What is 1 atomic mass unit (amu)?

1 amu is defined as exactly 1/12 of the mass of one Carbon-12 atom. It is equal to 1.6605 × 10⁻²⁴ grams. The symbol 'u' (unified atomic mass unit) is commonly used, as is 'Da' (dalton).

Why was Carbon-12 chosen as the standard?

Carbon-12 was adopted as the standard in 1961 by the IUPAC because it is stable, abundant, easily ionized in mass spectrometers, and its mass gives whole-number values for the mass numbers of most light elements.

What is the difference between atomic mass and mass number?

Mass number is the total number of protons and neutrons in an atom's nucleus — always a whole number. Atomic mass (also called relative atomic mass) is the weighted average mass of all naturally occurring isotopes of an element, measured in amu — often a decimal (e.g., Cl = 35.453).

Why is chlorine's atomic mass 35.5 and not a whole number?

Chlorine has two main isotopes: Cl-35 (34.969 u, 75.8% abundance) and Cl-37 (36.966 u, 24.2% abundance). The weighted average is (34.969 × 0.758) + (36.966 × 0.242) = 35.453 amu ≈ 35.5. The decimal reflects the fact that natural chlorine is a mixture of both isotopes.

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Atomic Mass Unit

PhysicalEasyNEET: 1-2 Q3D

Atomic Mass Unit — How Heavy Is One Atom?

Structure of Atom

Carbon-12 (12.000 u) is the standard for atomic mass. Add atoms of different elements to a 3D balance scale and discover how many Hydrogen atoms (1.008 u each) it takes to balance one Carbon-12 atom.

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Built by Animesh Singh · @neetlab

Element select karo aur + daba ke atoms add karo — dekho kitne atoms mein C-12 balance hota hai

How to use this lab

1Right pan pe atoms add karke C-12 (12.000 u) se compare karo.
2Average atomic mass weighted-average se kaise banta hai dekho.
3Isotopes ka contribution observe karo.

What you're seeing

1 amu (atomic mass unit) = 1/12 of one Carbon-12 atom's mass. Iska matlab hai ki C-12 exactly 12.000 amu ka hota hai. Baaki elements ki atomic mass C-12 se compare karke measure ki jaati hai. Hydrogen 1.008 amu (C-12 se 12x halka), Oxygen 15.999 amu (C-12 se thoda bhaari).

Key NEET facts

▸1 amu = 1/12 of C-12: 1.6605 × 10⁻²⁴ g — Itna chhota mass hai ki grams mein likhna mushkil hai — isliye amu scale banaya gaya.
▸Why Carbon-12?: Chosen as standard in 1961 — Stable, abundant, easily ionized in mass spectrometer, and gives whole-number masses for many elements.
▸Atomic mass ≠ mass number: Mass number = protons + neutrons — Mass number is always a whole number. Atomic mass is a weighted average of all isotopes — often a decimal.
▸Why is Cl = 35.5?: 75% Cl-35 + 25% Cl-37 — Weighted average: (34.969 × 0.7577) + (36.966 × 0.2423) = 35.453 amu.