The atomic number represents the number of protons within the nucleus of a single atom of an element. This value cannot change, so it can be used to derive other characteristics, such as the number of electrons and neutrons in an atom.
Steps
Part 1 of 2: Finding the Atomic Number
Step 1. Find a copy of the periodic table
At this link you can find one if you don't have another one at hand. All elements have a different atomic number and are sorted in the table by that value. If you'd rather not use a copy of the periodic table, you can memorize it.
Many chemistry books have the periodic table printed on the inside of the cover
Step 2. Find the items you are studying
Many periodic tables include the full names of the elements, as well as their respective chemical symbols (such as Hg for mercury). If you cannot find what you are interested in, search the internet for "chemical symbol" followed by the name of the element.
Step 3. Search for the atomic number
This value is usually found in the upper right or left corner of each element's box, but not all periodic tables respect this convention. However, it is always an integer.
If the number is expressed in decimals, it is probably the atomic mass
Step 4. Find confirmation by looking at adjacent items
The periodic table is sorted by atomic number. If the atomic number of the element you are interested in is "33", the element to its left should have "32" as its atomic number and the one to the right "34". If this scheme is respected, you have undoubtedly found the atomic number.
You may notice gaps after the elements with atomic numbers 56 (barium) and 88 (radium). There are actually no gaps; elements with missing atomic numbers are found in two rows below the rest of the table. They are separated in this way only to allow the table to be published in a smaller format
Step 5. Learn about the atomic number
This value has a simple definition: it is the number of protons present in an atom of an element. This is the fundamental definition of an element. The number of protons determines the total electric charge of the nucleus, which consequently determines the number of electrons that the atom can support. Since electrons are responsible for almost all chemical reactions, the atomic number indirectly affects many of the physico-chemical properties of the element.
In other words, all atoms with 8 protons are oxygen atoms. Two oxygen atoms may have different numbers of electrons (if one of them is an ion), but they will always have 8 protons
Part 2 of 2: Finding Related Information
Step 1. Find the atomic weight
Usually this value is reported under the name of each element of the periodic table, with an approximation of 2 or 3 decimal places. This is the average mass of an atom of the element, calculated based on the state of the element in nature. This value is expressed in "atomic mass units" (UMA).
Some scholars prefer the term "relative atomic mass" instead of atomic weight
Step 2. Calculate the mass number with a rounding
This value is the total of protons and neutrons in an atom of an element. It is very easy to find: just round up to the nearest whole number to the atomic weight shown on the periodic table.
- This method works because neutrons and protons have a UMA value very close to 1, while electrons are very close to 0 UMA. To calculate the atomic weight in decimals, precise measurements are used, but we are only interested in the integer that represents the sum of protons and neutrons.
- Remember, the atomic weight represents the average of a typical sample. A bromine sample has an average mass number of 80, but in reality a single bromine atom always has a mass of 79 or 81.
Step 3. Find the number of electrons
Atoms contain the same number of protons and electrons, so these values must be the same. Electrons are negatively charged, so they balance and neutralize protons, which are positively charged.
If an atom loses or acquires electrons, it becomes an ion, so it has an electrical charge
Step 4. Calculate the number of neutrons
Now that you know the atomic number (equal to the number of protons) and the mass number (equal to the sum of protons and neutrons), to find the number of neutrons of an element you just need to subtract the atomic number from the mass number. Here are some examples:
- A single helium atom (He) has a mass number of 4 and an atomic number of 2, so it must have 4 - 2 = 2 neutrons;
- A sample of silver (Ag) has an average mass number of 108 (according to the periodic table) and an atomic number of 47. On average, each silver atom within a naturally occurring sample has 108 - 47 = 61 neutrons.
Step 5. Learn about isotopes
These are specific forms of an element, with a precise number of neutrons. If a chemistry problem uses the term "boron-10" or "10B, "means boron atoms with a mass number of 10. Use this mass number instead of the value of" normal "boron atoms.
