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1.3Unit 1. Structure, Bonding and Introduction to Organic Chemistry

Atomic Structure — IAL Edexcel Modular (Chemistry) Revision Notes & Topical Questions

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1.3 Atomic Structure — IAL EDEXCEL MODULAR (CHEMISTRY) notes: State the relative masses, charges and positions of sub-atomic particles, work out particle numbers in atoms and ions, and interpret mass spectra to find isotopic abundances, Ar and Mr.Structure of the atom — a very sm…

  • Relative values
  • Behaviour in an electric field
  • Atomic number, Z
  • Mass number, A
  • Number of neutrons
  • Neutral atom

Section 1 of 9Sub-atomic particles

  • Structure of the atom
    • a very small, dense, positive nucleus surrounded by mostly empty space
    • The nucleus contains protons and neutrons, called nucleons, and holds almost all of the mass.
    • Electrons are found in orbitals (energy levels) around the nucleus.
ParticleRelative massRelative chargeLocation
Proton1+1Nucleus
Neutron10Nucleus
Electron1/1836 (negligible)−1Orbitals around nucleus
Sub-atomic particles
Relative values
  • Relative values masses and charges are compared with each other because actual values in grams and coulombs are too small to use
Behaviour in an electric field
  • Behaviour in an electric field protons deflect towards the negative plate and electrons deflect much more towards the positive plate
  • Neutrons are not deflected.

Section 2 of 9Atomic number, mass number and ions

Atomic number, Z
Atomic number, Z the number of protons in the nucleus, which decides which element the atom is
Mass number, A
Mass number, A the total number of protons and neutrons in the nucleus
Number of neutrons
A − Z
Neutral atom
  • Neutral atom the number of electrons equals the number of protons
  • Ions form when atoms gain or lose electrons.
Positive ion
  • Positive ion has fewer electrons than protons
  • Number of electrons = Z − charge.
Negative ion
  • Negative ion has more electrons than protons
Nuclide notation
  • Nuclide notation mass number is written as a superscript and atomic number as a subscript before the symbol, for example 2311Na has 11 protons, 12 neutrons and 11 electrons

Section 3 of 9Isotopes

Isotopes
Isotopes atoms of the same element with the same number of protons and electrons but different numbers of neutrons
Naming
  • Naming the symbol or name is followed by the mass number, such as carbon-12 and carbon-14
  • Carbon-12 has 6 neutrons and carbon-14 has 8 neutrons.
Chemical properties
  • Chemical properties identical, because chemical reactions depend on the electrons and isotopes have the same electron configuration
Physical properties
  • Physical properties differ slightly, because extra neutrons only add mass
  • Density, rate of diffusion, melting point and boiling point can differ.

Section 4 of 9The mass spectrometer

Vaporisation
  • Vaporisation the sample is heated and turned into a gas.
Ionisation
  • Ionisation high-energy electrons from an electron gun knock out electrons, forming positive ions.
Acceleration
  • Acceleration the ions are accelerated by an electric field towards negatively charged plates to the same kinetic energy.
Deflection
  • Deflection the ions pass into a flight tube where a magnetic field deflects them in a curve;
    • lighter or more highly charged ions are deflected more.
Detection
  • Detection ions striking the detector plate produce a current proportional to their abundance, and a mass spectrum is recorded.
  • Uses
    • finding relative isotopic masses and abundances, identifying unknown compounds, detecting drugs in athletes, forensic science, carbon-14 dating and analysing planetary atmospheres

Section 5 of 9Mass spectra of elements

Peaks
  • Peaks each peak is an isotope, with m/z equal to its isotopic mass for a 1+ ion
Peak height
  • Peak height shows the relative abundance of that isotope
Doubly charged ions
  • Doubly charged ions a 2+ ion appears at half of its mass on the m/z axis
Relative atomic mass Ar
Σ(isotopic mass × % abundance) ÷ 100
  • It is a weighted mean of the isotopic masses.
  • Chlorine is 75% 35Cl and 25% 37Cl, which gives Ar = 35.5.
Reverse calculations
  • Ar and the masses can be used to find the abundance of an unknown isotope

Section 6 of 9Mass spectra of molecules

Mass spectra of molecules
Molecular ion, M+
Molecular ion, M+ the whole molecule that has lost one electron
  • The peak with the highest m/z is the M+ peak and its m/z equals the relative molecular mass.
  • Lower m/z peaks come from fragments formed when bonds break.
Predicting fragment peaks
  • Predicting fragment peaks break single bonds, because only the positively charged fragment is detected
  • Ethanol gives CH3+ at 15, CH3CH2+ at 29, CH2OH+ at 31 and M+ at 46.
  • Propanone gives CH3CO+ at 43 and CH3+ at 15, with M+ at 58.
M+1 peak
  • M+1 peak a small peak one unit above M+ from molecules containing one carbon-13 atom
  • About 1.1% of carbon is carbon-13.
  • The more carbon atoms in the molecule, the taller the M+1 peak, so hexane has a larger M+1 peak than ethane.

Section 7 of 9Halogens in mass spectra

SpeciesPeaksHeight ratio
One Cl atomM+ and [M+2]+3 : 1
Two Cl atoms (such as Cl2)M+, [M+2]+, [M+4]+9 : 6 : 1
One Br atomM+ and [M+2]+1 : 1
Two Br atoms (such as Br2)M+, [M+2]+, [M+4]+1 : 2 : 1
Chlorine isotopes
  • 35Cl and 37Cl are in a 3 : 1 abundance ratio
  • Cl2 peaks at m/z 70, 72 and 74 come from 3/4 × 3/4, 2 × 3/4 × 1/4 and 1/4 × 1/4.
  • Peaks at m/z 35 and 37 come from Cl+ fragments.
Bromine isotopes
  • 79Br and 81Br are present in about equal abundance, so Br2 peaks occur at 158, 160 and 162

Section 8 of 9Key terms

TermMeaning
Atomic number (Z)The number of protons in the nucleus of an atom.
Mass number (A)The total number of protons and neutrons in the nucleus.
IsotopesAtoms of the same element with the same number of protons but different numbers of neutrons.
Relative isotopic massThe mass of an isotope relative to 1/12 of the mass of a carbon-12 atom.
Mass spectrometerAn instrument that separates ions by mass-to-charge ratio to give isotopic masses and abundances.
m/zThe mass-to-charge ratio of an ion.
Molecular ion peakThe peak of highest m/z caused by the whole molecule losing one electron, M⁺.

Section 9 of 9Exam technique

1
Giving the electron a relative mass of 1 — it is 1/1836, which is negligible
2
Saying isotopes have different chemical properties — they have the same electron configuration, so the same chemistry
3
Reading the M+1 peak as the molecular ion — the molecular ion is the peak at the highest m/z apart from small isotope peaks
4
Calculating Ar as a simple average of isotope masses — it must be weighted by abundance
5
Forgetting that fragments must be positive to appear in the spectrum

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