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

Correct — B, atomic number. The atomic number is the number of protons in an atom's nucleus, and it is what defines the element - change the proton count and you have a different element altogether. Melting point, colour and even atomic weight can vary, but the atomic number is fixed and unique for each element, so it is the most fundamental characteristic.

Why the others are wrong

• (a)
  Melting point — Melting point is a physical property that varies with the allotrope and conditions and is shared by many substances; it does not identify which element an atom is.
• (c)
  Colour — Colour changes with allotrope, impurity and physical state (carbon is black as graphite yet transparent as diamond), so it cannot fundamentally define an element.
• (d)
  Atomic weight — Atomic weight differs between isotopes of the same element and once caused ordering errors; it is the proton count (atomic number), not the weight, that truly defines an element.

The atomic number (Z) is the number of protons in the nucleus and equals the number of electrons in a neutral atom. It is unique to each element and fixes the element's place in the periodic table. Since Henry Moseley's work, the periodic table is ordered by atomic number rather than atomic weight.

Key facts

• Atomic number (Z) is the number of protons in the nucleus and, in a neutral atom, equals the number of electrons.
• Every element has a unique atomic number; changing it changes the element.
• Moseley showed the periodic table is correctly ordered by atomic number, not by atomic weight.
• Isotopes of an element share the same atomic number but have different mass numbers (atomic weights).

Common traps

• Confusing atomic number (protons) with atomic weight (protons plus neutrons).
• Thinking a physical property such as colour or melting point identifies an element.
• Forgetting that isotopes share an atomic number but differ in mass.

• practice — not a real PYQ

  The atomic number of an element is equal to the number of

  • (a)neutrons
  • (b)protons
  • (c)protons and neutrons
  • (d)electrons and neutrons
  Answer(b) protons - the atomic number is the proton count in the nucleus.
• practice — not a real PYQ

  The modern periodic table arranges elements in order of increasing

  • (a)atomic weight
  • (b)atomic number
  • (c)density
  • (d)valency
  Answer(b) atomic number - the basis of the modern periodic law (Moseley).