how do you find the mass number
4.17: Mass Number
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How can yous determine the mass of a chemical?
Ofttimes a student will need to weigh out a chemical for an experiment. If he or she uses a watch drinking glass (a pocket-sized, circular slice that will hold the solid chemical), the weight of the watch glass must exist adamant first. Then the solid is added to the glass and the weight of the glass plus the solid is measured. The balance reading will be the total of the glass plus the chemic.
History of Atomic Weight Determinations
Equally a part of his enquiry on atoms, John Dalton determined a number of diminutive weights of elements in the early 1800s. Atomic weights were the basis for the periodic table that Mendeleev developed. Originally all diminutive weights were based on a comparison to hydrogen, which has an atomic weight of one. After the discovery of the proton, scientists assumed that the weight of an atom was substantially that of the protons—electrons were known to contribute near nothing to the atomic weight of the element.
This arroyo worked until we learned how to determine the number of protons in an chemical element. We then saw that the atomic weight for an chemical element was often twice the number of protons (or more than). The discovery of the neutron provided the missing office of the moving-picture show. The atomic mass is now known to be the sum of the protons and neutrons in the nucleus.
Mass Number
Rutherford showed that the vast majority of the mass of an atom is full-bodied in its nucleus, which is equanimous of protons and neutrons. The mass number is defined as the total number of protons and neutrons in an atom. It can be calculated by adding the number of neutrons and the number of protons (atomic number) together.
Mass number = atomic number + number of neutrons
Consider Table \(\PageIndex{ane}\) beneath that shows data from the first six elements of the periodic table.
| Name | Symbol | Protons | Neutrons | Electrons | Atomic Number | Mass Number |
|---|---|---|---|---|---|---|
| Hydrogen | \(\ce{H}\) | i | 0 | 1 | ane | 1 |
| Helium | \(\ce{He}\) | 2 | 2 | two | two | 4 |
| Lithium | \(\ce{Li}\) | three | 4 | 3 | 3 | 7 |
| Beryllium | \(\ce{Be}\) | 4 | 5 | iv | iv | 9 |
| Boron | \(\ce{B}\) | 5 | 6 | five | 5 | xi |
| Carbon | \(\ce{C}\) | 6 | half dozen | 6 | six | 12 |
Consider the element helium. Its atomic number is 2, so it has two protons in its nucleus. Its nucleus also contains ii neutrons. Since \(ii + 2 = 4\), we know that the mass number of the helium cantlet is 4. Finally, the helium atom also contains ii electrons since the number of electrons must equal the number of protons. This instance may lead yous to believe that atoms take the same number of protons and neutrons, but farther examination of the tabular array above will bear witness that this is not the case. Lithium, for instance, has 3 protons and four neutrons, leaving information technology with a mass number of 7.
Knowing the mass number and the diminutive number of an atom allows you lot to decide the number of neutrons present in that atom by subtraction.
\[\text{Number of neutrons} = \text{mass number} - \text{atomic number}\nonumber \]
Instance \(\PageIndex{1}\)
Atoms of the chemical element chromium \(\left( \ce{Cr} \right)\) have an atomic number of 24 and a mass number of 52. How many neutrons are in the nucleus of a chromium atom?
Solution
To decide this, you lot decrease the atomic number from the mass number, equally shown above:
\[52 - 24 = 28 \: \text{neutrons in a chromium atom}\nonumber \]
The composition of any atom tin can be illustrated with a shorthand annotation using the atomic number and the mass number. Both are written before the chemical symbol, with the mass number written as a superscript and the atomic number written as a subscript. The chromium atom discussed above would be written as:
\[\ce{^{52}_{24}Cr}\nonumber \]
Another way to refer to a specific cantlet is to write the mass number of the atom afterward the name, separated past a hyphen. The above atom would exist written as chromium-52.
Summary
- The mass number is defined every bit the total number of protons and neutrons in an cantlet.
- The mass number = number of neutrons + atomic number.
Review
- Who starting time determined atomic weights for elements?
- What were the original atomic weights based on?
- Why were calculations based on numbers of protons not valid for determining atomic weights?
- A tin atom has an atomic number of 50 and a mass number of 118. How many neutrons are present in this atom?
- What is the mass number of a cobalt atom that has 27 protons and 30 neutrons?
Source: https://chem.libretexts.org/Bookshelves/Introductory_Chemistry/Introductory_Chemistry_%28CK-12%29/04:_Atomic_Structure/4.17:_Mass_Number
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