Let's learn how to write nuclide symbols.
A nuclide symbol consists of the element’s atomic symbol, as written in the periodic
table, its mass number in the upper left-hand corner, and its atomic number in the lower
left-hand corner.
Recall that the mass number is equal to the number of protons and neutrons in an element,
and the atomic number is equal to the number of protons.
Since the mass number, A, is the number protons plus the number of neutrons, and the atomic
number, Z, is equal to the number protons, then A minus Z will give you the number of
neutrons in a nuclide.
In other words, the number of neutrons is equal to the mass number minus the atomic
number.
Recall that isotopes are elements with the same number of protons but differing numbers
of neutrons.
Let's look at two uranium isotopes as an example.
Notice how they have the same atomic numbers, but different mass numbers.
The atomic number of every uranium atom is 92.
The element on the left is referred to as uranium-235, because of its mass number.
Similarly the uranium atom on the right is referred to as uranium-238.
The mass number of the nuclide is given in the upper left-hand corner.
Therefore, the mass number of uranium-235 is 235.
Its atomic number is given in the lower left-hand corner, 92.
This means that uranium-235 has 92 protons.
To determine the number of neutrons, we subtract the atomic number from the mass number.
235-92 is equal to 143 neutrons.
The mass number of uranium-238 is 238.
Its atomic number is 92.
The atomic number of every uranium isotope is 92.
Therefore, it contains 92 protons.
We can determine the number of neutrons by subtracting the atomic number from the mass
number.
238-92 is equal to 146.
Uranium-238 has 146 neutrons.
Let's look at a sample problem.
Astatine-75 has a mass number of 75 and atomic number of 33.
Its atomic number tells us that it has 33 protons.
To determine the number of neutrons, we subtract 33 from 75, and get 42 neutrons.
Tellurium-128.
Tellurium-128 has a mass number of 128, and atomic number 52.
This means that tellurium has 52 protons.
To determine the number of neutrons, subtract the atomic number from the mass number.
128-52 is equal to 76.
Sulfur-32.
Sometimes the atomic number is omitted from the nuclide symbol, because the atomic number
of every sulfur atom is the same.
We can get its atomic number from the periodic table.
Sulfur-32 has a mass number of 32, and atomic number of 16.
This means that sulfur has 16 protons, and sulfur-32 has 32-16 neutrons, or 16 neutrons.
In this sample problem we’re given the element’s symbol, tellurium, its atomic number and the
number of neutrons.
We know that the mass number is the sum of the number of protons and number of neutrons,
which is 78+117, which is equal to 195.
We write the mass number 195 in the upper left-hand corner of the nuclide symbol.
The number of protons is equal to the atomic number, 78.
We write the atomic number, 78, in the lower left-hand corner of the nuclide symbol.