The atomic number of an element is the number of protons in the nucleus of a single atom of that element. The atomic number of an element or isotope doesn't change, so you can use it to find other data, such as the number of neutrons.
Step
Part 1 of 2: Finding the Atomic Number
Step 1. Prepare a periodic table of elements
We provide it here, if you don't have one yet. Each element has a different atomic number, so there are no shortcuts to studying it. You have to look at the periodic table or memorize it.
Most chemistry textbooks include the periodic table on the cover
Step 2. Find the element you are studying
Most periodic tables include the element's full name, as well as its symbol (such as Hg for Mercury). If you're having trouble finding it, search the internet for "element symbols" and their names.
Step 3. Find the atomic number
Although it may be written elsewhere, the atomic number of an element is usually located in the upper left or right corner of the element grid. This number is always written as an integer.
If the number you see is a decimal number, it is likely the mass number
Step 4. Make sure by paying attention to nearby elements
The periodic table of elements is arranged in order of atomic number. If the atomic number of your element is "33," the atomic number of the element to the left should be "34." If the pattern is like this, the number you get is indeed the atomic number.
You may find the atomic numbers are not consecutive between 56 (Barium) and 88 (Radium). Actually the atomic numbers of them are sequential and are located in two rows of elements at the bottom of the table. The elements are listed separately so that they can be included in a shorter periodic table
Step 5. Understand about atomic number
The atomic number of an element has a simple definition: the number of protons contained in one atom of that element. This is the basic definition of an element. The number of protons determines the electric charge in the nucleus of an atom, thus determining how many electrons can fit into it. Since electrons are responsible for almost all chemical reactions, the atomic number determines almost all the physical and chemical properties of an element.
In other words, every atom that has eight protons is an oxygen atom. Two oxygen atoms may have different numbers of neutrons or, if one of them is an ion, different numbers of electrons. But all oxygen atoms will always have eight protons
Part 2 of 2: Finding Related Data
Step 1. Find the atomic weight
The atomic weight is usually listed below the name of the element in the periodic table, usually with two or three numbers after a comma. This value is the average mass of one atom of an element, in its natural form in nature. This weight is expressed in "atomic mass units" (amu).
Some experts refer to it as "relative atomic mass," not atomic weight
Step 2. Find the mass number
The mass number is the total number of protons and neutrons in one atom of an element. This value is easy to find: pay attention to the atomic weights listed in the periodic table, and pay attention to the integers nearby.
- This is possible because protons and neutrons are very close to 1 amu, while electrons are very close to 0 amu. Atomic weights are written accurately using decimal numbers, but we only need to pay attention to whole numbers which mean the number of protons and neutrons.
- Remember, if you are using atomic weights, you are using average data from the same sample of elements. Samples of bromine have an average mass number of 80, however, a single bromine atom almost always has a mass of 79 or 81.
Step 3. Find the number of electrons
An atom has the same number of protons and electrons. So, this number will be the same. Electrons have a negative charge so they can neutralize protons which are positively charged.
An atom will turn into an ion which is a charged atom if it loses or gains electrons
Step 4. Count the number of neutrons
Now, you know the atomic number = number of protons, and mass number = number of protons + number of neutrons. To find the number of neutrons in an element, you just need to subtract the mass number from the atomic number. Here are some examples:
- A single helium (He) atom has a mass number of 4 and an atomic number of 2. So the number of neutrons is 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. So, on average, each silver atom has 108 - 47 = 61 neutrons.
Step 5. Understand about isotopes
Isotopes are certain forms of an element, with a certain number of neutrons. If the chemistry question says "boron-10" or "10B, " is the element boron with a mass number of 10. Use this mass in the problem, not the mass number that is usually used for boron in general.
