NDA (II) 2016 GAT Q64: There are six electrons, six protons and six neutrons in an

Q: What is the answer to NDA (II) 2016 GAT Q64?

A. Correct — A, 6. The atomic number of an element is defined as the number of protons in the nucleus of its atom, and nothing else. The stem gives that number directly: six protons, so the atomic number is 6. The other two figures are there to be used and then set aside. Six electrons simply tells you the atom is electrically neutral, since a neutral atom carries as many negative electrons as positive protons — useful confirmation, but the electron count is not the definition. Six neutrons contributes to the mass number, which is protons plus neutrons and therefore 12, not to the atomic number. Put together, an atom with Z equal to 6 and a mass number of 12 is carbon-12, the very isotope on which the modern scale of atomic masses is based.

NDA (II) 2016 General Ability Test · Question 64 of 150

Question text verified against the official NDA question paper.

There are six electrons, six protons and six neutrons in an atom of an element. What is the atomic number of the element ?

(a)6
(b)12
(c)18
(d)24

Answer

Why

Correct — A, 6. The atomic number of an element is defined as the number of protons in the nucleus of its atom, and nothing else. The stem gives that number directly: six protons, so the atomic number is 6. The other two figures are there to be used and then set aside. Six electrons simply tells you the atom is electrically neutral, since a neutral atom carries as many negative electrons as positive protons — useful confirmation, but the electron count is not the definition. Six neutrons contributes to the mass number, which is protons plus neutrons and therefore 12, not to the atomic number. Put together, an atom with Z equal to 6 and a mass number of 12 is carbon-12, the very isotope on which the modern scale of atomic masses is based.

Why the others are wrong

(b)12 — This is the mass number, obtained by adding the six protons to the six neutrons. Mass number and atomic number are different quantities, and confusing them is the single commonest slip in this topic; the mass number is written as a superscript, the atomic number as a subscript.
(c)18 — This adds up all three particle counts, six plus six plus six. Electrons sit outside the nucleus and have negligible mass, so they belong in neither the atomic number nor the mass number.
(d)24 — This doubles the mass number, and corresponds to nothing in the definitions. It is offered only to catch a candidate who is multiplying rather than reading.

Concept

Three numbers describe an atom and each has a fixed meaning. The atomic number Z counts the protons and defines which element the atom is — change it and you have a different element altogether. The mass number A counts protons plus neutrons, and atoms of the same element with different neutron counts are isotopes. In a neutral atom the number of electrons equals the number of protons; gain or lose electrons and you get an ion, but the element does not change.

The question is really a definition check dressed up as arithmetic, and the trap is that all three numbers given are the same, so almost any operation produces a plausible-looking figure. Resist the pull to calculate. Ask only what the atomic number is defined as, take the proton count, and stop. Then use the leftover data as a check: equal electrons and protons means the atom is neutral, and protons plus neutrons gives the mass number 12, which identifies the atom as carbon-12.

Key facts

The atomic number is the number of protons in the nucleus and it alone fixes the identity of the element.
The mass number is the sum of protons and neutrons; here that is 12.
A neutral atom has equal numbers of protons and electrons.
Atomic number 6 with mass number 12 is carbon-12, the isotope on which the unified atomic mass unit is defined.

Reading the three counts correctly

Atomic number Z = number of protons→Z = 6, so the answer is option (a) and the element is carbon

Mass number A = protons + neutrons→A = 6 + 6 = 12, which is the trap offered as option (b)

Neutral atom: electrons = protons→6 electrons confirm the atom carries no net charge

Symbol→Carbon-12, written with 12 above and 6 below the letter C

Only the proton count answers the question; the rest of the data is confirmation.

Study next

Structure of the atom — protons, neutrons and electrons and where each sitsIsotopes and isobars, and how they differ from one anotherElectronic configuration and the filling of shellsIons, and how gaining or losing electrons changes charge but not identityThe modern periodic table arranged by atomic number

Common traps

Reporting the mass number when the atomic number is asked for.
Adding the electron count into either number — electrons contribute negligible mass and no nuclear charge.

NDA GAT asks atomic structure almost every session, often by giving the counts and asking for one derived quantity, so keep the two definitions crisply separate.

Related PYQs

UPSC_1996_GS1_Q1271996Open & attempt →

The alpha particle carries two positive charges. Its mass is very nearly equal to that of

(a) two protons
(b) an atom of helium
(c) sum of masses of two positrons and two neutrons
(d) two positrons as each positron carries a single positive charge

Answer(b) an atom of helium

Works the same accounting in the other direction — two protons and two neutrons make an alpha particle, whose mass matches a helium atom because electrons weigh almost nothing.

NDA_GAT_2020_I_II_Q942020Open & attempt →

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

(a) Melting point
(b) Atomic number
(c) Colour
(d) Atomic weight

Answer(b) Atomic number

States why the proton count is the number asked for here — atomic number, not mass, is what makes an element the element it is.

Practice

practice — not a real PYQ
An atom has 11 protons, 12 neutrons and 11 electrons. Its mass number is
- (a)11
- (b)12
- (c)23
- (d)34
Answer(c) 23 — mass number is protons plus neutrons, that is 11 + 12.
practice — not a real PYQ
Two atoms having the same atomic number but different mass numbers are called
- (a)isobars
- (b)isotopes
- (c)isomers
- (d)allotropes
Answer(b) isotopes — same element, same proton count, different numbers of neutrons.