The structure of the nucleus
The number of protons and neutrons found in the nucleus of an atom. and The number of protons in the nucleus of an atom. Also called the proton number. are two important pieces of information about a The central part of an atom. It contains protons and neutrons, and has most of the mass of the atom. The plural of nucleus is nuclei..
A nucleus can be represented using the symbol notation:
\(_{Z}^{A}\textrm{X}\)
Where:
- A is the mass number
- Z is the atomic number
- X is the chemical symbol of the element
- A - Z = number of neutrons N
For example, chlorine (Cl) can be shown as:
Cl is the chemical symbol for chlorine
A chlorine nucleus will contain:
- Number of protons Z = 17
- Number of protons + neutrons A = 35
- Number of neutrons N = 35 - 17 = 18
The chlorine atom will also have 17 electrons, as an atom is neutral.
Atoms and isotopes
An element's atomic number defines it.
An element with 17 protons will always be chlorine.
However an element's mass numbers can vary, which means that it can have different numbers of neutrons.
So although chlorine has a mass number of 35 which means it has 18 neutrons, it can also have a mass number of 37, which means it has 20 neutrons.
The different types of chlorine are called isotopes of chlorine.
There are three isotopes of hydrogen: hydrogen, deuterium (hydrogen-2) and tritium (hydrogen-3):
Carbon has three isotopes: \(_{6}^{12}\textrm{C}\), \(_{6}^{13}\textrm{C}\) and \(_{6}^{14}\textrm{C}\). They all contain six protons, but six, seven and eight neutrons respectively.
\(_{7}^{14}\textrm{N}\) and \(_{6}^{14}\textrm{C}\) are not isotopes.
They have the same mass number, A, but different atomic number, Z, and different chemical symbol - they are not the same element.
If the number of protons changes, then it is a different element.
Question
How many protons does \(_{6}^{14}\textrm{C}\) contain?
The atomic number is 6 so \(_{6}^{14}\textrm{C}\) contains six protons.
Question
How many neutrons does \(_{6}^{14}\textrm{C}\) contain?
Number of neutrons = mass number - atomic number = 14 - 6 = 8 neutrons.
Question
Write down the number of particles in the following nuclei:
\(_{30}^{64}\textrm{Zn}\)
\(_{92}^{238}\textrm{U}\)
\(_{1}^{3}\textrm{H}\)
\(_{30}^{64}\textrm{Zn}\)
- Number of protons Z = 30
- Number of protons + neutrons A = 64
- Number of neutrons N = 64 - 30 = 34
- Since this is a nucleus there are no electrons
There are 30 protons and 34 neutrons.
\(_{92}^{238}\textrm{U}\)
- Number of protons Z = 92
- Number of protons + neutrons A = 238
- Number of neutrons N = 238 - 92 = 146
- Since this is a nucleus there are no electrons
There are 92 protons and 146 neutrons.
\(_{1}^{3}\textrm{H}\)
- Number of protons Z = 1
- Number of protons + neutrons A = 3
- Number of neutrons N = 3 - 1 = 2
- Since this is a nucleus there are no electrons
There is 1 proton and 2 neutrons.