Chalk−1

Chemistry · General chemistry I · Worked example

Read an isotope label in a structure

Acetic acid can be made with carbon-13 in its methyl group, written [13CH3]C(=O)O as a SMILES string. How many neutrons does the labeled carbon have, what stays the same, and what does Chalk report for the molecule?

Read the label

¹³C is carbon with mass number 13. Carbon’s atomic number is 6, so the labeled atom has 13−6=7 neutrons, one more than carbon-12.

13−6=7

See what does not change

The label changes no bond and no charge. The molecule is still acetic acid, with the same atoms joined the same way and the formula C2H4O2.

See what does change

The labeled molecule is heavier than one with carbon-12 there by about one atomic mass unit, the extra neutron. Ordinary acetic acid has a molar mass of 60.05 g/⁢mol, which is already an average, because about 1⁢% of natural carbon is carbon-13.

What Chalk reports

Chalk draws the structure with the mass number on the carbon and keeps the formula C2H4O2. For the molar mass it counts the labeled carbon at carbon-13’s own mass, 13.00335 u, from NIST’s table of isotopic masses. That replaces carbon’s average 12.01 u, and every other atom keeps its usual atomic mass. Analyze → Composition says: “61.05 g/mol for the labeled molecule (¹³C at its isotopic mass)”, against 60.05 g/⁢mol for ordinary acetic acid. Chalk writes no name for a labeled structure.

Compare with the unlabeled molecule

Draw ordinary acetic acid beside it and choose Analyze → Compare. Chalk reports: “Isotopologues: the same atoms joined the same way, with different isotopes.”

Result

7 neutrons. The bonds and the formula are unchanged and the molecule is about one unit heavier. Chalk draws the label, reports 61.05 g/⁢mol for the labeled molecule, and writes no name.

Your turn

1. One like the example

How many protons, neutrons and electrons are in a neutral atom of carbon-14?

Show the answer and explanation

6 protons, 8 neutrons and 6 electrons.

Carbon’s atomic number is 6, so there are 6 protons, and a neutral atom has as many electrons as protons. The mass number 14 counts protons plus neutrons, so there are 14−6=8 neutrons.

N=14−6N=8

2. A variation

How many protons, neutrons and electrons are in a chloride ion made from chlorine-37, ³⁷Cl⁻?

Show the answer and explanation

17 protons, 20 neutrons and 18 electrons.

Chlorine’s atomic number is 17, so there are 17 protons, and 37−17=20 neutrons. The charge of 1− means one electron more than the protons, 18. The charge changes the electrons, never the protons or neutrons.

N=37−17N=20

3. Somewhere new

Heavy water is ²H₂¹⁶O, with deuterium in place of both hydrogens. How many neutrons does one molecule have, and how many does ordinary ¹H₂¹⁶O have?

Show the answer and explanation

10 neutrons, against 8 in ordinary water.

Each deuterium, ²H, has 2−1=1 neutron, and oxygen-16 has 16−8=8. One molecule of heavy water has 2⁢(1)+8=10. Ordinary hydrogen has no neutron, so ordinary water has only the 8 in its oxygen. The two extra neutrons are why heavy water is about 2 g/⁢mol heavier.

N=2⁢(2−1)+(16−8)N=10

Keep exploring

Import [2H]O[2H], heavy water, the same way: Chalk draws it with the mass number 2 on each hydrogen, and reports 20.03 g/⁢mol for the labeled molecule, about 2 g/⁢mol more than ordinary water. Typing D2O instead is refused, because D is not a SMILES atom, and Chalk asks for [2H].

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Study it, then try it

Practice the key terms as flashcards, then open the example in its tool, change a value and keep a useful result on your board.

Study the 7 key terms Draw ¹³C-labeled acetic acid in the Skeletal tool Draw ordinary acetic acid beside it See the labeled molecule in every representation Open worked example on a board Periodic table: atomic masses and isotopes

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