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S1.2Structure 1 — Models of the particulate nature of matter

The nuclear atom — IB Chemistry HL Revision Notes & Topical Questions

5 min read 10 parts Lesson 2 of 6 1 quiz

📚 IB Chemistry HL Hub / Topical Revision
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Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
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Lesson summary

S1.2 The nuclear atom — IB CHEMISTRY HL notes: Describe the nuclear model, find the particles in atoms, ions and isotopes, calculate relative atomic mass and read mass spectra.Nucleus — small, dense and positively charged because it holds the protons; it contains most of the atom…

  • Rutherford alpha scattering
  • Relative values
  • Actual charge
  • Data booklet
  • Atomic number (Z)
  • Mass number (A)

Part 1 of 8The nuclear model of the atom

The nuclear model of the atom
Nucleus
  • Nucleus small, dense and positively charged because it holds the protons;
    • it contains most of the atom's mass.
Nucleons
  • Nucleons protons and neutrons, the heaviest subatomic particles, are located in the nucleus.
Electrons
  • Electrons negatively charged, moving around the nucleus in a "cloud" of negative charge outside it.
Binding
  • Binding the atom is held together by electrostatic attraction between the nucleus and the electrons.
Rutherford alpha scattering
  • Rutherford alpha scattering most alpha particles passed straight through the gold foil, so the atom is mostly empty space.
  • A small proportion were deflected, showing a concentrated positive charge.
  • About 1 in 8000 bounced back, showing a tiny dense nucleus holding nearly all the mass.

Part 2 of 8Subatomic particles

ParticleRelative chargeRelative massLocation
Proton+11Nucleus
Neutron01Nucleus
Electron−1Negligible (about 1/1836)Outside the nucleus
Subatomic particles
Relative values
  • Relative values masses and charges are compared with each other because actual values are extremely small.
Actual charge
  • Actual charge an electron has charge −1.602 × 10−19 C and a proton the same magnitude positive.
Data booklet
  • Data booklet these values are given in the IB Data Booklet.

Part 3 of 8Atomic number, mass number and ions

Atomic number, mass number and ions
Atomic number (Z)
  • Atomic number (Z) the number of protons in the nucleus; it equals the number of electrons in a neutral atom.
Mass number (A)
  • Mass number (A) the total number of protons and neutrons (nucleons).
Neutrons
  • Neutrons number of neutrons = A − Z.
Protons
  • Protons the same for every atom and ion of an element, for example lithium always has 3.
Neutral atom
  • Neutral atom equal numbers of protons and electrons, so no overall charge.
Cation
  • Cation formed by losing electrons, so it has fewer electrons than protons.
Anion
  • Anion formed by gaining electrons, so it has more electrons than protons.
Charge
  • Charge the difference between the number of protons and electrons.
Periodic table
  • Periodic table gives the mass number and atomic number of each element.

Part 4 of 8Isotopes

Isotopes
Isotopes
Isotopes atoms of the same element with the same number of protons and electrons but a different number of neutrons.
Different mass numbers
  • Different mass numbers carbon-12 and carbon-14 (written 12C or C-12) have 6 and 8 neutrons.
Same chemical properties
  • Same chemical properties they have the same electron configuration.
Different physical properties
  • Different physical properties mass, density, melting and boiling point and rate of diffusion vary with mass number.
Heavy water
  • 2H2O has a higher density and boiling point than ordinary water.

Part 5 of 8Relative atomic mass

1Relative atomic mass (Ar)
Relative atomic mass (Ar) the ratio of the average mass of the atoms of an element to one twelfth of the mass of a carbon-12 atom.
2Standard
  • Standard carbon-12 is assigned exactly 12; one unified atomic mass unit is about 1.66 × 10−27 kg.
3Average
  • Average it uses the masses of all the isotopes weighted by abundance.
4Abundance
  • Abundance percentage abundance is given or read from a mass spectrum.
5Equation
  • Ar = Σ / 100.
    • (% abundance × isotopic mass)
6Rounding
  • Rounding round only after the whole calculation is complete, to the number of decimal places asked.
1
Record the m/z and relative intensity of each peak.
2
Add the intensities to get the total.
3
Work out each isotope's percentage abundance.
4
Multiply each abundance by its mass and divide by 100.
5
Add the contributions to get Ar.

Part 6 of 8Mass spectrometer stages (HL)

1
Vaporisation: the sample is vaporised.
2
Ionisation: it is bombarded with electrons to form positive ions.
3
Acceleration: the ions are accelerated by an electric field.
4
Deflection: a magnetic field deflects ions, those with smaller m/z more.
5
Detection: each ion gives a signal recorded as mass-to-charge ratio, m/z.
1Not assessed
  • Not assessed the specific details of these processes are not examined.
2Use
  • Use the spectrum gives isotopic abundances and relative atomic mass.

Part 7 of 8Interpreting mass spectra (HL)

Element spectrum
  • Element spectrum each peak is an isotope;
    • its height gives relative abundance and its m/z gives the isotope mass.
Molecular ion
  • Molecular ion the peak [M]+ at highest m/z gives the relative molecular mass of a molecule.
Chlorine pattern
  • 35Cl and 37Cl occur in about a 3 : 1 ratio.
  • A molecule with one Cl atom shows [M]+ and [M+2]+ peaks in a 3 : 1 ratio.
Bromine pattern
  • 79Br and 81Br occur in about a 1 : 1 ratio.
  • A molecule with one Br atom shows [M]+ and [M+2]+ peaks of almost equal height.
Fragmentation
  • Fragmentation an unstable molecular ion breaks into a positive fragment and a neutral radical.
  • m/z 15 is CH3+.
  • m/z 29 is C2H5+.
  • m/z 43 is C3H7+ or CH3CO+.
  • m/z 77 is C6H5+.

Part 8 of 8More from the specification

More from the specification
Cell culture as an investigative means
  • Cell culture as an investigative means maintaining cell lines in nutrient media provides an in vitro means to study division and drug cytotoxicity.
Subatomic nucleon composition
  • Protons and neutrons reside in the nucleus and are collectively called nucleons, carrying charges of +1 and 0 respectively.
Charge measurement in coulombs
  • The elementary charge of a proton is +1.602 x 10-19 C, while an electron carries -1.602 x 10-19 C.
Nuclide notation and chemical symbol
  • Standard notation (AZ X) displaying the elemental chemical symbol, mass number A, and atomic number Z.
    • (e.g. Mg for magnesium)
Determining subatomic particles
  • For any unknown nuclide, number of neutrons equals mass number minus atomic number (N = A - Z).
Coulomb
  • The unit of charge; one ampere for one second.
Symbol
  • Shorthand for an element, e.g. Na.
Chemical symbol
  • The one- or two-letter code for an element.

Key terms

TermMeaning
Atomic numberNumber of protons in the nucleus (Z).
Mass numberNumber of protons plus neutrons (A).
NucleonA proton or a neutron.
IsotopesAtoms of one element with different numbers of neutrons.
Relative atomic massAverage mass of an atom compared with one twelfth of carbon-12.
m/zMass-to-charge ratio of an ion in mass spectrometry.
Molecular ionThe ion formed when a molecule loses one electron; its m/z gives the relative molecular mass.

Avoid these mistakes

1
Changing the number of protons when an ion forms — only electrons are gained or lost.
2
Counting electrons as nucleons — mass number is the number of protons plus neutrons only.
3
Saying isotopes differ chemically — they have the same electron configuration and so the same chemical properties.
4
Rounding before the end of the Ar calculation.

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