Q.What is an atom and a moleule ? What is the order of magnitude of mass of one atom ? What are isotopes?

Step 1. An atom is the smallest particle of an element that still shows that element's chemical identity — it cannot be divided further by ordinary chemical means.

Step 2. A molecule is a group of two or more atoms held together by chemical bonds; it is the smallest unit of a compound (or of an element that naturally exists as bonded atoms, such as O2) that displays that substance's properties.

Step 3. A single atom's mass is extraordinarily small — of the order of 10^-24 to 10^-23 gram (for instance, one hydrogen atom weighs about 1.6736 x 10^-24 g), which is why chemists use a relative atomic-mass scale instead of grams for everyday work.

Step 4. Isotopes are atoms of the SAME element (same number of protons, hence same atomic number and same chemical behaviour) that differ in the number of neutrons in the nucleus, and therefore differ in atomic mass — carbon-12, carbon-13 and carbon-14 are isotopes of carbon.

Recall the standard definitions of atom, molecule and isotope, and the typical order of magnitude of an atom's mass in grams.

• Confusing 'atomic mass' (relative, unitless on the u scale) with the actual mass of an atom in grams, which is what this question specifically asks for.
• Thinking isotopes of an element have different chemical properties — they don't; only the mass (and some nuclear behaviour) differs.