Chemistry 3.1.1 - Atomic structure

Definitions to learn

Relative atomic mass (Ar)

The weighted average mass of an atom, taking into account all naturally occurring isotopes, relative to 1/12th the mass of an atom of carbon-12

Relative molecular mass (Mr)

The weighted average mass of a molecule, taking into account all naturally occurring isotopes, relative to 1/12th the mass of an atom of carbon-12

Isotope

Atoms of the same element with the same number of protons and a different number of neutrons

Mass number

The sum of the number of protons and the number of neutrons in an atom

Atomic number

The number of protons in an atom

Electrospray ionisation key features

• Sample dissolved in a volatile solvent

• Injected through a fine hypodermic needle

• Tip of needle is attached to the positive terminal of a high-voltage power supply

• Particles are ionised by gaining a proton

• X_(g) + H⁺ → XH⁺_(g) 

Electron impact ionisation key features

• Sample is vaporised

• High energy electrons from an electron gun fired at sample

• Electron gun knocks off an electron from each particle, forming a positive ion:

• X_(g) → X⁺_(g) + e^- 

Atomic orbital

A region of space where there is a high probability of finding an electron

First ionisation energy

The enthalpy change required for X_(g) → X⁺_(g) + e^- 

Plum pudding atomic model key features

A sphere of evenly spread positive charge dotted with electrons

Acceleration features

• All ions are accelerated such that they have the same kinetic energy

• Ions are accelerated by a negatively charged acceleration plate

Ion drift

• Lighter ions travel faster and meet the detector first

• Heavier ions travel slower and take longer to meet the detector

Detection

• Ions hit the detector and gain an electron, generating a current

• X_(g) + e^- → X⁺_(g)  

• The more abundant a particular ion, the greater the current generated

Electrospray vs electron impact ionisation use

• Electrospray used for larger molecules; has no fragmentation

• Electron impact used for elements and smaller molecules

• Electron impact often results in fragmentation

Why is the 1st ionisation energy of Al lower than that of Mg?

The electron is removed from the 3p orbital, which is higher in energy than the 3s

Why is the 1st ionisation energy of S lower than that of F?

In S, electrons are paired in the 3p orbital so there is repulsion.