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Atoms, molecules and relative atomic mass
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Most elements are a mix of isotopes. Relative atomic mass is the average mass, weighted by how common each isotope is.
Atoms and molecules
An atom is the smallest particle of an element that can take part in a chemical reaction.
A molecule is two or more atoms chemically bonded together, for example O2 or H2O.
Atomic number: number of protons. Mass number: protons + neutrons. Isotopes: atoms of the same element with different numbers of neutrons.
A molecule is two or more atoms chemically bonded together, for example O2 or H2O.
Atomic number: number of protons. Mass number: protons + neutrons. Isotopes: atoms of the same element with different numbers of neutrons.
Relative atomic mass (A_r)
A_r is the weighted mean mass of the isotopes of an element, compared with 1/12 of the mass of a carbon-12 atom.
A_r = (mass number × % abundance + mass number × % abundance) ÷ 100
A_r = (mass number × % abundance + mass number × % abundance) ÷ 100
Worked example: chlorine
Chlorine is 75% chlorine-35 and 25% chlorine-37.
A_r = (35 × 75 + 37 × 25) ÷ 100 = (2625 + 925) ÷ 100 = 35.5
The answer is closer to 35 because chlorine-35 is more common.
A_r = (35 × 75 + 37 × 25) ÷ 100 = (2625 + 925) ÷ 100 = 35.5
The answer is closer to 35 because chlorine-35 is more common.
Boron is 20% boron-10 and 80% boron-11. Calculate its relative atomic mass.
- (10 × 20 + 11 × 80) ÷ 100
- (200 + 880) ÷ 100
Answer: 10.8
An atom is the smallest particle of an element that can react; a molecule is bonded atoms. Isotopes differ in neutrons. A_r is the weighted mean of isotope masses: multiply each mass number by its percentage, add, divide by 100.
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