Carbon can be burned in excess oxygen to form carbon dioxide according to the following equation: solid carbon plus O2 gas turns into CO2 gas. How many elements, atoms, and molecules are present in the equation?
In order to distinguish between elements, atoms, and molecules, let’s review some key definitions. A molecule is two or more atoms covalently bonded together. An element is a type of atom based on the number of protons. And we can think of atoms as the building blocks, the units of the different elements that make up the different molecules. By knowing these definitions, as well as understanding the notation of the chemical equation above, we can determine the number of elements, atoms, and molecules present.
First, let’s determine the number of molecules. Keeping in mind our definition of a molecule, that it’s two or more atoms covalently bonded together, we can see that CO2 or carbon dioxide is indeed a molecule as it contains one carbon atom covalently bonded to two oxygen atoms. A second molecule present is O2 or molecular oxygen. This fits our definition as well as there are two oxygen atoms covalently bonded together. Note that the two or more atoms that are covalently bonded together do not have to be of different types. You can have two or more atoms of the same element bonded together. And it counts as a molecule.
Lastly, the solid carbon is not considered a molecule because there is only one atom present. So as part of our final answer, we can say that there are two molecules, CO2 and O2. Next, let’s look at the number of atoms present in the equation. To start, how many atoms are there in the CO2 molecule? In the CO2 molecule, there are three atoms, one carbon atom bonded to two oxygen atoms. The little two next to the O implies that there are two oxygen atoms, whereas the absence of a number next to the C implies that there’s just one carbon atom. One carbon atom plus two oxygen atoms is three atoms total.
How many atoms are there on the left side of the equation? Well, there are two atoms in O2, two oxygen atoms, as denoted by the subscript two. And there’s a single carbon atom in the solid carbon. So in total, we have one plus two plus three equals six atoms. So part of our final answer will indicate that there are six atoms. Lastly, how many different elements are there in this equation, or how many different types of atoms are there in this equation? Another way of asking this question is to ask, how many different entries from the periodic table do we need to draw from in order to write out this equation?
Of the six atoms that we have in this equation, there are only two different kinds of atoms, carbon atoms and oxygen atoms. All of the carbon atoms have the same number of protons, six. And all of the oxygen atoms have the same number of protons, eight. If there were a third element present in the equation, we would see a third chemical symbol listed, for example, a capital N for nitrogen or a capital N followed by a lowercase “a” for sodium. But as the equation is written, there’s only two symbols, C for carbon and O for oxygen.
So our final answer can include two elements. So, after counting the number of groups of covalently bonded atoms, the number of building blocks, and the different types of atoms that are present, we can confidently say that our final answer is two elements, six atoms, and two molecules.