Chemistry · Some Basic Concepts Of Chemistry · NEET
1 amu is a tiny unit of mass. It equals exactly 1/12 (one-twelfth) of the mass of a single carbon-12 atom. We use it because a real atom weighs far too little to measure in grams easily. In grams, 1 amu = 1.66056 x 10^-24 g. For NEET, remember it is a RELATIVE unit: it compares every atom to carbon-12.
Older systems used hydrogen (mass = 1) or oxygen (mass = 16). But scientists agreed in 1961 to use carbon-12 as the world standard. Carbon-12 is a stable, common isotope of carbon and gives simpler, more consistent atomic mass values. Once carbon-12 is fixed at exactly 12 u, all other atomic masses are decided by comparing them to it.
There is no difference in size. 'amu' is the old name and 'u' (unified atomic mass unit) is the new, official name. NCERT says 'amu' has been replaced by 'u', which is called the unified mass. So 12 amu and 12 u mean exactly the same mass. In NEET, both symbols are correct.
By definition, one whole carbon-12 atom has a mass of exactly 12 u. We then split this into 12 equal parts, and one part is defined as 1 u (1 amu). So the '12' is the mass in u, and it is also how many 1-u pieces fit into one carbon-12 atom. Both meanings point to the same idea.
On the carbon-12 scale, we measure the mass of a hydrogen atom (1.6736 x 10^-24 g) and divide it by 1 amu (1.66056 x 10^-24 g). This gives about 1.008 u, not a perfect 1. Atomic masses are almost never whole numbers because of how nuclear mass compares to the carbon-12 standard and because of isotope mixing. This is covered in detail in the next concept.
Almost, but not exactly. A proton and a neutron each have a mass very close to 1 u, so people often approximate them as 1 u for quick counting. But 1 amu is officially tied to carbon-12, NOT to a proton or neutron. For NEET calculations, treat 1 amu strictly as 1/12 of a carbon-12 atom.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
1 amu = 1.66056 x 10^-24 g, which is the same as 1.66056 x 10^-27 kg. This is an extremely small mass because atoms are extremely light.
It was agreed upon internationally in 1961. Before that, oxygen (=16) was used as the standard. Carbon-12 replaced it to give one consistent global scale.
Yes. 1 amu = 1 u = 1 dalton (Da). They are three names for the same unit of atomic-scale mass. NEET usually uses u or amu.
Because a single atom weighs about 10^-24 grams, which is too small and clumsy to use. The amu gives simple, easy numbers (like carbon = 12, oxygen = 16) that are easy to compare and calculate with.
amu measures the mass of ONE atom (or one molecule as u). When you take the same number in grams instead of u, you get the mass of one MOLE. For example, carbon: 12 u for one atom, 12 g for one mole.
