Select the option that correctly matches the contents of the first column with the contents of the second column.

A.	Elements combine in fixed ratios	I.	Dalton's atomic theory
B.	Atoms are indivisible	II.	The same number of molecules
C.	Sulphate and oxalate ions are anions, whereas magnesium and	III.	Law of constant proportions
D.	Gram atomic mass of an element and the gram molecular mass of a compound contain	IV.	Ammonium ions are cations

The correct answer is A - III, B - I, C - IV, D - II

Key Points

• According to Dalton's theory, matter consists of indivisible atoms and all the atoms of an element have identical properties including identical mass.
  • ​It explained the law of chemical combination.
  • This theory also stated that compounds are formed when atoms of different elements combine in a fixed ratio.
•  According to the law of constant proportions, a given compound always contains exactly the same proportion of elements by weight, for example, carbon dioxide (CO₂) contains both carbon and oxygen compounds combined in the fixed ratio of 3:8 by weight.
  • This is also called as law of definite proportions.
• Cations are positively charged ions i.e, more protons than electrons and are formed when a metal loses its electrons.
  • For example, Ca²⁺, K⁺, Ag⁺, Zn²⁺, Na⁺ etc.
• ​Anions are negatively charged ions i.e., more electrons than protons, formed when a non-metal gains electrons.
  • ​For example, OH^-, Cl^- etc.
• Sulphate is a compound made of sulfur and oxygen, and it is an anion with the formula SO₄^2- whereas oxalate is an anion with the formula C₂O₄^2-. and Ammonium ion (NH₄⁺) is a cation.
• Atomic mass is the mass exactly the same as the one-twelfth mass of one carbon-12 atom and molecular mass is the sum of atomic masses of all the elements present in a molecule.
• So, the gram atomic mass of an element and the gram molecular mass of a compound contains the same number of molecules. Hence option 2 is correct.