Atomic Models and Spectrometry: Key Concepts for Chemistry Students

John Dalton's model of the atom

Atoms were tiny, indivisible solid spheres that could not be broken down.

J.J. Thomson's discovery

He discovered the electron and proposed the plum pudding model: a ball of positive charge with electrons embedded in it.

Rutherford and Marsden's experiment

The alpha scattering experiment, where alpha particles were fired at thin gold foil.

Results of the alpha scattering experiment

Most particles passed straight through, some were deflected, and a few were reflected backwards.

Conclusions from the alpha scattering experiment

Atoms are mostly empty space, contain a small dense nucleus, and the nucleus is positively charged.

Niels Bohr's atomic model

Electrons orbit the nucleus at specific distances in fixed energy levels (electron shells).

Three fundamental particles of the atom

Protons, neutrons, and electrons.

Relative charge and mass of an electron

Charge = -1, relative mass = 1/1836.

Relative charge and mass of a proton

Charge = +1, relative mass = 1.

Relative charge and mass of a neutron

Charge = 0, relative mass = 1.

Location of protons and neutrons

In the nucleus, held together by the strong nuclear force.

Necessity of the strong nuclear force

It is much stronger than the electrostatic repulsion between protons and holds the nucleus together.

Reason atoms are electrically neutral

Because the number of protons equals the number of electrons.

Instrument for determining relative atomic masses

The mass spectrometer.

Scale for measuring relative atomic masses

A scale where the mass of a carbon-12 atom is exactly 12.

Time-of-flight mass spectrometer vacuum

To prevent ions from colliding with air molecules.

Electrospray ionisation

Sample dissolved in volatile solvent, passed through a fine needle connected to a high voltage. Produces tiny droplets, solvent evaporates, leaving positively charged ions (usually +1).

Electron impact ionisation

Sample is vaporised, high-energy electrons fired at it, knocking off one electron to form +1 ions: X(g) + e- → X+(g) + 2e-.

Acceleration in TOF mass spectrometry

Positive ions are accelerated by an electric field so they all gain the same kinetic energy.

Ion drift in TOF mass spectrometry

Ions travel through a region with no electric field. Lighter ions with the same charge move faster than heavier ions.

Detector in TOF mass spectrometry

Ions hit the detector, generating a current proportional to the abundance. Lighter ions arrive first.

Calculation of relative atomic masses

As weighted averages of isotopes' mass numbers, taking into account relative abundances, relative to carbon-12.

Chlorine's Ar explanation

Because it is a weighted average of its isotopes, Cl-35 and Cl-37.

Isotopes in mass spectrum

Because isotopes have different masses, producing distinct peaks.

Mass spectrometry identifying elements

Each element produces a characteristic pattern ('fingerprint') based on its isotopes.

Electron shells

Regions around the nucleus where electrons with fixed energies are found.

Principal quantum number

A number (n) that indicates the main energy level of an electron. Larger n means higher energy and further from the nucleus.

Sub-shells in the 1st shell

1 sub-shell: 1s.

Sub-shells in the 2nd shell

2 sub-shells: 2s and 2p.

Sub-shells in the 3rd shell

3 sub-shells: 3s, 3p, and 3d.

Sub-shells in the 4th shell

4 sub-shells: 4s, 4p, 4d, 4f.

Types of sub-shells

s, p, d, f.

s sub-shell orbitals and electron capacity

1 orbital, holds 2 electrons.

p sub-shell orbitals and electron capacity

3 orbitals, holds 6 electrons.

d sub-shell orbitals and electron capacity

5 orbitals, holds 10 electrons.

f sub-shell orbitals and electron capacity

7 orbitals, holds 14 electrons.