3.6:
Atomic Weight
An element's atomic weight, sometimes called atomic mass, is approximately equal to the sum of the number of protons and neutrons. This is expressed in atomic mass units, abbreviated as amu, or Daltons, and is represented near its chemical symbol on the periodic table.
But, most elements have several isotopes that differ in the number of neutrons and so in their masses.
For such elements, the atomic weight is the average mass of all of the isotopes of an element, weighted according to their relative abundance or the percentage of each isotope found in nature.
For instance, chlorine has two naturally occurring isotopes. The mass of chlorine-35 is 34.97 amu and of chlorine-37 is 36.97 amu.
When multiplied by their relative abundances, 75.77% for chlorine-35 and 24.23% for chlorine-37, and added up, the final atomic weight of 35.45 amu is obtained.
Since chlorine-35 is more abundant in nature than chlorine-37, the atomic weight of chlorine is close to 35 amu.
3.6:
Atomic Weight
Protons and neutrons have approximately the same mass, about 1.67 × 10-24 grams. Scientists arbitrarily define this amount of mass as one atomic mass unit (amu) or one Dalton. Electrons are much smaller in mass than protons, weighing only 9.11 × 10-28 grams, or about 1/1800 of an atomic mass unit. As a result, they do not contribute much to an element's overall atomic mass. This means that, when considering atomic mass, it is customary to ignore the mass of any electrons and calculate the atom's mass based on the number of protons and neutrons alone.
However, since most naturally occurring elements are composed of isotopes, it is important to consider the mass and natural abundance of isotopes when determining the atomic mass of such elements. The atomic mass in such cases is calculated by summing the masses of all the element's isotopes, each multiplied by their natural fractional abundance.
Chemists often use the unit "mole" to determine the number of atoms of a compound that participate in a chemical reaction. A mole of an element is its atomic weight in grams, while a mole of a compound is the sum of the atomic weights of its components, called the molecular weight. An often-used example is calculating a mole of glucose with the chemical formula C6H12O6. The atomic weight of carbon (C) is 12.011 grams, and there are six carbons in glucose for a total atomic weight of 72.066 grams. Doing the same calculations for hydrogen (H) and oxygen (O), the molecular weight of glucose equals 180.156 grams.
This text is partially adapted from Openstax, Biology for AP® courses. Section 2.1 Atoms, Isotopes, Ions, and Molecules: The Building Blocks and Openstax, Anatomy and Physiology 2e, Section 2.4 Inorganic compounds Essential to Human Functioning