon-12 is much more abundant than Carbon-14.
3. Weighted Average:
* The atomic mass listed on the periodic table is the weighted average of the masses of all the isotopes. This means the mass of each isotope is multiplied by its natural abundance, and then all the products are added together.
Formula:
Atomic Mass = (Mass of Isotope 1 * Abundance of Isotope 1) + (Mass of Isotope 2 * Abundance of Isotope 2) + ...
Example:
Let's consider chlorine (Cl), which has two major isotopes:
* Chlorine-35: Mass = 34.9689 amu, Abundance = 75.77%
* Chlorine-37: Mass = 36.9659 amu, Abundance = 24.23%
Calculation:
Atomic Mass of Cl = (34.9689 amu * 0.7577) + (36.9659 amu * 0.2423)
= 26.4957 amu + 8.9568 amu
= 35.4525 amu
Therefore, the atomic mass of chlorine listed on the periodic table is approximately 35.45 amu.
Note:
* Atomic masses are usually expressed in atomic mass units (amu).
* The atomic masses listed on the periodic table are generally rounded to two or three decimal places.
* Some elements have no stable isotopes and therefore have no natural abundance. For these elements, the atomic mass is based on the longest-lived isotope.
