Chemistry · Some Basic Concepts Of Chemistry · NEET
No. This is the most common mistake. Two samples with the same mass can have very different numbers of atoms. For example, 1 g of lithium has far more atoms than 1 g of silver, because lithium is very light (7 g/mol) and silver is very heavy (108 g/mol). Always convert mass to moles first using moles = mass / molar mass. Never compare grams directly.
Step 1: Convert every option to moles. Use moles = mass / molar mass for a mass, moles = volume / 22.7 L for a gas at STP, or use the mole value if it is already given. Step 2: Multiply moles by Avogadro number (6.022 x 10^23) only if you need the actual count. Step 3: For ATOMS, also multiply by the number of atoms in one molecule or formula unit. The biggest final value wins.
It is how many atoms are inside one molecule or one formula unit. O2 has 2 atoms. Li is monatomic, so 1 atom. H2 has 2 atoms. CO2 has 3 atoms (1 C + 2 O). Na2CO3 has 6 atoms (2 Na + 1 C + 3 O). NaOH has 3 atoms. When the question asks for MOLECULES you skip this step. When it asks for ATOMS you must include it.
Because moles = mass / molar mass. If the mass is fixed (say 1 g), a smaller molar mass in the bottom of the fraction gives a bigger number of moles, and more moles means more particles. That is why for 'which 1 g sample has the most atoms', the lightest monatomic element wins. In NEET 2020, 1 g of Li (molar mass 7) beat O2, Ag and Mg.
Convert litres to moles first. At STP, 1 mole of any gas takes 22.7 L (the older value is 22.4 L; NEET now uses 22.7 L, so read the numbers in the option). So moles = volume in litres / 22.7. In NEET 2024, 2.271 L of helium at STP = 0.1 mole, which was much smaller than 4 moles of helium.
Bring every option to the same unit: moles (or number of particles). A value in atomic mass units (u) is a trap - 4 u of helium is just 1 single atom, not a mole. In NEET 2024, '4 u of helium' equals only 1 atom while '4 mol of helium' equals 4 x Avogadro atoms, the maximum.
Which one of the following has the maximum number of atoms? (at. masses: O = 16, Li = 7, Ag = 108, Mg = 24)
In which of the following is the number of water molecules maximum?
The highest number of helium atoms is present in
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Always convert each option to moles first, then multiply by atoms per molecule or formula unit. The largest result has the most atoms. Never compare grams directly.
NEET now uses 22.7 L per mole at STP (0 degrees C, 1 bar). The old value 22.4 L is for 1 atm. Read the numbers in the option - NEET usually writes them so the answer is clear either way.
Because lithium has the smallest molar mass (7 g/mol) and is monatomic. Smaller molar mass means more moles for the same 1 g, and monatomic means every particle is one atom, so it gives the maximum atoms.
Almost. For molecules, stop at moles x NA. For atoms, do one more step: multiply by the number of atoms in one molecule or formula unit (O2 = 2, CO2 = 3, Na2CO3 = 6).
A value written in 'u' (atomic mass units) is not a mole. For example 4 u of helium is just 1 single atom. Also do not assume equal mass means equal atoms.
