The next few chapters will be used to attempt to classify and study matter in different states of aggregation. First, though, we have to ask "What is matter made of ?"
During the evolution of physics, we have managed to explore ever-smaller dimensions and to look deeper and deeper into matter to explore its elementary building blocks. This general way of learning more about a system by finding its subsystems is called "reductionism"; and it has proven an incredibly fruitful guiding principle during the last four or five centuries of modern science.
For now, we will assume that atoms are the elementary building blocks of matter, but we will learn in the second semester of this physics course that atoms themselves are composite particles. The substructure of atoms, however, can only be resolved with accelerators and other tools of modern nuclear physics. So for our purposes it is completely reasonable to make the assumption of atoms as elementary matter building blocks.
The word "atom", by the way, comes from the Greek "atomos", which means indivisible. The old Greeks believed that there were only 4 different kinds of atoms, earth, water, air, and fire.
Now we know that there are at least 115 different elements, which are classified in the periodic table of elements.
Question: How big is an atom?
Answer: Atoms have a diameter of about 10-10 m. This length is also often call an Angstrom (Å):
For now let's just accept the answer, and in the second physics semester we will learn the supporting arguments.
Now we can also ask just how many atoms are contained in macroscopic objects. And this will yield some interesting answers.
Experiments show that 12 grams of carbon, the carbon isotope 12C to be precise, contain 6.022$\cdot$ 1023 atoms. This number is also called the Avogadro Number:
How
much carbon is this? To find out, we have to know the density of
carbon. The answer is
different for diamond and graphite, both forms of carbon. A 12 g
diamond with NA atoms has 60
carat (0.2 gram = 1 carat) and a volume of about 3.5 cubic
centimeters, about 1.5 times bigger than the Hope-Diamond. The
wedding ring in this picture has a little more than 1 carat and
therefore about 1022 atoms.
Definition:
The SI-unit for amount
of mass is Gram-Mole, or shorter, Mole.
One Mole of a substance contains NA = 6.022$\cdot$ 1023 atoms or molecules.
The mass of one Mole of a substance is given by the atomic mass number (or: relative atomic mass). Listed in the periodic table of elements is the atomic mass number for each element. As a rule of thumb, this relative mass number is roughly equal to the number of protons and neutron contained in the nucleus of the atom. For molecules, one simply has to add up the relative mass numbers of all atoms in the molecule in order to obtain the relative mass number of the molecule.
© MultiMedia Physics, 1999