☰ Contents · Physics

Structure of matter and the size of molecules

Lessons 1–2 · 2 lessons · P. Habibullayev, A. Boydedayev, A. Bahromov, K. Suyarov, J. Usarov, M. Yuldasheva. Physics Grade 9, revised and expanded 3rd edition. G‘afur G‘ulom Publishing House, Tashkent, 2019
2

Mass and size of a molecule

Textbook: pp. 7–11
GoalKnow the concepts of molecule, atomic mass unit, relative atomic and molecular mass; estimate the size and mass of a molecule.
New words
molecule · molekulaatomic mass unit (u) · massa atom birligi (u)relative molecular mass · nisbiy molekulyar massaoil-film method · moy parda usuli
Explanation

A molecule is the smallest particle of a substance that keeps its chemical properties; it consists of one or more atoms (a water molecule H₂O has two hydrogen atoms and one oxygen atom). The size of molecules can be estimated by the oil-film method: a drop of oil of volume V spreads over water into a layer one molecule thick, so d = V/S. Atoms and molecules have diameters of about 10⁻¹⁰ m and cannot be seen with an optical microscope; a scanning tunnelling microscope can image them. Because masses in kg are awkward, the atomic mass unit is used: 1 u is 1/12 of the mass of a ¹²C carbon atom, 1 u ≈ 1.66·10⁻²⁷ kg. Relative atomic mass is the atom’s mass divided by 1 u (a dimensionless number); relative molecular mass M_r is the sum of the relative atomic masses of the atoms in the molecule, e.g. M_r(H₂O) = 2·1 + 16 = 18.

Worked examples
Relative molecular mass of methane CH₄: M_r = 12 + 4·1 = 16 (C = 12, H = 1). So a methane molecule has a mass of about 16 u, i.e. 16·1.66·10⁻²⁷ kg ≈ 2.7·10⁻²⁶ kg.
A drop of oil of volume 0.5 mm³ spread over 0.25 m² of water. The molecule diameter is d = V/S = (0.5·10⁻⁹ m³)/(0.25 m²) = 2·10⁻⁹ m. This is an upper estimate: the film may not be one molecule thick everywhere, so a real molecule is smaller.
Class activity

Scale thinking: if 10⁻¹⁰ m is drawn as 1 mm (everything enlarged 10⁷ times), how thick would a 0.1 mm notebook page appear? Work it out in your notebook and discuss in class.

Practice
1
What is the atomic mass unit (u) and how is it defined?
2
Find the relative molecular mass of carbon dioxide CO₂ (C = 12, O = 16).
3
Roughly what is the mass of an O₂ molecule? (1 u ≈ 1.66·10⁻²⁷ kg, M_r = 32)
4
Why does the oil-film method give only an estimate of the molecule diameter?