The nuclear symbol consists of three parts: the symbol of the element, the atomic number of the element and the mass number of the specific isotope.
Here is an example of a nuclear symbol:
The element symbol, Li, is that for lithium.
The three, subscripted left, is the atomic number and the seven, superscripted left, is the mass number.
The atomic number is:
The number of protons in the nucleus of the atom.
The mass number is:
The number of protons and neutrons in the nucleus of the atom.
Here is one last example:
The 22 is the atomic number for titanium and 48 is its mass number. The number of neutrons is 48 minus 22 = 26.
By the way, here's a nice atomic number question: What is the atomic number of the element whose name is formed by deleting the first letter of the name of another element? Answer is below.
Now, write the nuclear symbol for the chlorine isotope with 18 neutrons. Answer.
Here are two tips:
1) The element name (or symbol) uniquely determines the atomic number. In the example just above Ti is the only element with an atomic number of 22. So, if you need the atomic number, and all you know is the specific element, go to a periodic table and find its atomic number.
2) Suppose you are asked to write a nuclear symbol from scratch and the teacher requires it be a realistic one. Do this:
a) Select an element, making sure it is a naturally occuring one. This will determine its atomic number.
b) Take the element's atomic weight and round it off to the nearest whole number. More often than not, this will be the mass number of the most abundant stable isotope
Let's try an example. Write the nuclear symbol for silver. Answer.
68. (The element is Erbium and the one whose first letter is deleted is Terbium.)