Explain the numbers of protons and neutrons in atomic nuclei and electron numbers in atoms and ions.
Using definitions explain how the number of protons and neutrons in atomic nuclei and electron numbers in atoms can be calculated from the full symbols of elements given in the periodic table. Use sulfur as an example. Plus the features that make an atom of sulfur unique.
The protons and neutrons in atoms and ions make up the atomic nucleus, and are hence called nucleons. The atomic number of an atom is the number of protons in its nucleus. This is the number by which the standard periodic table is ordered.
The atomic mass number is also given in the standard periodic table and, approximately, is equal to the number of protons plus the number of neutrons. It is only approximately true as the atomic mass number is not necessarily a whole number.
So it is therefore possible, using a standard periodic table, to get the number of protons from the atomic number and the number of neutrons by subtracting the number of protons from the atomic mass.
Written as formulae we have
#Protons + #Neutrons `approx ` Atomic mass (1) (`approx ` for approximately)
#Protons = Atomic number (2)
(1) and (2) together implying that
#Neutrons `approx ` rounded(Atomic mass) - Atomic number (3)
To take the element Sulphur (32 isotope) as an example:
Sulphur has an atomic number of 16, as it is no. 16 in the standard periodic table. It therefore has 16 protons (and in fact, since proton and electron numbers are equal in atoms, 16 electrons as well). Sulphur's atomic mass is given as 32.06, which rounded is equal to 32. Therefore we can calculate the number of neutrons by subtracting the number of protons from this rounded atomic mass number, giving number of neutrons as 16.