RRB Group D Atoms & Molecules

Every atom has three main particles - protons (positive charge), neutrons (no charge), and electrons (negative charge). The nucleus contains protons and neutrons, while electrons orbit around it. Atomic number equals the number of protons. Mass number equals protons plus neutrons

- Atomic Number (Z) = Number of protons - Mass Number (A) = Number of protons + Number of neutrons - Number of neutrons = Mass Number - Atomic Number - Number of molecules = Given mass / Molecular mass - Avogadro's number = 6.022 × 10²³

Carbon-14 has mass number 14 and atomic number 6, so neutrons = 14-6 = 8. Shortcut Trick #2 - Molecule Counting: To find number of molecules, use the formula: Number of molecules = (Given mass × Avogadro's number) / Molecular mass. Remember Avogadro's number as 6 × 10²³ for quick calculations

Students often confuse atomic mass with mass number. Atomic mass is the average mass considering all isotopes, while mass number is the total protons plus neutrons in a specific isotope. In exams, always use mass number for calculations unless specifically asked for atomic mass. Another frequent error is forgetting to multiply by Avogadro's number when converting moles to molecules

moles give you the amount, but molecules require multiplication by 6.022 × 10²³.