Which one among the following statements with respect to the atomic number of an atom is correct?

Correct — C, the number of protons is same as atomic number. The atomic number, written Z, is defined as the number of protons in the nucleus, and it is what fixes the identity of an element. Every carbon atom has six protons; give it a seventh and it stops being carbon. In a neutral atom the electron count equals the proton count, so Z also gives the number of electrons, but the definition is the proton count and it holds for ions too, where the electron count no longer matches. The sum of protons and neutrons is a different quantity called the mass number, written A, and the neutron count is A minus Z.

• (a)The number of neutrons is same as atomic number — The neutron count is independent of the atomic number, which is why isotopes exist. Carbon-12 and carbon-14 both have six protons but six and eight neutrons respectively, and both are carbon.
• (b)The sum of electrons and neutrons is same as atomic number — Electrons plus neutrons is not a quantity used in chemistry at all. In a neutral atom the electron count alone equals the atomic number, but adding the neutrons to it produces nothing meaningful.
• (d)The sum of protons and neutrons is same as atomic number — Protons plus neutrons is the mass number A, not the atomic number. Confusing the two is the standard error: for sodium, Z is 11 and A is 23, and the difference of 12 is the neutron count.

Two numbers describe a nucleus. The atomic number Z is the proton count and fixes which element the atom is; the mass number A is the total of protons and neutrons and fixes which isotope it is. Atoms with the same Z but different A are isotopes; atoms with the same A but different Z are isobars. Because Z decides the electron count in a neutral atom, it also decides the electronic configuration and therefore all the chemistry, which is why the modern periodic table is ordered by atomic number rather than by atomic mass.

All four options are built from the same three particle counts rearranged, so the item is a definition check with no room for reasoning around it. The habit that saves marks is to keep the two symbols apart in your head: Z is protons, A is protons plus neutrons, and neutrons are A minus Z. Moseley's work on X-ray spectra in 1913 is what established the atomic number as the correct ordering principle for the periodic table, replacing Mendeleev's ordering by atomic mass and removing several anomalies at a stroke.

• Atomic number Z equals the number of protons, and it determines the identity of the element.
• Mass number A equals protons plus neutrons; the neutron count is A minus Z.
• In a neutral atom the electron count equals Z, but in an ion it does not.
• Isotopes share Z and differ in A; isobars share A and differ in Z.
• Moseley showed in 1913 that the periodic table should be ordered by atomic number, not atomic mass.

• Answering with the mass number, which is the most common slip on this definition.
• Treating electron count as the definition, which fails for ions.
• Assuming the neutron count is fixed for an element.

A pure definition item in which the four options are permutations of protons, neutrons and electrons.

• practice — not a real PYQ

  An atom has 17 protons and 18 neutrons. Its mass number is

  • (a)17
  • (b)18
  • (c)35
  • (d)1

  Answer(c) 35 — mass number is protons plus neutrons, so 17 + 18.

• practice — not a real PYQ

  Two atoms with the same mass number but different atomic numbers are called

  • (a)isotopes
  • (b)isobars
  • (c)isotones
  • (d)allotropes

  Answer(b) isobars — argon-40 and calcium-40 are the standard example.