The density, , measures how compact an object is. That is, how much mass it contains per unit volume. For example,

To understand why the density tells us something about the chemical composition of an object, we need to consider the structure of an atom. An atom is bascially composed of three types of particles, positively charged particles known as protons, chargeless particles known as neutrons, and negatively charged particles known as electrons. The protons and neutrons are found in the center of the atom which is referred to as the nucleus and the electrons are found in a cloud surrounding the nucleus. A schematic representation of an atom is shown below:

The nucleus is tiny compared to the overall size of the atom. An atom is roughly 10**(-8) cm in diameter while the nucleus is only around 10**(-13) cm in diameter. That is, the nucleus accounts for around 10**(-15) of the volume of the atom. This is amazing since mass of the proton ~ mass of the neutron ~ 1,836 times the mass of an electron. Most of the mass of an atom is contained in the nucleus.

What are the implications of this? Well, let us consider what makes the various types of material (atoms) that we see in the Universe differ from one and another. That is, what makes hydrogen different from lead or iron?

The difference lies in the number of protons that each type of atom contains in its nucleus. Hydrogen is the simplest element and contains 1 proton in its nucleus. Helium is the next simplest element and contains 2 protons in its nucleus. See the Periodic Table on page 129-130 of the text for a complete listing of the elements. The element's type is determined by only the number of protons in the nucleus. It does not depend upon the number of neutrons in the nucleus. So what about atoms which contain different amounts of neutrons and electrons?

The density of an atom is determined by the type of element. If the atom contains many neutrons and protons packed into its nucleus, it will be dense. For example, a common isotope of Lead has 82 protons and 126 neutrons and a bar of lead is dense, it would have a of around 11 g cm. A lighter substance, for example, water (which is composed of 2 hydrogens -- 1 proton -- and oxygen -- 8 protons + 8 neutrons) has a compatively low density. Liquid water in this room would have a density of around 1 g cm.

Based on this idea, we see that the density of the planets tells us something about their chemical composition (types of atoms). However, if we used this argument indiscriminantly, we would infer that the Sun and the Jovian planets were made primarily of water. This is not true. The Sun and Jovian planets are hydrogen and helium. Why did the argument go awry?

Answer: Well, we left something out.

Question: What did we leave out?

We failed to take account of the mass of the object. What happens is this since gravity has the form, F = - GMm/R,

more massive objects, have stronger gravities. As a result, more massive objects get more compressed than less massive objects. This compression (decrease in the size of the object), means that the density of the object will be greater. Thus, even for two objects which have the same composition, if one is more massive, then the objects will have different densities. This effect needs to be accounted for when converting densities into chemical compositions. A quantity known as the

is defined which is free of this mass dependence. The uncompressed densities of Mercury, Venus, Earth, the Moon, and Mars are 5.3 g cm, 3.95 g cm, 4.03 g cm, 3.3 g cm, and 3.71 g cm, respectively. Mercury is significantly denser than the Earth.