An atom is an element. The two words are synonymous, so if you're looking for the number of atoms in an element, the answer is always one, and only one. Scientists know of 118 different elements, which they categorize in the periodic table, a diagram that arranges them in increasing order according to the number of protons in their nuclei. This arrangement allows you to answer a significant question at a glance: "What is the number of protons in a particular element?" To answer that, you simply need to look at the place the element occupies in the chart. The place number corresponds to the number of protons.
TL;DR (Too Long; Didn't Read)
If you have a sample that contains atoms of a single element, you can find the number of atoms by weighing it.
Elements That Form Diatomic Molecules
Some atoms can form covalent bonds with other atoms of the same element to form diatomic molecules. The best known is oxygen (O). A single oxygen atom is highly reactive, but when it forms a bond with another oxygen atom to form O2, the combination is more stable. This is the form in which oxygen exists in the earth's atmosphere. Four other elements can combine in this way at standard temperature and pressure. They include nitrogen (N), which is the most abundant element in the atmosphere, hydrogen (H), chlorine (Cl) and fluorine (F). Two other elements, bromine (Br) and iodine (I), can form diatomic molecules at higher temperatures. All diatomic molecules contain two atoms.
Noble Gases and Metals
Some atoms, such as sodium and phosphorous, are so reactive that they are never found free in nature. However, two groups of elements, the noble gases and noble metals, are stable and can exist in samples that contain only non-bound atoms of that element. For example, a container full of argon gas (Ar) contains only argon atoms, and a bar of pure gold contains only gold (Au) atoms. If you have a large sample of a noble gas or metal, you can calculate how many atoms it contains by weighing it.
In addition to these gases and metals, carbon (C) can also exist in the free state. Diamond and graphite are the two most common forms. Among non-metals, carbon is unique in its ability to exist in this way.
To calculate the number of atoms in a sample, you need to find how many moles of the element the sample contains. A mole is a unit chemists use. It's equal to Avogadro's number (6.02 X 1023) of atoms. By definition, the weight of one mole of an element (its molar mass) is equal to its atomic weight in grams. The atomic weight for each element is on the periodic table right under the element's symbol. The atomic weight of carbon is 12 atomic mass units (amu), so the weight of one mole is 12 grams.
If you have a sample that contains only atoms of a particular element, weigh the sample in grams and divide by the atomic weight of the element. The quotient tells you the number of moles. Multiply that by Avogadro's number, and you'll find out how many atoms the sample contains.
1. How many atoms are there in one ounce of pure gold?
An ounce is 28 grams, and the atomic weight of gold is 197. The sample contains 28 ÷ 197 = 0.14 moles. Multiplying this by Avogadro's number tells you the number of atoms in the sample = 8.43 x 1022 atoms.
2. How many oxygen atoms are there in a gas sample that weighs 20 grams?
The same procedure applies to finding the number of atoms in a diatomic gas, even though the atoms have combined to form molecules. The atomic weight of oxygen is 16, so one mole weighs 16 grams. The sample weighs 20 grams, which is equal to 1.25 moles. Therefore, the number of atoms is 7.53 x 1023.