NDA (II) 2018 General Ability Test · Question 62 of 146

Correct — B, Two. Atomic number 8 is oxygen, so a neutral atom has eight electrons distributed as two in the first shell and six in the second, written 2,6. The outermost shell is two electrons short of the octet, so oxygen gains two electrons and becomes the oxide ion, O²⁻, with the stable arrangement of neon. Sodium does the opposite: atomic number 11, arrangement 2,8,1, it loses its single outer electron to become Na⁺. Two sodium atoms are therefore needed to supply the two electrons one oxygen atom takes, and the compound formed is sodium oxide, Na₂O. The number asked for is the number oxygen gains, which is two.

• (a)One — One electron is what a halogen needs. Chlorine, arrangement 2,8,7, is a single electron short of the octet, which is why sodium and chlorine combine one to one as NaCl. Oxygen is two short, not one.
• (c)Three — Three is the count for a group-15 element such as nitrogen, arrangement 2,5, which forms the nitride ion N³⁻ and combines with sodium as Na₃N. Oxygen sits one group to the right of nitrogen and needs one electron fewer.
• (d)Four — Four is carbon's shortfall, arrangement 2,4 — and carbon is so far from a stable ion that it shares electrons rather than transferring them, forming covalent bonds instead of ionic ones.

Atoms combine so as to reach the electron arrangement of the nearest noble gas, which for the light elements means eight electrons in the outermost shell. A metal with few outer electrons gives them away and becomes a positive ion; a non-metal a few short of eight takes them and becomes a negative ion. The number an atom gains or loses is its valency, and the formula of the compound follows from balancing the charges. Sodium has valency 1 and oxygen valency 2, so two sodium ions pair with one oxide ion.

Work such items in three fixed steps rather than by recall. First turn the atomic number into an electron arrangement — 8 gives 2,6. Second, count the shortfall from eight in the outer shell — that is 2, and it is the answer to the question actually asked. Third, if the formula is wanted, balance the charges — two Na⁺ against one O²⁻ gives Na₂O. The step students skip is the first, jumping straight to 'oxygen is divalent' from memory; the arrangement takes five seconds to write and protects you when the question hands you an unfamiliar atomic number instead of a familiar element.

• Oxygen has atomic number 8 and the electron arrangement 2,6, giving it a valency of 2.
• Gaining two electrons produces the oxide ion O²⁻, which has neon's stable arrangement of 2,8.
• Sodium has atomic number 11 and the arrangement 2,8,1, and loses one electron to form Na⁺.
• Balancing one O²⁻ against two Na⁺ gives the formula Na₂O for sodium oxide.
• Metals lose electrons and form positive ions; non-metals gain electrons and form negative ions.

• Answering with the number of electrons in the outer shell (six) instead of the number needed to complete it (two).
• Reading the question as asking for the formula and answering with the subscript on sodium.
• Assuming every non-metal needs one electron because chlorine does.

NDA gives an atomic number and asks for the valency, the number of valence electrons, the ion formed or the formula of a compound — always one step of the same chain.

• practice — not a real PYQ

  An element X has atomic number 12. The formula of the compound it forms with chlorine is

  • (a)XCl
  • (b)XCl₂
  • (c)X₂Cl
  • (d)XCl₃

  Answer(b) XCl₂ — atomic number 12 gives the arrangement 2,8,2, so X loses two electrons to form X²⁺, and two chloride ions are needed to balance it.

• practice — not a real PYQ

  How many electrons does an atom of nitrogen (atomic number 7) need to gain to complete its octet?

  • (a)One
  • (b)Two
  • (c)Three
  • (d)Five

  Answer(c) Three — nitrogen's arrangement is 2,5, so it is three electrons short of eight in the outer shell.