An element is nature's basic building block. It is the simplest form of matter, different from compounds and mixtures. An element is made of one, and only one, type of atom.
Remember that while an element is made of only one type of atom the atom itself consists of protons, electrons and neutrons. The number of protons in an element defines that element. For example, the atomic number of calcium is 20, which means it has 20 protons. Only an element with the atomic number 20 would be an element of calcium. An element with a different atomic number would be a different element.
Know that the Periodic Table contains information about the chemical properties of all known elements, and is organized for easy reference. The atomic number is the number of protons and the atomic weight (or mass number) gives the number of protons and neutrons in a single atom. For example, the Atomic Number of gold (Au) is 79 so it has 79 protons. The Atomic Weight is 197. To find the number of neutrons subtract the number of protons from the atomic weight. So: 197-79=118 neutrons.
Recognize that while the Periodic Table gives the atomic structure, it does not tell us how to find the number of atoms in an element. For that we have to visit Avogadro's number and the mole!
Revise Avogadro's number. Avogadro's number—6.02*10^23—refers to the number of objects in one mole. The theory is simple to understand. It tells us the number of atoms of an element needed to find the measure of that element equal to its atomic weight. In other words, a measure of an element equal to its atomic weight contains a fixed number of atoms.
Find the atomic weight of the element you are studying. Measure (in grams) a quantity of that element equal to its atomic weight. For example, let us look at the element helium. Its atomic weight is 4.00. So, 4 grams of helium contain one mole of helium atoms.
Multiply the number of moles by Avogadro's number to find the number of atoms. So in our example of the element helium, 4 grams of helium contain 6.02_10^23 helium atoms. Two moles of helium contain 1.204_10^24 helium atoms.
A mole represents a number. Just as the word 'dozen' refers to the number 12, the word 'mole' refers to Avogadro's number, 6.02*10^23.
You can work with fractions of moles. One gram of helium, for example, is 1/4 mole. To find the number of atoms in one gram of helium multiply 6.02*10^23 by 1/4.