How to Find Atomic Number
Find a copy of the periodic table., Find the element you're studying., Look for the atomic number., Confirm based on nearby elements., Understand atomic number., Find the atomic weight., Understand atomic mass., Round to the mass number., Calculate...
Step-by-Step Guide
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Step 1: Find a copy of the periodic table.
We've got one here if you don't have one handy.
Every single element has a different atomic number, so there are no quick rules to learn.
It's either find a copy of the periodic table, or memorize one.
Most chemistry textbooks have a periodic table printed on the inside cover. -
Step 2: Find the element you're studying.
Most periodic tables include the full name of the elements, as well as the chemical symbol (such as Hg for Mercury).
If you're having trouble finding it, search online for "chemical symbol" followed by the name of the element. , The atomic number is usually in the top left or top right corner of the element's box, but it could be in a different spot.
This is always a whole number.
If the number includes a decimal point, you're probably looking at atomic mass instead. , The periodic table is arranged in order of atomic number.
If your element's atomic number is "33," the element to the left should be "32" and the element to the right should have a "34." If this pattern holds true, you have definitely found the atomic number.
You may see gaps after elements 56 (Barium) and 88 (Radium).
There's no actual gap; the elements with those atomic numbers are located in two rows below the rest of the chart.
They are only separated like this to fit the periodic table in a more narrow shape. , An element's atomic number has a simple definition: the number of protons in one atom of that element.This is the fundamental definition of an element.
The number of protons determines the total electric charge of the nucleus, which determines how many electrons the atom can support.
Because electrons are responsible for almost all chemical interactions, the atomic number indirectly sets almost all of the element's physical and chemical properties.
To put it another way, every atom with eight protons is an oxygen atom.
Two oxygen atoms might have different numbers of neutrons or (if one is an ion) different numbers of electrons, but they will always have eight protons each. , The atomic weight is typically printed below the element name on the periodic table, usually to two or three decimal places.
This is the average mass of one atom of the element, weighted to what you would expect to find in nature.
This number is in "atomic mass units" (AMU).
Some scientists prefer the term "relative atomic mass" instead of atomic weight., Atomic mass is a closely related concept to atomic weight.
The only difference is, you're looking at a specific atom instead of the average of a sample.A gram of iron contains many atoms with slightly different masses, so you look up the "relative weight" to find the average.
If you pick out a single iron atom, you don't care about the average — you only care about the exact mass of that specific atom.
In practice, chemistry problems dealing with single atoms usually tell you the atomic mass.
Just understand this concept so you aren't surprised when it's different than atomic weight. , The mass number is the total number of protons and neutrons in one atom of an element.
This is easy to find: just take the atomic weight printed on the periodic table, and round to the nearest whole number.This works because neutrons and protons are both very close to 1 AMU, and electrons are very close to 0 AMU.
The atomic weight uses precise measurements to get the decimal value, but we're only interested in the whole numbers that tell us the number of protons and neutrons.
Remember, if you use the atomic weight, you're getting an average for a typical sample.
A bromine sample has an average mass number of 80, but as it turns out, a single bromine atom almost always has a mass number of 79 or
81., Now you know that atomic number = number of protons, and mass number = number of protons + number of neutrons.
To find the number of neutrons in an element, subtract the atomic number from the mass number.
Here are a couple example:
A single helium (He) atom has a mass number of 4 and an atomic number of
2.
It must have 4
- 2 = 2 neutrons.
A sample of silver (Ag) has an average mass number of 108 (based on the periodic table) and an atomic number of
47.
On average, each silver atom in the sample has 108
- 47 = 61 neutrons. , An isotope is a specific form of an element, with a certain number of neutrons.
If a chemistry problem mentions "boron-10" or "10B," it's talking about boron elements with a mass number of
10.Use this mass number instead of the values for "typical" boron.
Isotopes never change the atomic number.
Every isotope of an element has the same number of protons. -
Step 3: Look for the atomic number.
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Step 4: Confirm based on nearby elements.
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Step 5: Understand atomic number.
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Step 6: Find the atomic weight.
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Step 7: Understand atomic mass.
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Step 8: Round to the mass number.
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Step 9: Calculate the number of neutrons.
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Step 10: Understand isotopes.
Detailed Guide
We've got one here if you don't have one handy.
Every single element has a different atomic number, so there are no quick rules to learn.
It's either find a copy of the periodic table, or memorize one.
Most chemistry textbooks have a periodic table printed on the inside cover.
Most periodic tables include the full name of the elements, as well as the chemical symbol (such as Hg for Mercury).
If you're having trouble finding it, search online for "chemical symbol" followed by the name of the element. , The atomic number is usually in the top left or top right corner of the element's box, but it could be in a different spot.
This is always a whole number.
If the number includes a decimal point, you're probably looking at atomic mass instead. , The periodic table is arranged in order of atomic number.
If your element's atomic number is "33," the element to the left should be "32" and the element to the right should have a "34." If this pattern holds true, you have definitely found the atomic number.
You may see gaps after elements 56 (Barium) and 88 (Radium).
There's no actual gap; the elements with those atomic numbers are located in two rows below the rest of the chart.
They are only separated like this to fit the periodic table in a more narrow shape. , An element's atomic number has a simple definition: the number of protons in one atom of that element.This is the fundamental definition of an element.
The number of protons determines the total electric charge of the nucleus, which determines how many electrons the atom can support.
Because electrons are responsible for almost all chemical interactions, the atomic number indirectly sets almost all of the element's physical and chemical properties.
To put it another way, every atom with eight protons is an oxygen atom.
Two oxygen atoms might have different numbers of neutrons or (if one is an ion) different numbers of electrons, but they will always have eight protons each. , The atomic weight is typically printed below the element name on the periodic table, usually to two or three decimal places.
This is the average mass of one atom of the element, weighted to what you would expect to find in nature.
This number is in "atomic mass units" (AMU).
Some scientists prefer the term "relative atomic mass" instead of atomic weight., Atomic mass is a closely related concept to atomic weight.
The only difference is, you're looking at a specific atom instead of the average of a sample.A gram of iron contains many atoms with slightly different masses, so you look up the "relative weight" to find the average.
If you pick out a single iron atom, you don't care about the average — you only care about the exact mass of that specific atom.
In practice, chemistry problems dealing with single atoms usually tell you the atomic mass.
Just understand this concept so you aren't surprised when it's different than atomic weight. , The mass number is the total number of protons and neutrons in one atom of an element.
This is easy to find: just take the atomic weight printed on the periodic table, and round to the nearest whole number.This works because neutrons and protons are both very close to 1 AMU, and electrons are very close to 0 AMU.
The atomic weight uses precise measurements to get the decimal value, but we're only interested in the whole numbers that tell us the number of protons and neutrons.
Remember, if you use the atomic weight, you're getting an average for a typical sample.
A bromine sample has an average mass number of 80, but as it turns out, a single bromine atom almost always has a mass number of 79 or
81., Now you know that atomic number = number of protons, and mass number = number of protons + number of neutrons.
To find the number of neutrons in an element, subtract the atomic number from the mass number.
Here are a couple example:
A single helium (He) atom has a mass number of 4 and an atomic number of
2.
It must have 4
- 2 = 2 neutrons.
A sample of silver (Ag) has an average mass number of 108 (based on the periodic table) and an atomic number of
47.
On average, each silver atom in the sample has 108
- 47 = 61 neutrons. , An isotope is a specific form of an element, with a certain number of neutrons.
If a chemistry problem mentions "boron-10" or "10B," it's talking about boron elements with a mass number of
10.Use this mass number instead of the values for "typical" boron.
Isotopes never change the atomic number.
Every isotope of an element has the same number of protons.
About the Author
Melissa Peterson
Professional writer focused on creating easy-to-follow practical skills tutorials.
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