CDS (II) 2024 General Knowledge · Question 19 of 120

Correct — B, Electrons. A chemical reaction is a rearrangement of electrons — atoms give them up, take them on, or share them, and nothing happens to the nucleus at all. So it is the electrons, and above all the ones in the outermost shell, that decide how an element behaves: how many bonds it forms, whether it is a metal or a non-metal, and how vigorously it reacts. The decisive test is the behaviour of isotopes. Chlorine-35 and chlorine-37 have different numbers of neutrons and therefore different nuclei and different masses, yet they are chemically indistinguishable — they form the same compounds in the same proportions, because they have the same seventeen electrons arranged the same way. That single comparison eliminates neutrons and the nucleus together. Protons are a subtler case: their number fixes the element and therefore fixes how many electrons a neutral atom has, so they set the chemistry indirectly, but the reactions themselves are electron business.

• (a)Protons — The proton count is the atomic number and it decides which element an atom is, so it fixes the electron arrangement indirectly. But protons stay locked in the nucleus and take no part in bonding; changing the proton number would not modify an element's chemistry, it would produce a different element altogether.
• (c)Neutrons — Neutrons add mass and affect nuclear stability, not chemistry. Isotopes of one element differ only in neutron number and yet behave identically in chemical reactions, which is the clearest possible proof that neutrons do not govern chemical properties.
• (d)Nucleus — The nucleus holds the protons and neutrons and carries almost the whole mass of an atom, but it is untouched by chemical change. Reactions that alter the nucleus are nuclear reactions, a different subject with different energies altogether.

An atom has a tiny dense nucleus of protons and neutrons, with electrons occupying shells around it. The atomic number, which is the proton count, identifies the element; the mass number, protons plus neutrons, gives the mass. In a neutral atom the electron count equals the proton count, and those electrons fill shells from the innermost outward. The electrons in the outermost shell, called valence electrons, are the ones exposed to other atoms and the ones that take part in bonding.

The habit worth building here is to separate chemical change from nuclear change. Burning, rusting, dissolving, neutralising — all of these are electron rearrangements at ordinary energies. Radioactive decay and fission are nuclear, occur at energies thousands of times greater, and are unaffected by chemical form. Once that line is drawn, the answer to this item is immediate, and so are several neighbouring facts: elements in the same group of the periodic table share chemical properties because they have the same number of valence electrons; the noble gases are unreactive because their outer shells are already full; and heavy-water chemistry works because deuterium, an isotope of hydrogen, behaves chemically like ordinary hydrogen.

• Chemical reactions involve only the electrons, chiefly those in the outermost shell; the nucleus is unchanged.
• Isotopes of an element differ in neutron number but have identical chemical properties.
• The number of protons is the atomic number and identifies the element; in a neutral atom it also equals the number of electrons.
• Elements in the same group of the periodic table behave alike because they have the same number of valence electrons.
• The noble gases are chemically inert because their outermost shells are already complete.

• Choosing protons because they define the element; defining an element is not the same as governing its reactions.
• Choosing the nucleus because it carries most of the mass; mass has nothing to do with chemical behaviour.
• Forgetting the isotope test, which settles the item in one line.

Usually as a which-particle item, or through valency and electronic configuration; the isotopes-behave-alike point is a favourite second step.

• practice — not a real PYQ

  Two isotopes of the same element have identical chemical properties because they have the same

  • (a)number of neutrons
  • (b)mass number
  • (c)electronic configuration
  • (d)nuclear density

  Answer(c) electronic configuration — isotopes differ only in neutrons, and since reactions involve electrons alone, their chemistry is the same.

• practice — not a real PYQ

  The noble gases are chemically almost inert because their atoms have

  • (a)no electrons in the outermost shell
  • (b)completely filled outermost shells
  • (c)more neutrons than protons
  • (d)unusually heavy nuclei

  Answer(b) completely filled outermost shells — with no partly filled outer shell there is nothing to gain, lose or share, so they rarely react.