Atoms and Molecules

6.02 × 10²³

Step 1: The mole is defined as the amount of substance containing as many particles as there are atoms in exactly 12g of carbon-12. Step 2: This number of atoms in 12g of carbon-12 is experimentally determined to be 6.02 × 10²³. Step 3: This number is called Avogadro's number and represents one mole of any substance. Step 4: Therefore, exactly 12g of carbon-12 contains 6.02 × 10²³ atoms by definition. Other options are either too large or too small for this fundamental constant.

2

Step 1: Valency is the combining capacity of an element with other elements. Step 2: Oxygen typically forms two bonds with other atoms to achieve a stable configuration. Step 3: In compounds like H2O, oxygen combines with 2 hydrogen atoms, and in CO2, oxygen needs 2 bonds with carbon. Step 4: Therefore, oxygen has a valency of 2. Valencies of 1, 3, or 4 would not match oxygen's typical bonding pattern in common compounds.

¹H, ²H, ³H

Step 1: Isotopes are atoms of the same element with the same atomic number but different mass numbers. Step 2: Hydrogen has atomic number 1 (1 proton) but can have different numbers of neutrons. Step 3: ¹H (protium) has 0 neutrons, ²H (deuterium) has 1 neutron, and ³H (tritium) has 2 neutrons. Step 4: All have 1 proton, making them hydrogen isotopes. H, He, Li are different elements; H⁺, H⁻, H are ions and atom; H₂, H₃, H₄ represent molecules, not isotopes.

Number of protons + neutrons

Step 1: Mass number (A) represents the total number of particles in the nucleus of an atom. Step 2: The nucleus contains only protons and neutrons, which are called nucleons. Step 3: Both protons and neutrons have approximately the same mass (about 1 atomic mass unit each). Step 4: Therefore, mass number = number of protons + number of neutrons. Electrons have negligible mass compared to nucleons and are not included in mass number calculation.