Matter, Atoms and Molecules, Laws of Chemical Combination, Atomic Structure and Models

What to remember

• Matter has mass and occupies space. Pure substances are elements and compounds; mixtures are physical combinations. Chemical combination follows the laws of conservation of mass, constant proportions and multiple proportions.
• One mole contains 6.022 x 10²³ particles (Avogadro number) and has a mass equal to the atomic or molecular mass in grams; 1 mole of any gas at STP occupies 22.4 litres.
• An atom has a small, dense, positive nucleus (protons and neutrons) with electrons in shells; atomic number Z = protons, mass number A = protons + neutrons. Electrons fill shells in the order K, L, M, N with a maximum of 2n² electrons.

Matter and its classification

Matter is anything that has mass and occupies space. It exists as solid, liquid and gas (and plasma). Solids have a fixed shape and volume; liquids a fixed volume but no fixed shape; gases neither. Particles of matter are very small, have spaces between them, are in constant motion, and attract each other. Interchange of states: melting, freezing, evaporation, condensation, sublimation.

A compound has properties different from its elements and can be separated only by chemical means; a mixture keeps the properties of its parts and can be separated by physical methods (filtration, distillation, evaporation, magnet).

Laws of chemical combination

• 1. Law of conservation of mass (Lavoisier): in a chemical reaction the total mass of reactants equals the total mass of products. Mass is neither created nor destroyed.
• 2. Law of constant (definite) proportions (Proust): a pure compound always contains the same elements in the same fixed proportion by mass, whatever its source. In water, hydrogen and oxygen are always in the ratio 1 : 8 by mass.
• 3. Law of multiple proportions (Dalton): when two elements form more than one compound, the masses of one element that combine with a fixed mass of the other are in a simple whole-number ratio. Example: CO and CO₂ have oxygen masses of 16 and 32 for 12 of carbon, a ratio of 1 : 2.
• 4. Gay-Lussac's law of gaseous volumes: gases react in simple whole-number ratios of volumes at the same temperature and pressure.
• 5. Avogadro's law: equal volumes of all gases at the same temperature and pressure contain the same number of molecules.

Worked example: 12 g of carbon burns with 32 g of oxygen to give 44 g of carbon dioxide (mass conserved). 3 g of carbon needs 8 g of oxygen (constant proportion 3 : 8).

Dalton's atomic theory

Matter is made of tiny indivisible particles called atoms. Atoms of one element are alike in mass and properties; atoms of different elements differ. Atoms combine in small whole-number ratios to form compounds. Atoms are neither created nor destroyed in a chemical reaction. Later discoveries showed that atoms are divisible into sub-atomic particles and that isotopes of the same element differ in mass.

Atoms, molecules, ions and the mole

• Symbols and formulae: symbols are one or two letters (H, O, Na from natrium, K from kalium, Fe from ferrum, Cu from cuprum, Ag from argentum, Au from aurum, Pb from plumbum).
• Molecule: a group of two or more atoms bonded together that can exist independently. Atomicity: the number of atoms in a molecule: He, Ne are monatomic; O₂, N₂, H₂, Cl₂ are diatomic; O₃ is triatomic; P₄ tetra-atomic; S₈ octa-atomic.
• Ion: a charged atom or group of atoms. A cation is positive (Na⁺); an anion is negative (Cl⁻). Polyatomic ions: NH₄⁺, SO₄²⁻, NO₃⁻, OH⁻.
• Atomic mass unit (u): 1/12 of the mass of one carbon-12 atom. Atomic masses: H = 1, C = 12, N = 14, O = 16, Na = 23, Cl = 35.5 (approx.).
• Molecular mass: sum of the atomic masses. H₂O = 18 u, CO₂ = 44 u, NaCl = 58.5 u (a formula unit mass for an ionic solid).
• Mole: the amount of substance containing 6.022 x 10²³ elementary particles (Avogadro's number, Nₐ).
• Moles = given mass / molar mass = number of particles / Nₐ = volume at STP / 22.4 L.

Worked example 1: 18 g of water is 1 mole, so it has 6.022 x 10²³ molecules.

Worked example 2: 0.5 mole of CO₂ has mass 0.5 x 44 = 22 g.

Worked example 3: 5.6 litres of any gas at STP is 5.6/22.4 = 0.25 mole.

Worked example 4: 1 mole of water molecules has 3 moles of atoms (2 H + 1 O).

Structure of the atom: sub-atomic particles

• Atomic number (Z) = number of protons = number of electrons in a neutral atom. Mass number (A) = protons + neutrons. Neutrons = A − Z. For ²⁷Al (Z = 13): neutrons = 14.
• Isotopes: same Z, different A (hydrogen's protium, deuterium, tritium; carbon-12 and carbon-14; chlorine-35 and chlorine-37). Same chemical properties. Isobars: same A, different Z (⁴⁰Ar and ⁴⁰Ca). Isotones: same number of neutrons.

Atomic models

Rutherford's observations: most alpha particles passed straight through; a few were deflected at small angles; very few rebounded straight back. The nucleus is about 10⁻¹⁵ m, much smaller than the atom (about 10⁻¹⁰ m).

Electronic configuration

Bohr-Bury scheme: the maximum number of electrons in a shell is 2n²: K (n = 1) = 2, L = 8, M = 18, N = 32. The outermost shell holds at most 8 electrons (2 for K), the octet rule. The outermost electrons are the valence electrons, and valency is the number of electrons lost, gained or shared to reach a full outer shell.

Noble gases (He, Ne, Ar, Kr, Xe, Rn) have complete outer shells and are chemically inert.

Subshells and orbitals: each shell has subshells s, p, d, f holding 2, 6, 10 and 14 electrons. Aufbau principle: orbitals fill in order of increasing energy: 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s... Pauli exclusion principle: no two electrons in an atom have the same four quantum numbers; an orbital holds at most two electrons of opposite spin. Hund's rule: in orbitals of equal energy, electrons occupy separate orbitals singly before pairing. Examples: Na = 1s² 2s² 2p⁶ 3s¹; Cl = 1s² 2s² 2p⁶ 3s² 3p⁵. Chromium (3d⁵ 4s¹) and copper (3d¹⁰ 4s¹) show half-filled and fully-filled stability.

The four quantum numbers: principal (n, shell size), azimuthal (l, shape of the subshell), magnetic (m, orientation) and spin (s, ±1/2).

Exam traps

• Mass number is not the same as atomic mass; the number of neutrons is A − Z.
• Isotopes have the same Z; isobars have the same A.
• A compound is not a mixture; air and sea water are mixtures.
• The law of constant proportions concerns mass ratios in a compound; multiple proportions concerns different compounds of the same elements.
• Avogadro's number is a count; the molar volume 22.4 L applies to gases at STP only.
• Rutherford discovered the nucleus; Chadwick the neutron; Thomson the electron.
• The Bohr orbit capacity is 2n²; the outer shell never exceeds 8.
• Atoms are not truly indivisible, despite Dalton's theory.

One-liners

• 1. Lavoisier gave the law of conservation of mass.
• 2. Proust gave the law of constant proportions.
• 3. Atomic mass unit is 1/12 of a carbon-12 atom.
• 4. Avogadro's number is 6.022 x 10²³.
• 5. One mole of a gas at STP occupies 22.4 L.
• 6. Electron was discovered by J.J. Thomson.
• 7. Neutron was discovered by Chadwick.
• 8. Rutherford's gold foil experiment showed the nucleus.
• 9. Mass number = protons + neutrons.
• 10. Shell capacity is 2n².
• 11. Sodium configuration is 2, 8, 1.
• 12. Noble gases have complete outer shells.

Practice questions

1. The law of conservation of mass was given by

   1. Proust
   2. Avogadro
   3. Dalton
   4. Lavoisier
   Answer

   D. Lavoisier

   Lavoisier showed that mass is conserved in a chemical reaction.

2. In water, hydrogen and oxygen are always present in the mass ratio

   1. 1 : 8
   2. 1 : 16
   3. 1 : 2
   4. 2 : 1
   Answer

   A. 1 : 8

   Every pure sample of water has 1 part hydrogen to 8 parts oxygen by mass.

3. The law of constant proportions was proposed by

   1. Rutherford
   2. Proust
   3. Gay-Lussac
   4. Lavoisier
   Answer

   B. Proust

   Proust stated that a compound has a fixed ratio of elements by mass.

4. Which of the following is a mixture?

   1. Sodium chloride
   2. Water
   3. Air
   4. Carbon dioxide
   Answer

   C. Air

   Air has nitrogen, oxygen and other gases in variable proportion.

5. The value of Avogadro's number is

   1. 6.022 x 10^22
   2. 6.022 x 10^23
   3. 1.6 x 10^-19
   4. 3.0 x 10^8
   Answer

   B. 6.022 x 10^23

   One mole of any substance has 6.022 x 10^23 particles.

6. The chemical symbol of iron, taken from its Latin name ferrum, is

   1. I
   2. Ir
   3. F
   4. Fe
   Answer

   D. Fe

   Fe is from ferrum.

7. One mole of any gas at STP occupies a volume of

   1. 22.4 litres
   2. 44.8 litres
   3. 2.24 litres
   4. 11.2 litres
   Answer

   A. 22.4 litres

   Molar volume at STP is 22.4 L.

8. The atomic mass unit is defined as

   1. 1/12 of the mass of a carbon-12 atom
   2. the mass of a proton plus an electron
   3. the mass of a hydrogen atom
   4. 1/16 of the mass of oxygen
   Answer

   A. 1/12 of the mass of a carbon-12 atom

   The unified atomic mass unit is based on carbon-12.

9. The electron was discovered by

   1. Rutherford
   2. Goldstein
   3. Chadwick
   4. J.J. Thomson
   Answer

   D. J.J. Thomson

   Thomson discovered the electron using cathode rays.

10. The neutron was discovered by

    1. Goldstein
    2. James Chadwick
    3. Bohr
    4. Dalton
    Answer

    B. James Chadwick

    Chadwick discovered the neutral particle in the nucleus.

11. The nucleus of the atom was discovered in the experiment on scattering of alpha particles by

    1. a thick lead block
    2. a hydrogen gas
    3. a thin gold foil
    4. a glass plate
    Answer

    C. a thin gold foil

    Rutherford's gold foil experiment.

12. The atomic number of an element is equal to the number of

    1. electrons in the outer shell
    2. protons in the nucleus
    3. protons plus neutrons
    4. neutrons in the nucleus
    Answer

    B. protons in the nucleus

    Z = number of protons.

13. Isotopes of an element have the same

    1. atomic mass exactly
    2. number of neutrons
    3. atomic number
    4. mass number
    Answer

    C. atomic number

    Isotopes differ in the number of neutrons.

14. The maximum number of electrons that the M shell can hold is

    1. 8
    2. 32
    3. 18
    4. 2
    Answer

    C. 18

    2n^2 = 2 x 9 = 18.

15. The electronic configuration of sodium (Z = 11) is

    1. 2, 8, 1
    2. 2, 8, 2
    3. 2, 9
    4. 2, 7, 2
    Answer

    A. 2, 8, 1

    11 electrons fill K = 2, L = 8, M = 1.

16. The valency of chlorine (Z = 17) is

    1. 1
    2. 7
    3. 2
    4. 0
    Answer

    A. 1

    Configuration 2, 8, 7: it gains 1 electron to complete the octet.

17. Which of the following is a noble gas with a complete outer shell?

    1. Oxygen
    2. Sodium
    3. Chlorine
    4. Neon
    Answer

    D. Neon

    Neon has configuration 2, 8.

18. Plum pudding model of the atom was proposed by

    1. Bohr
    2. J.J. Thomson
    3. Rutherford
    4. Chadwick
    Answer

    B. J.J. Thomson

    Thomson pictured a positive sphere with electrons embedded.

19. According to Bohr's model, an electron in a stationary orbit

    1. falls into the nucleus
    2. has no fixed energy
    3. radiates energy continuously
    4. does not radiate energy
    Answer

    D. does not radiate energy

    Energy changes only in jumps between orbits.

20. The chemical symbol of potassium is

    1. Po
    2. Pt
    3. P
    4. K
    Answer

    D. K

    K comes from the Latin kalium.

21. The molecular mass of water (H = 1, O = 16) is

    1. 17 u
    2. 20 u
    3. 18 u
    4. 16 u
    Answer

    C. 18 u

    2 x 1 + 16 = 18 u.

22. The molecular mass of carbon dioxide (C = 12, O = 16) is

    1. 44 u
    2. 12 u
    3. 28 u
    4. 32 u
    Answer

    A. 44 u

    12 + 2 x 16 = 44 u.

23. The number of molecules in 18 g of water is

    1. 3.011 x 10^23
    2. 6.022 x 10^23
    3. 12.04 x 10^23
    4. 1.8 x 10^22
    Answer

    B. 6.022 x 10^23

    18 g is 1 mole, which has Avogadro's number of molecules.

24. The mass of 0.5 mole of CO2 (molar mass 44 g/mol) is

    1. 44 g
    2. 11 g
    3. 88 g
    4. 22 g
    Answer

    D. 22 g

    0.5 x 44 = 22 g.

25. The number of moles in 5.6 litres of a gas at STP is

    1. 0.5
    2. 0.25
    3. 2.5
    4. 4
    Answer

    B. 0.25

    5.6 / 22.4 = 0.25 mole.

26. The number of neutrons in aluminium with atomic number 13 and mass number 27 is

    1. 13
    2. 40
    3. 14
    4. 27
    Answer

    C. 14

    Neutrons = A - Z = 27 - 13 = 14.

27. The number of moles in 44 g of carbon dioxide is

    1. 2
    2. 0.5
    3. 44
    4. 1
    Answer

    D. 1

    44 g / 44 g per mole = 1 mole.

28. The total number of atoms in one mole of water molecules is

    1. 3 x 6.022 x 10^23
    2. 2 x 6.022 x 10^23
    3. 6.022 x 10^23
    4. 18 x 6.022 x 10^23
    Answer

    A. 3 x 6.022 x 10^23

    Each H2O molecule has 3 atoms.

29. 3 g of carbon combines completely with how much oxygen to form carbon dioxide?

    1. 4 g
    2. 32 g
    3. 8 g
    4. 16 g
    Answer

    C. 8 g

    C : O in CO2 is 12 : 32 = 3 : 8.

30. The Rutherford model could not explain the stability of the atom because

    1. electrons have no charge
    2. a revolving electron should lose energy and spiral into the nucleus
    3. the nucleus is not charged
    4. atoms have no space
    Answer

    B. a revolving electron should lose energy and spiral into the nucleus

    An accelerating charge radiates energy, so the orbit would shrink.

31. The number of electrons in the outermost shell of calcium (Z = 20) is

    1. 2
    2. 8
    3. 20
    4. 1
    Answer

    A. 2

    Configuration is 2, 8, 8, 2.

32. The element with electronic configuration 2, 8, 4 has atomic number

    1. 14
    2. 12
    3. 4
    4. 10
    Answer

    A. 14

    The total of 2 + 8 + 4 = 14 electrons.

33. The valency of oxygen (Z = 8) is

    1. 6
    2. 8
    3. 4
    4. 2
    Answer

    D. 2

    Configuration 2, 6; it gains 2 electrons.

34. CO and CO2 are formed from the same elements. The masses of oxygen that combine with 12 g of carbon are in the ratio

    1. 2 : 3
    2. 1 : 2
    3. 1 : 1
    4. 1 : 4
    Answer

    B. 1 : 2

    16 g in CO and 32 g in CO2 shows the law of multiple proportions.

35. The maximum number of electrons in a d subshell is

    1. 6
    2. 10
    3. 14
    4. 2
    Answer

    B. 10

    s, p, d, f hold 2, 6, 10, 14.

36. Chlorine-35 and chlorine-37 are

    1. isotones
    2. allotropes
    3. isotopes
    4. isobars
    Answer

    C. isotopes

    They have the same atomic number but different mass numbers.

37. Which of the statements is/are correct? 1. Isotopes have the same number of protons. 2. Isobars have the same atomic number.

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    A. 1 only

    Isobars have the same mass number but different atomic numbers.

38. Which of the statements is/are correct? 1. A compound can be separated into its elements by physical methods. 2. A mixture shows the properties of its components.

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    B. 2 only

    Compounds need chemical methods.

39. Which of the statements is/are correct? 1. The neutron is neutral. 2. The mass of an electron is nearly equal to the mass of a proton.

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    A. 1 only

    An electron is about 1/1836 of the mass of a proton.

40. Which of the statements is/are correct? 1. The maximum capacity of a shell is 2n^2. 2. The outermost shell can hold more than 8 electrons in the Bohr-Bury scheme.

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    A. 1 only

    The outermost shell holds at most 8 electrons.

41. Which of the statements is/are correct? 1. Gay-Lussac's law deals with volumes of gases. 2. Avogadro's law states that equal volumes of gases at the same conditions have equal numbers of molecules.

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    C. Both 1 and 2

    Both are correct.

42. Which of the statements is/are correct? 1. Mass number is always smaller than atomic number. 2. Rutherford showed that the nucleus is negatively charged.

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    D. Neither 1 nor 2

    The nucleus is positive, and A is at least equal to Z.

43. Which of the pairs is correctly matched?

    1. Electron - Bohr
    2. Proton - Chadwick
    3. Nucleus - Rutherford
    4. Neutron - Goldstein
    Answer

    C. Nucleus - Rutherford

    Proton - Goldstein, neutron - Chadwick, electron - Thomson.

44. Which of the statements is/are correct? 1. Oxygen is diatomic (O2) and ozone is triatomic (O3). 2. Helium is monatomic.

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    C. Both 1 and 2

    Both are correct statements.