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

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

5 min read 11 parts Lesson 2 of 6 1 quiz

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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 SL notes: Describe the nuclear model, deduce the numbers of subatomic particles in atoms and ions, and explain isotopes and relative atomic mass.Evidence — in the Rutherford, Geiger and Marsden experiment, alpha particles fired at thin gold fo…

  • Most pass through
  • Replaced model
  • Relative values
  • Actual masses
  • Atomic number Z
  • Mass number A

Part 1 of 9Nuclear model of the atom

Evidence
  • Evidence in the Rutherford, Geiger and Marsden experiment, alpha particles fired at thin gold foil mostly passed straight through.
Most pass through
  • Most pass through the atom is mostly empty space.
  • A few deflect strongly — there is a small, dense, positively charged nucleus.
Replaced model
  • Replaced model the plum pudding model.
Nucleus
  • Nucleus positively charged because it contains protons, and dense because it holds almost all the mass.
Electrons
  • Electrons occupy the space outside the nucleus as a cloud of negative charge.
Size
  • Size the nucleus has a radius of about 10−15 m and the atom about 10−10 m.
Stability
  • Stability the atom is held together by electrostatic attraction between the nucleus and the electrons.

Part 2 of 9Subatomic particles

ParticleRelative massRelative chargeLocation
Proton1+1Nucleus
Neutron10Nucleus
Electron1/1836 (negligible)−1Space around the nucleus
Subatomic particles
Nucleons
  • Nucleons protons and neutrons, the particles in the nucleus.
Relative values
  • Relative values comparisons only, since the real charge of a proton is +1.602 × 10−19 C.
Behaviour
  • Behaviour the electrons determine chemical behaviour.
Actual masses
  • Actual masses proton 1.672622 × 10−27 kg and electron 9.109383 × 10−31 kg.
    • (IB data booklet)

Part 3 of 9Atomic number, mass number and notation

Atomic number Z
Atomic number Z the number of protons in the nucleus, which defines the element.
Mass number A
Mass number A the total number of protons and neutrons.
Notation
  • AZX shows both numbers for element X.
Neutrons
  • Neutrons number of neutrons = A − Z.
Neutral atom
  • Neutral atom has equal numbers of protons and electrons, so the atomic number also gives the number of electrons.

Part 4 of 9Ions

Protons and neutrons
  • Protons and neutrons do not change when an ion forms.
Cation
  • Cation has lost electrons, so it has fewer electrons than protons.
Anion
  • Anion has gained electrons, so it has more electrons than protons.
Electron count
  • Electron count electrons = Z − charge, so 32S2− has 16 protons, 16 neutrons and 18 electrons.

Part 5 of 9Isotopes

Definition
Definition atoms of the same element with the same number of protons but different numbers of neutrons.
Examples
  • 12C, 13C and 14C have 6 protons and 6, 7 and 8 neutrons.
PropertySame or differentReason
Chemical propertiesSameSame electron configuration
Mass and densityDifferentDifferent number of neutrons
Melting and boiling pointSlightly differentDifferent mass
Rate of diffusionDifferentLighter isotopes move faster
Nuclear stabilityMay differSome isotopes are radioactive

Part 6 of 9Relative atomic mass

Definition
Definition the weighted mean mass of the atoms of an element relative to one twelfth of the mass of a carbon-12 atom.
Cause
  • Cause most elements exist as a mixture of isotopes.
Units
  • Ar has no units and is usually not a whole number.
Formula
  • Ar = Σ ÷ 100.
    • (isotopic mass × % abundance)
Chlorine
  • 35Cl (75%) and 37Cl (25%) give Ar of about 35.5.
Rounding
  • Rounding round only after the full calculation is finished.

Part 7 of 9Calculating Ar from abundance data

1
Identify each isotope and its mass or m/z value from the data or spectrum.
2
Read the percentage abundance or relative intensity of each isotope.
3
Multiply each isotopic mass by its abundance.
4
Add the products.
5
Divide by the total abundance (100 if percentages are used).
6
Quote Ar to the required decimal places without units.

Part 8 of 9Mass spectrometer stages

1
Vaporise the sample.
2
Ionise it to positive ions.
3
Accelerate the ions in an electric field.
4
Separate the ions by mass-to-charge ratio (m/z).
5
Detect the ions and record their abundance.
1Output
  • Output the mass spectrum shows m/z against relative abundance, and the heights give isotope abundances.
2Molecular ion
  • M+ forms when a whole molecule loses one electron, and its m/z gives the relative molecular mass.

Part 9 of 9More from the specification

More from the specification
Mass spectrometry as an analytical means
  • Mass spectrometry as an analytical means measuring the mass-to-charge ratios of ionized fragments provides a definitive means to determine molecular structures.
Nucleons and atomic mass
  • Protons and neutrons reside in the nucleus and are collectively called nucleons, determining nearly the entire mass of an atom.
Relative charges vs SI coulombs
  • Protons and electrons carry equal and opposite elementary charges (+/- 1.602 x 10-19 C), represented conventionally as +1 and -1 respectively.
Nuclide identification
  • The elemental chemical symbol specifies atomic number, while mass number gives total nucleon count.
    • (such as Li for lithium or Mg for magnesium)
Relative atomic mass (Ar)
  • The weighted average mass of naturally occurring isotopes of an element relative to 1/12th of a carbon-12 atom.
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.
Physical property
  • A property observed or measured without changing the material.

Key terms

TermMeaning
AtomThe smallest particle of an element that keeps its chemical identity, with a dense nucleus surrounded by electrons.
NucleonA particle in the nucleus, either a proton or a neutron.
Atomic number (Z)The number of protons in the nucleus.
Mass number (A)The total number of protons and neutrons in the nucleus.
IsotopesAtoms of the same element with different numbers of neutrons.
Relative atomic mass (Ar)The weighted mean mass of an element's atoms relative to one twelfth of a carbon-12 atom.
Molecular ionThe ion formed when a whole molecule loses one electron in a mass spectrometer.

Avoid these mistakes

1
Saying the nucleus contains electrons — it contains only protons and neutrons.
2
Changing the number of protons when forming an ion — only electrons are gained or lost.
3
Saying isotopes differ chemically — they have the same electron configuration so the same chemistry.
4
Giving Ar units or rounding early — Ar is a ratio and has no units, and rounding is done last.

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