Density Curriculum

Section 3—Lesson 9: How Do Atomic and Molecular Bonds Contribute to Density?

Background Information

Atoms, Molecules, and Compounds

Before diving into this lesson, it may help to clarify some terms and relationships with your students.

Atoms are made up of protons, neutrons, and electrons. An atom is mostly empty space. Atoms can bond to other atoms by atomic bonds to create molecules. Atoms can also bond to other atoms to create compounds. When we refer to atomic bonds, we are referring to the bonds between atoms to create molecules or compounds, not what holds protons, neutrons and electrons together within atoms.

At the next level, molecules can bond to other molecules by molecular bonds. However, not all matter is made up of molecules. Atoms can also combine directly into compounds. A compound is made up of different elements where there are bonds between the atoms (atomic bonds), and the proportion of one element to the other(s) remains the same. For instance, table salt (NaCl) is made up of chlorine and sodium atoms. If you take a piece of it, it will always have the same ratio of sodium atoms to chlorine atoms, but it does not break down into molecules. Molecules can also form compounds, but not all matter consists of molecules.

The Strength of Atomic and Molecular Bonds

The second cause of density also involves imagining that you are zooming in to the micro-level. It has to do with how atoms are bonded (with atomic bonds) to other atoms (either the same type or different types) to create molecules of pure substances or compounds. It also has to do with how molecules are bonded (with molecular bonds) to other molecules. (For a more complete description of atoms, molecules, pure substances, and compounds, see the Points of Clarification section, below.) Some atoms are bonded to other atoms very closely (such as in a metal). Other atoms are bonded loosely and there is more space between the atoms, and thus fewer atoms are packed into a given amount of space. The same thing goes for the molecular bonds. With tighter or stronger bonds, there are more molecules, and therefore more atoms, per unit of space. It is the strength of the bonds that counts—the bonds themselves do not contribute mass or matter because they are not things (they are electrical attraction).

The Structure of Atomic and Molecular Bonds

Density is also affected by how atoms are bonded to each other to form molecules or compounds. The electrical charge of an atom impacts how other atoms are able to bond with it. For instance, the atoms in a water molecule bond in a structure that resembles mouse ears.

Similarly, when molecules are bonded to other molecules, there are certain ways that they can come together. For instance, when two water molecules bond together, the "mouse ear" structures^* can only connect in certain ways.

*This "mouse ear" structure is also known as "polar" structure. The polarity of molecules gives water some special physical and chemical properties, most of which are outside the scope of this module.

Atomic and Molecular Bonds in Solids

In a solid, how atoms are bonded to form molecules or compounds contributes significantly to density. Different atoms can be bonded in very different ways to create different substances. For instance, graphite and diamond are both made up of only carbon atoms. However, how the carbon atoms are bonded to each other is quite different (and corresponds with differences in density). The same thing holds for how molecules are bonded to other molecules.

As students will learn in Lesson 12, when solids are heated, the mass of their atoms doesn't change; instead, the bonds vibrate (to use an analogy) like a spring, and can get as far apart as the "springy-ness" of the bonds can stretch.

This lesson also raises the question of whether air can exist between bonds. There is no air in between atomic bonds (or in the atoms themselves, for that matter.) Except for the monatomic gases or "noble gases" (such as helium and neon) all other gases form molecules consisting of at least two atoms (diatomic).

These molecules would not fit in the space between the atomic bonds of any substance. Even the smallest atom that can travel alone, helium, probably wouldn't fit. The diameter of a helium atom is approximately 0.98 angstroms. Most atoms range from between 1.0 to 2.4 angstroms in diameter. Realize too, that the space between two bonded atoms is smaller than the space between two packed atoms (sitting next to each other). This is because they share an electron in their outer shell. The length of bonds varies, but for example, the average bond length between two hydrogen atoms is 0.74 angstroms. Atomic bonds appear to range from approximately 0.74 to 2.54 angstroms. In addition to it being unlikely that an atom or molecule would fit between the bonds, there are various chemical reasons why atoms bond or push apart. Therefore, even if an atom could fit within a bond, it is unlikely that it would pack there.

Molecules are collections of atoms that are held together by covalent bonds. Again, given the ways that molecules bond, even if there were enough space in between the bonds for the molecules (and a few monatomic atoms) to fit there, it is unlikely that they would pack into those spaces.

Atomic and Molecular Bonds in Liquids

Scientists don't understand the bonds in liquids very well and they are studying them to try to understand them better. However, they do know that there are different amounts of space between the bonds of different liquids.

When liquids are heated, the mass of the atoms in the liquid doesn't change. Scientists aren't sure how to think about what happens to the bonds, but we know that the atoms vibrate and therefore can get further apart. If there is air in between the atoms or molecules, such as in carbonated soda or in water when it boils, it is considered a case of mixed density.

Liquids are not necessarily denser than solids. Most of us think of water when we are told to think of a liquid. However, if we compare water in its liquid state to water in its solid state and extend that reasoning to make assumptions about the densities of liquids as compared to solids, it will lead to misconceptions. Water is really different from other liquids. Ask yourself, "Does ice sink or float in water?" It floats. This means that it is less dense ("more spread out") than in its liquid form. Why? When water crystallizes, it adopts a structure that incorporates little pockets of empty space, like a honeycomb. When the honeycomb melts the little pockets collapse and the liquid occupies less volume. The little pockets are full of vacuum, not air (a common misconception) although most ice does trap little bubbles of air when it freezes. This is different from most other substances, which do not adopt a honeycomb structure when moving from a liquid to a solid.

Atomic and Molecular Bonds in Gases

For gases, the major source of density is how spread out the atoms or molecules are (even though their atoms weigh different amounts and some of them are more than one atom bonded together). The important difference is how far the atoms are spread out (typically, there are other gases in between them). This difference is so much larger than the differences between the bonds of the particular atoms (when there are bonds—helium is individual atoms) or molecules that the strength of the bonds is inconsequential. The effect is outweighed.

However, when you compare different gases, the difference in the mass of their atoms helps to determine which gases sink in relation to others. When a gas is spread out there can be other kinds of atoms and molecules in between—such as nitrogen, oxygen, water, etc. Therefore, they are considered in this module to be a form of mixed density.

Gases can expand a lot because they typically consist of lone, unbonded atoms or molecules. Gases typically do not have bonds between individual atoms or molecules, which allows them to spread out much farther than those of a liquid or solid. How close together or spread out the atoms or molecules are determines the density of a gas to a large extent, because the mass of the individual atoms of molecules doesn't change, while the amount of volume they take up can change dramatically.

Points of Clarification

Students tend to be confused about whether or not air plays a role in density. In order to help students see that in many cases, air or the molecules that make up the air have no role in density; this curriculum presents two causes (atomic mass and atomic or molecular bonds) where "air molecules" do not make any real contribution to differences in density.

The third cause (introduced in Lesson 10) focuses on mixed density. Here, "air molecules" can contribute to overall density. Mixed density of an object refers to instances when the total density includes the density of more than one material or substance.

The three causes are, in a sense, a simplification. Molecular bonds are grouped with atomic bonds to make it easier for students to reason about the three causes. However, there are instances where the causes overlap. For instance, the structure of molecules affects how spread out they are, and in the case of the molecules in many plastics (polymers), they are long and curly. When they fit next to each other there can be spaces (with vacuum, gases or liquids in them). These can be cases of mixed density. In other cases, molecules or atoms are spread out, but it is not due to mixed density. Instead of air in between the molecules or atoms, there is simply space.

With older students, you may wish to raise some of these puzzles after they have learned the three causes of density. This will help them to see that the three causes are a simplification. The "causes" are tools for analysis, but need to be understood in terms of the perspective that they present. Teachers can decide whether to gloss over these fine distinctions or to help students understand them, depending upon the particular learning needs of the group and curriculum objectives.