| Atomic and Molecular Weights | |||||||||||||||||||||||||||
Each atomic symbol and chemical formula not just represent an element or a compound. They also contain other meaningful quantities -- the atomic and molecular weights.
Atomic Mass Unit (amu)Atoms weigh very very light. Scientists before were able to compare the relative of oxygen compared to hydrogen. They discovered that each 100 grams of water contains about 88.9 grams of oxygen and 11.1 grams of hydrogen. Thus, dividing 88.9/11.1 gives you approximately 8. They concluded that oxygen contains 8 times as massive as hydrogen in water. Sooner they found out that there are actually 2 hydrogen atoms in one molecule of water! So, they say claim that oxygen atom weighs 16 times (8 x 2) as much as hydrogen atom.Naturally, hydrogen being the lightest of all elements became the basis for this so called atomic weights. The atomic weights are relative weights compared to the standard weight of a model atom. As time progresses, capabilities of measuring exact atomic weights were already around. Scientists now were able to find what really is the weight of one oxygen atom and one hydrogen atom: O weighs 2.6560 x 10-23 grams Since the weights were very small, scientists coined the atomic mass unit(amu). 1 amu = 1.66054 x 10-24 grams Using the above as a conversion factor, we can find what is the weight of 1 hydrogen atom in terms of atomic mass unit: (1.6735 grams x 10-24) (1 amu)/(1.66054 x 10-24 grams) = 1.0078 amu Scientists decided to standardized atomic weights by basing them to the Carbon-12 isotope which they assigned an exact weight of 12 amu.
Average Atomic Mass (Atomic Weight)What we now see in periodic tables are relative weights of each element compared to C-12 isotope. But, the weights are actually average atomic masses of all known isotopes of that element. For instance a certain element X has 3 isotopes A, B and C in existence. A (15 amu) is 20% in abundance (i.e. 20% of it occurs naturally), B (14.5 amu) is 50% and C(14.7 amu) exists as 30%. The atomic weight that you will see for X in the periodic table is computed as:15 amu (0.2) + 14.5 amu (0.5) + 14.7 amu (0.3) = 14.66 amu Thus, atomic weights are average atomic masses of all known isotopes in account of their relative abundances. Try this exercise: Carbon occurs in two different isotopes, the C-12 and C-13. C-12 is exactly 12 amu, while C-13 is 13.00335 amu. If the relative abundance for C-12 and C-13 are 98.892% and 1.108% respectively, find the atomic weight for carbon. Click here for the answer. By now, you should be thinking of atomic weights in periodic table in terms of the other isotopes of the element. Let's try another exercise: Chlorine has two naturally occuring isotopes, the 35Cl and the 37Cl. Cl-35 weighs 34.969 amu and has a relative abundance of 75.53% while Cl-37 weighs 36.966 amu and occurs 24.47 percent in nature. Find the atomic weight for Cl. The answer is here.
Formula and Molecular WeightsTaking the formula weight is easy. Just sum up all the atomic weights for each atom in a chemical formula. What is the difference between formula weight and molecular weight? Not much. However, molecular weight refers to molecular formulas, that is, it is a special term for the formula weight of a molecular compound such as Diphosphorus pentoxide (P2O5). It is therefore not right when you speak of the molecular weight of Sodium chloride (NaCl) because it is not a molecular compound, but an ionic compound whose structure is a three-dimensional crystal consisting of sodium ions (Na+) and chloride ions (Cl-). Thus, we should speak of the formula weight of Sodium chloride not of its molecular weight.Let's have an example. Let's take Calcium chlorate, Ca(ClO3)2. Here is how it is computed:
Now, try this yourself: Find the molecular weight of glucose, C6H12O6. Use the following atomic weights: C = 12.0 amu, H = 1.0 amu and O = 16.0 amu. Solve it and check your answer here.
Percentage by MassA compound is composed of atoms. Each atom contributes to the formula weight of a compound. Thus, in water for instance, O atom contributes a mass of 16.0 amu while H atoms make up the remaining 2.0 amu in order to have 18.0 amu, the molecular weight of H2O. We can therefore, calculate the percentage by mass of each atom as follow:
The above shows the percentage composition of water. You're turn. Try solving this exercise: Find the percentage by mass of sulfur in H2SO4. Use the following atomic weights: H= 1.0 amu, S = 32.1 amu, O= 16.0 amu.. As usual here is the answer. |
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