Relative Atomic Mass Of Chlorine

1. Making sense of measurements with ratios

Chlorine is a chemical element with atomic number 17 which means there are 17 protons and 17 electrons in the atomic structure. The chemical symbol for Chlorine is Cl. Atomic Mass of Chlorine Atomic mass of Chlorine is 35.453 u. Isotope abundances of chlorine. In the above, the most intense ion is set to 100% since this corresponds best to the output from a mass spectrometer. This is not to be confused with the relative percentage isotope abundances which totals 100% for all the naturally occurring isotopes.

What is the relative atomic mass of chlorine

A cup of milk tea with pearls gives you 342 kcal of energy. What does this number mean? The paranoid will find it worrying, since it is a three-figure sum. The delusional BBT lovers will say that it’s less than a thousand and so it’s nothing much.

Alternatively, we can make sense of the number by thinking about it as a ratio. Comparing milk tea to a slice of bread, one cup gives you 5 slices’ worth of energy. On the other hand, a large packet of fries from McDonald’s gives you 7 slices’ worth of energy. Now I am worried.

2. Making sense of the mass of an atom with the ratio: relative atomic mass

The mass of an atom is so small that it is mind boggling to express it in grams. The mass of a hydrogen atom is about 1.67×10^-30 g.

Instead, to describe the mass of a single atom, scientists use the relative atomic mass, which is a ratio. Like how we compared milk tea to bread slices, we compare an atom to a 1/12th “slice” of a carbon-12 atom. For example, a hydrogen atom has 1 slice’s worth of mass, a carbon atom 12 slices’ worth, and a magnesium atom 24 slices’ worth. Therefore, we say that hydrogen, carbon, and magnesium have a relative atomic mass of 1, 12, and 24 respectively.

Note that the relative atomic mass has no unit, as it is a ratio.

Relative atomic mass, A_r is defined as the ratio of the mass of an atom compared to 1/12th of the mass of an atom of carbon-12.

3. Relating relative atomic mass to the structure of an atom

The nucleus of carbon-12 has 12 nucleons, comprising 6 protons and 6 neutrons. Thus, 1/12th of its mass is about the mass of 1 nucleon. This is why hydrogen — with only 1 nucleon — has a relative atomic mass of 1.

Simply put, the relative atomic mass of an atom is equal to the number of nucleons it has.

4. The periodic table, your enabler

Good news! The relative atomic mass of every element is given in the periodic table. Just don’t mix up the proton (atomic) number with the relative atomic mass. Refer to the key to know which is which.

5. Relative atomic mass of an element: not your average kind of average

If you inspect your periodic table hard enough, you will realise that chlorine is an odd ball. It is the only element in the O-Level periodic table with a relative atomic mass that is not a whole number. This is because relative atomic mass is a sneaky concept. It is actually an average, though it does not explicitly tell you so.

In a sample of chlorine, most atoms exist as the chlorine-35 isotopes. However, a significant portion of 25% exist as the chlorine-37 isotopes. Therefore, the relative atomic mass of chlorine is the average of the relative molecular mass of all the chlorine atoms, be it chlorine-35 or chlorine-37.

In fact, for the other elements, their relative atomic mass only appears to be a whole number because they are rounded offto the nearest 0.5 in the O-Level periodic table. Referring to the A-Level periodic table, the relative atomic mass is more precise as it is rounded to the nearest 0.1. This shows that all elements have different isotopes. Their relative atomic mass is the weighted average of that of each isotope.

6. Relative molecular mass

A molecule is a chemical combination of atoms. In a similar vein, the relative molecular mass is simply the sum of the relative atomic mass of each of every atom in a molecule.

Relative molecular mass, M_r of XY = A_r of X + A_r of Y

How To Calculate Relative Atomic Mass Of Chlorine

Let’s try to find the relative molecular mass of a molecule of sucrose, C₁₂H₂₂O₁₁.