Atoms, Structure and Mass 2

What is electrostatic attraction?

• the force of attraction between negative particles and positive particles, keeping the atom bonded together

Describe the nucleus.

• approximately 10 000-100 000 times smaller than the size of the atom

• contributes around 99.97% of the atom’s mass → atomic nuclei are extremely dense

• nucleons - subatomic particles in the nucleus (protons & neutrons)

• protons - 1.673 x 10^-27 kg

• neutrons almost identical mass to protons

• proton

  • charge of +1

  • mass relative to a proton - 1

  • mass (kg) - 1.673 × 10^-27

• neutron

  • charge of 0

  • mass relative to a proton - 1

  • mass (kg) = 1.675 ×10^-27

• electron

  • charge of -1

  • mass relative to a proton - 1/1800

  • mass (kg) - 9.109 × 10^-31

What are isotopes?

• different versions of elements that contain a different number of neutrons

• the reason mass numbers contain decimals

• isotopes have identical chemical properties but different physical properties (mass/density)

• 1 unit of mass → 1/12 of the mass of a carbon-12 isotope (12 units)

What is the relative isotopic mass?

the mass of an individual isotope of each element compared to the standard that 1 unit of mass is 1/12 of the mass of a C-12 isotope

What is the relative atomic mass?

• average relative mass of an atom in the naturally occurring mixture of isotopes

  • takes into account the relative abundances of each isotope in natural samples of the element

What is a mass spectrometer?

used to identify the mass of elements, ions, isotopes/molecules, and their relative abundances

What is the first step of mass spectometry?

1. ionisation

• the sample needs to be vaporised first, before being passed into the ionisation chamber → an electrically heated metal coil gives off a stream of electrons (vaporisation)

• atoms/molecules in the sample are bombarded by this stream of electrons and (sometimes) the collision will knock an electron from the particle → resulting in a positively charged ion

• most of the ions formed will have a +1 charge, as it is difficult to remove a second electron from an already positive ion

What is the second step of mass spectrometry?

2. acceleration

• the positively charged ions are repelled from the ionisation chamber (positively charged)

• then pass through negatively charged slits which focus and accelerate this into a beam

What is the third step of mass spectrometry?

3. deflection

• the stream of positively charged ions are then deflected by a magnetic field

• the amount ions are deflected by depends on:

  • the mass of the ion (lighter ions will be deflected more than heavier ones)

  • the charge of the ion (ions with a greater charge than +1 are deflected more)

• these properties can be considered as a mass/charge ratio (m/z)

What is the last step of mass spectrometry?

4. detection

• by varying the strength of the magnetic field, the different ion streams (after deflection) can be focused on the ion detector, in order of increasing mass/charge ratio (lightest ions would be deflected the least)

• when an ion hits the detector, the charge is neutralised → generating an electrical current → current is proportional to the abundance of the ion & sent to computer for analysis

• mass spectrum is generated → showing the different m/z values of ions present & their relative abundance

What is the simplified steps of mass spectrometry?

1. vaporised chemical sample is fed into the ionisation chamber where a high energy electron beam is fired at the sample to ionise the atoms and form cations with a +1 charge

2. the cations are passed through negatively charge slits which accelerate them towards the electromagnet

3. accelerated cations are passed through a magnetic field that deflects them based on their mass/charge ratio. heavier particles are experience lesser deflection, while lighter particles experience more

4. upon reaching the detector, cations have their charge neutralised. this produces signals based on the mass/charge ratio detected, which allows the machine to register the relative abundance of each isotope present