Which one of the following is the most fundamental characteristic of an element?

The correct answer is B, Atomic number. The atomic number, the number of protons in the nucleus, decides which element an atom is, and no two elements share it. Every atom with six protons is carbon and every atom with eight is oxygen, however many neutrons or electrons it may have. The atomic number also fixes the number of electrons in a neutral atom, and so its chemical behaviour. Henry Moseley showed this in 1913 from X-ray studies, and the modern periodic law states that the properties of elements are a periodic function of their atomic numbers. Option A is wrong because melting point depends on the form and purity of a substance; carbon as diamond and as graphite behave very differently. Option C is wrong because colour changes with form, size of particles and compounds, so it cannot identify an element. Option D is wrong because atomic weight varies between isotopes of the same element, and Mendeleev's ordering by atomic weight left some pairs, such as argon and potassium, out of place. Exam tip: atomic number (Z) = protons = identity of the element; Moseley's modern periodic law is based on Z, Mendeleev's on atomic mass.