chapter 1 atomic structure

relative atomic mass

average mass of one atom compared with 1/12 the mass of carbon 12

relative isotopic mass

mass of an atom of the isotope compared to 1/12 the mass of carbon 12

uses of mass spectroscopy

• determine the number of fundamental particles in atoms and ions using mass number, atomic number and charge

• explain the existence of isotopes

how the abundance of each isotope is determined in a mass spectrometer.

Ions hit the detector and accept electrons causing current to flow. Bigger current = higher abundance of that ion

why the inside of a mass spectrometer is operated under vacuum.

To stop the ions produced colliding with air or other particles/ so that. there are only the ions produced from the sample present

Describe how the ions are accelerated in the TOF mass spectrometer.

the ions are attracted towards the negatively charged plate

Describe how ions are formed in a time of flight (TOF) mass spectrometer.

A high voltage is applied to the sample (in a volatile solvent). where the Atoms / Molecules lose an electron

or

atoms get bombarded with electrons using an electron gun and 1e- gets knocked out of the atom

Explain why it is necessary to ionize molecules when measuring their mass in a TOF mass spectrometer.

Ions, not molecules, will interact with and be accelerated by the electric field. Therefore, Only the ions will create a current when hitting the detector

stages of TOF mass spectrometry

• ionisation

• acceleration

• ion drift

• detection

step 1 ionisation

The sample is dissolved in a volatile solvent The solvent is injected into the mass spectrometer using a fine hollow needle to create a mist The needle is attached to a high-voltage power supply, so as the sample is injected, the particles are ionised by losing electrons. Most of the positive ions will have a 1+ charge as it is difficult to remove further electrons The solvent evaporates until the mist contains only positively charged ions

step 2 acceleration

The positively charged ions are attracted towards a negatively charged plate They accelerate towards it using an electric field This ensures all ofthe positive ions have the same kinetic energy +

Since all the positive ions will have the same kinetic energy,their velocity will depend on their mass Lighter ions will move faster and heavier ions will move slowe

step 3 ion drift

The positively charged ions will pass through a hole in the negatively charged plate and move into a flight tube This is where the name 'Time of Flight' comes from The time of flight of each ion in this tube depends on their velocity

step 4 detection

When the ions have passed through the flighttube they hit a negatively charged 'detector' plate As they hitthis electric plate,they gain an electron This gaining of an electron causes a current to be produced The size ofthe currentis proportionalto the abundance ofthose ions hitting the plate and gaining an electron Lighter ions will reach the detector firstfollowed by heavier ions The signalfrom the detector is transferred to a computer, which produces the mass spectrum

electron configuration

what is shielding

shielding is the electron repulsion of the valence electrons which counter acts the attraction between the valence electrons and the nucleus

factors affecting the atomic radius

as the number of protons increase the nuclear charge increases. This makes the atomic radius smaller due to there being less repulsion hence less shielding.

ionisation energy

energy required to remove 1 electron from the ground state in each atom in a mole of gaseous atoms og that element to form a mole of gaseous ions of charge 1+

trend in period 3 of ionisation energy

the number of protons increase which increases the nuclear charge. this decreases the atomic radius across the group and makes the element require more energy to remove the outermost electron.

trend in group 2 ionisation energy

down the group the number of energy levels increase with the electronic repulsion and shielding.this means that the ionization energy decreases because the attraction between the valence electron and the nucleus is weaker. therefore, less energy is needed to remove the electron