Carbon and oxygen are similar in a few ways. They are both elements, which are the fundamental building blocks of all matter. This means that they both have protons and electrons. They also both have neutrons. Because they are elements, they are both on the periodic table; carbon is number 6 on the periodic table and oxygen is number 8 on the periodic table.

Their weight, number of atoms and density depend on a lot of things. I am going to use the word "mass" to describe their weight even though these are different things. Weight can change depending on gravity. Mass just describes how much of "something" we have. So our "something" weighs more on Jupiter than on Earth, but the mass is the same.

I should also introduce "atomic weight", which is how many grams are in a mole of atoms. Atoms are really, really small. A single atom is over 100,000 times smaller than the thickness of a single strand of hair! That also means a single atom is not very heavy, and so we'd have to have a lot of atoms to give us 1 lb of carbon or oxygen. That's why we use the mole.

The mole is just a number that represents 6.02x10²³ (or 602 sextillion or 602,000,000,000,000,000,000,000). You could have a mole of tacos (if you had a mole of tacos, you could feed every person on Earth 1000 tacos per day for 23 million years); a mole of dollars (this would take 19 million years to spend even if you spent 1 billion dollars per second); or a mole of atoms. Carbon and oxygen have different atomic weights: 12 grams per mole for carbon and 16 grams per mole for oxygen.

So to answer the first part of your second question, the atomic mass of carbon and oxygen are different. This means that if you weighed 1 mole of carbon and 1 mole of oxygen, oxygen would have a higher weight. However, a pound of oxygen would have the same weight as a pound of carbon. The second part of your second question (the number of atoms in carbon and oxygen) depends on how much carbon and oxygen you have. As mentioned, if you had a mole of carbon and a mole of oxygen, you'd have the same number of atoms (1 mole). However, if you had 14 grams of carbon and 14 grams of oxygen, you'd have more than 1 mole of carbon and less than 1 mole of oxygen! So they would have a different number of atoms.

So to answer the first part of your second question, the atomic mass of carbon and oxygen are different. This means that if you weighed 1 mole of carbon and 1 mole of oxygen, oxygen would have a higher weight. However, a pound of oxygen would have the same weight as a pound of carbon. The second part of your second question (the number of atoms in carbon and oxygen) depends on how much carbon and oxygen you have. As mentioned, if you had a mole of carbon and a mole of oxygen, you'd have the same number of atoms (1 mole). However, if you had 10 grams of carbon and 10 grams of oxygen, you'd have more than 1 mole of carbon and less than 1 mole of oxygen! So they would have a different number of atoms.

The third part of your question is tricky. As it turns out, carbon and oxygen exist in many different forms. The oxygen that you breathe in is actually two oxygen atoms bonded together. Carbon commonly bonds with itself to form graphite or diamond. The density of gaseous oxygen is 1.429 g/L. The density of graphite is 2160 g/L. And the density of diamond is a whopping 3515 g/L. But this isn't really a fair comparison, since graphite and diamond are both solids, which are typically really dense compared to gasses. So if you make oxygen really cold (-218.79 oC or -361.82 oF, which is way colder than -89.2 oC or -128.6 oF measured in Antarctica), you can make solid oxygen. Solid oxygen's density is about 1500 g/L – still smaller than graphite or diamond.

Carbon and oxygen are somewhat close to each other on the periodic table, and they're both important elements that make up most of the human body and other life forms by mass. Carbon atoms have 6 protons while oxygen atoms have 8 protons, and it's the number of protons in an atom, also known as the atomic number, that determines which elements are which. They are both on the right side of the periodic table and are nonmetals (usually). An oxygen atom has more mass (weight) than a carbon atom because it has more protons and neutrons. Atoms are made up of protons and neutrons, which are heavy, and electrons, which are very light. And then molecules are made up of a bunch of atoms bonded together, possibly of different elements. When you're thinking of the number of atoms or density you're probably thinking in terms of molecules.

For example, one very common gas that only contains oxygen atoms is a molecule made up of two oxygen atoms, called O₂. Graphite, used in pencil leads, is made up of all carbon atoms. Carbon dioxide contains one carbon and two oxygen atoms so it's written CO₂.

Oxygen and carbon are similar in that they are both low atomic weight elements and nonmetals (the upper right side of the periodic table). They are abundant elements in the universe and especially here on earth and are fundamental building blocks in living things. Carbon and oxygen also bond well together and may bond in single or double bonds to generate many useful functional groups including carbonyls, ethers, esters. They form these bonds in nature and we can also form these types of bonds in the lab. Apart from these general classifications, carbon and oxygen are quite distinct elements and share fewer common properties than, for example, elements that fall in the same group (column).

Carbon and oxygen are both nonmetallic elements of the same period. Being nonmetallic they tend to engage in covalent bonding rather than ionic bonding, and being in the same period they're both fairly light (but not *that* light) elements. The process of nuclear synthesis also tends to produce them readily, making them the third (oxygen) and fourth (carbon) most abundant elements in the universe, after hydrogen (first) and helium (second). However, there really isn't a whole lot else between them that is similar; they're actually pretty different chemically. For example, at the Earth's surface, oxygen is always a gas, and the two forms of carbon that we are familiar with (graphite and diamond) are both solid.

Carbon atoms are made up of 6 protons, 6 electrons, and 6 neutrons, whereas Oxygen atoms are made of 8 protons, 8 neutrons, and 8 electrons. This may not sound like a big difference at the atomic level, but it is responsible for all of the differences between Oxygen and Carbon. Oxygen is gas Carbon is a solid, pure Oxygen is flammable pure Carbon isn't, Carbon is black and Oxygen is colorless, the list goes on. It's amazing but true that all of these differences can be traced back to the difference between six and eight.