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Chemistry · Practice book · Senior 4 · Unit 1 of 17

Structure of an atom and mass spectrum

Senior 4 Chemistry · 14 questions with answers. Try each question on paper, then tap it to see the answer.

Unit 1 Structure of an atom and mass spectrum

Key unit competence: Use simple mass spectra to calculate the relative atomic mass of different elements.

Key ideas

  • Discoveries: J.J. Thomson found the electron with cathode rays (1897); Goldstein found positive rays (protons); Rutherford's gold-foil experiment (1911) showed a tiny, dense, positive nucleus; Chadwick discovered the neutron (1932).
  • Particles: proton (+1, mass 1), neutron (0, mass 1), electron (−1, mass 1/1836). Mass number A = protons + neutrons; atomic number Z = protons. Nuclide notation: [A,Z]X.
  • Isotopes have the same Z but different numbers of neutrons; they have the same chemical properties but different physical properties (mass, density, rate of diffusion).
  • Mass spectrometer stages: vaporisation, ionisation (electron bombardment forms positive ions, X+), acceleration (electric field), deflection (magnetic field: lighter or more highly charged ions are deflected more; deflection depends on m/z) and detection.
  • A mass spectrum plots relative abundance against m/z. Relative atomic mass: Ar = Σ(isotopic mass × abundance) ÷ Σ(abundance).
  • Uses of mass spectrometry: finding relative atomic and molecular masses, identifying compounds (molecular ion peak M+ and fragmentation pattern), carbon dating, detecting drugs in sport, space probes and pollution monitoring.

Common misconceptions

  • "Isotopes have different chemical properties." No: Isotopes have the same electron configuration, so the same chemical properties.
  • "Heavier ions are deflected more in a mass spectrometer." No: Lighter ions (smaller m/z) are deflected MORE by the magnetic field.
  • "Relative atomic mass is the mass of the most abundant isotope." No: It is a weighted average of all isotopes using their abundances.

Worked examples

Example 1. Boron gives peaks at m/z 10 (19.9%) and 11 (80.1%). Calculate its relative atomic mass.

Solution

Ar = (10 × 19.9 + 11 × 80.1) ÷ 100

= (199 + 881.1) ÷ 100

= 1080.1 ÷ 100 = 10.8.

Example 2. Describe the five main stages in a mass spectrometer.

Solution

Vaporisation: the sample is turned into gas.

Ionisation: high-energy electrons knock electrons off atoms, forming positive ions.

Acceleration: an electric field speeds up the ions.

Deflection: a magnetic field bends the ions; lighter or more highly charged ions bend more.

Detection: ions hit a detector, which records m/z and abundance.

Practice questions

  1. State the charge and relative mass of a proton and an electron.

    Show answer

    Proton: +1, 1. Electron: −1, 1/1836.

    Common mistake: Giving the electron mass as 1.

  2. Define relative atomic mass.

    Show answer

    The weighted average mass of the atoms of an element compared with 1/12 of the mass of a carbon-12 atom.

    Common mistake: Saying 'mass of one atom in grams'.

  3. What is the purpose of the magnetic field in a mass spectrometer?

    Show answer

    To deflect the ions according to their m/z.

    Common mistake: Saying it accelerates ions.

  4. Explain why isotopes of chlorine react in the same way.

    Show answer

    They have the same number and arrangement of electrons.

    Common mistake: Saying they have the same mass.

  5. Write the symbol of the isotope with 8 protons and 10 neutrons.

    Show answer

    188O.

    Common mistake: Writing 108O.

  6. Give two uses of mass spectrometry.

    Show answer

    Determining Ar/Mr, identifying compounds, carbon dating, drug testing (any two).

    Common mistake: Saying 'measuring temperature'.

  7. Lithium: Li-6 7.4%, Li-7 92.6%. Calculate Ar.

    Show answer

    (6 × 7.4 + 7 × 92.6) ÷ 100 = 6.93.

    Common mistake: Taking a simple average.

  8. Why are the ions accelerated before deflection?

    Show answer

    So all ions have the same kinetic energy and enter the magnetic field as a beam.

    Common mistake: Saying to make them heavier.

  9. What does the height of a peak in a mass spectrum show?

    Show answer

    The relative abundance of that ion (isotope).

    Common mistake: Saying it shows the mass.

  10. What conclusion did Rutherford draw from the few alpha particles that bounced back?

    Show answer

    There is a tiny, very dense, positively charged nucleus.

    Common mistake: Saying electrons repelled them.

Matching and drag-and-drop

  1. Matching

    Match each item in column A with the correct answer in column B. Write the letter next to each number.

    Column AColumn B
    1. Ions are accelerated in a mass spectrometer by:a. 3517Cl and 3717Cl
    2. Who discovered the electron?b. Ne2+
    3. Which ion would be deflected more: Ne+ (m/z 20) or Ne2+ (m/z 10)?c. J.J. Thomson
    4. Which pair are isotopes?d. an electric field
    Show answer

    1 → d (an electric field) · 2 → c (J.J. Thomson) · 3 → b (Ne2+) · 4 → a (3517Cl and 3717Cl)

    • Negatively charged plates attract the positive ions.
    • Thomson used cathode-ray tubes in 1897.
    • Smaller m/z gives more deflection.
    • Same atomic number 17.

  2. Drag and drop

    Choose the correct word or phrase from the box for each gap. One word in the box is not needed.

    Chadwick  ·  vaporised  ·  water treatment  ·  dating ancient objects
    1. In a mass spectrometer, atoms are first .
    2. The neutron was discovered by .
    3. Carbon-14 is used in .
    Show answer

    1: vaporised · 2: Chadwick · 3: dating ancient objects

    • The sample must be a gas.
    • James Chadwick in 1932.
    • Its half-life allows age estimation.

  3. Matching

    Match each item in column A with the correct answer in column B. Write the letter next to each number.

    Column AColumn B
    1. Ionisation in a mass spectrometer forms:a. electron configuration
    2. A mass spectrum plots abundance against:b. a small, dense, positive nucleus
    3. Isotopes have the same chemical properties because they have the same:c. mass-to-charge ratio (m/z)
    4. Rutherford's gold-foil experiment showed that the atom has:d. positive ions
    Show answer

    1 → d (positive ions) · 2 → c (mass-to-charge ratio (m/z)) · 3 → a (electron configuration) · 4 → b (a small, dense, positive nucleus)

    • Electrons are knocked off the atoms.
    • Each peak is one m/z value.
    • Chemistry depends on electrons.
    • Most alpha particles passed through; a few bounced back.

  4. Drag and drop

    Choose the correct word or phrase from the box for each gap. One word in the box is not needed.

    detect banned drugs in urine samples  ·  two isotopes  ·  measure height  ·  neutrons
    1. A spectrum shows two peaks for an element at m/z 63 (69%) and 65 (31%). The element has .
    2. Isotopes of an element differ in their number of .
    3. Mass spectrometry is used in sport to .
    Show answer

    1: two isotopes · 2: neutrons · 3: detect banned drugs in urine samples

    • Each peak is one isotope.
    • Same Z, different A.
    • Drug molecules give characteristic spectra.

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