Density of Earth:

 The density of the Earth is 5.51 grams per cubic centimeter. This is an average, of course. If you could mix up the Earth so all of its mass was evenly distributed, and then took a 1 cm cube and put it on the scale, it would weigh 5.51 grams. This makes Earth the densest planet in the Solar System.The surface of the Earth is mostly water and rock, so scientists have long suspected that our planet's high density comes from large quantities of iron in the core. This suspicion was confirmed in 1936 by seismologist Inge Lehman, who was measuring how seismic waves passed through the Earth. He detected that waves bounced off the solid iron core of the Earth.

Earth is the densest planet in the Solar System, and tiny Mercury, at 5.43 g/cm³. One interesting fact is that Mercury would actually be denser than Earth; however, our planet has much more mass, and so it compacts itself more tightly than Mercury. If Mercury and Earth were the same size, Mercury would be denser. All the numbers that were provided are so close to each other, however, that they can each be considered valid. Some assorted numbers given would be: 5.5, 5.52, and 5.15 g/cm³ Density is found by dividing the mass by the volume  ρ = mv . A scientist named Henry Cavendish assembled an apparatus that consisted of a suspended metal rod with two lead balls hanging from it. He placed masses of metal near these balls in order to measure the force of attraction between them. Correspondingly, he could then find the attraction on a mass the size of the Earth and then determine its density.

The mass of the Earth is found to be 6 sextillion, 587 quintillion short tons (or 5.98 * 10²¹ metric tons). Since the Earth is a sphere, the formula 4/3π r³ is used to find the volume. The volume of the Earth is considered to be 1.08 * 10¹² km³ (or 2.5988 * 10¹¹ miles³).Since the Earth's mass is so great, it makes a gravity that compresses it more than every other kind of inner planet.