Atomic number and mass number: counting the particles in an atom
Definition
Atomic number
The number of protons in the nucleus of an atom, which is unique to each element.
Definition
Mass number
The total number of protons and neutrons in the nucleus of an atom.
- Every atom of a given element has the same number of protons, and no other element shares that number.
- Carbon always has 6 protons, so carbon's atomic number is 6.
- A neutral atom holds as many electrons as protons, so the atomic number also gives the electron count.
- The mass number counts the protons and neutrons together, because those two particles carry essentially all the mass.
- On a periodic table entry the larger number is the relative atomic mass, not the mass number, and the smaller is the atomic number.
Key Idea
- The atomic number identifies which element the atom is.
- The mass number belongs to one particular isotope, and is written in nuclide notation rather than printed on the table.
Isotopes: same element, different numbers of neutrons
Definition
Isotope
Atoms of the same element with the same number of protons but different numbers of neutrons.
- Isotopes of one element share an atomic number, because they contain the same number of protons.
- They have different mass numbers, because they contain different numbers of neutrons.
- Chlorine-35 and chlorine-37 are both chlorine, so each atom has 17 protons.
- A chlorine-35 atom has 35−17=1835 - 17 = 1835−17=18 neutrons.
- A chlorine-37 atom has 37−17=2037 - 17 = 2037−17=20 neutrons.
- Extra neutrons change the mass but not the chemical reactions, because the electron arrangement is unchanged.
Common Mistake
- Do not change the number of protons to make an isotope, because that changes the element.
- Do not read the mass number as the number of protons, because it counts protons and neutrons together.
Counting the particles from the two numbers
- The number of protons is equal to the atomic number.
- The number of electrons in a neutral atom is also equal to the atomic number.
- The number of neutrons is the mass number minus the atomic number: neutrons=mass number−atomic number\text{neutrons} = \text{mass number} - \text{atomic number}neutrons=mass number−atomic number
- Sodium with atomic number 11 and mass number 23 has 11 protons, 11 electrons and 23−11=1223 - 11 = 1223−11=12 neutrons.
- Chlorine-37 with atomic number 17 has 17 protons, 17 electrons and 37−17=2037 - 17 = 2037−17=20 neutrons.
- Check that your proton count matches the element named in the question before you accept the answer.
Example
- An atom has atomic number 8 and mass number 16.
- Protons =8= 8=8, so the element is oxygen.
- Electrons =8= 8=8, because the atom is neutral.
- Neutrons =16−8=8= 16 - 8 = 8=16−8=8.
Why relative atomic mass is often not a whole number
Definition
Relative atomic mass
The weighted mean mass of an element's atoms, taking the abundance of each isotope into account, compared with one-twelfth of the mass of a carbon-12 atom.
- Every individual atom has a whole-number mass number, because an atom cannot hold part of a neutron.
- A real sample of an element contains a mixture of isotopes in fixed proportions.
- Relative atomic mass averages those isotopes, weighting each one by how common it is.
- Chlorine is about three-quarters chlorine-35 and one-quarter chlorine-37, which averages to 35.535.535.5.
- The value sits closer to 353535 than to 373737 because chlorine-35 is the more abundant isotope.
- A value such as 35.535.535.5 describes the sample, never any single atom in it.
Self review
- What does the mass number tell you?
- Why do all atoms of one element have the same atomic number?
- How do isotopes of the same element differ?
- How many protons, neutrons and electrons are in an atom with atomic number 8 and mass number 16?
- Why can the relative atomic mass of chlorine be 35.535.535.5 when each chlorine atom has a whole-number mass number?
