3.1.1.2 Mass number and isotopes

relative atomic mass

(mean mass of an atom of an element)/(1/12 x mean mass of 1 atom of C12)

mass number

sum of protons and neutrons, represented by A

atomic number

number of protons, represented by Z

isotopes

element with same number of protons but different number of neutrons, with different mass number - different physical properties
all react the same chemically as they have the same number of electrons

ion

atom that loses/gains electrons which results in a charged particle

mass spectrometry

used to identify different isotopes and find the overall relative atomic mass of an element

time of flight mass spectrometry

records time taken for ions of each isotope to reach detector, spectra can be produced showing each present isotope

ionisation

sample can be ionised in 2 ways - electron impact and electrospray ionisation

electron impact

vaporised sample injected at low pressure
electron gun fires high energy electrons at sample
knocks out outer electron
form positive ion with different charges e.g. Ti(g) -> Ti+(g) + e-

electrospray ionisation

sample dissolved in a volatile, polar solvent
injected through fine needle producing aerosol (fine mist)
tip of needle has high voltage - molecule M gains proton H+ from solvent forming MH+
M(g) + H+ -> MH+
solvent evaporates while MH+ ions move towards the negative plate

use of electron impact

for elements/substances with low formula mass, can cause large organic molecules to fragment

use of electrospray ionisation

used for large organic molecules - fragmentation doesn't occur

acceleration

positive ions accelerated towards negative charged detection plate due to electrostatic forces to a constant kinetic energy
velocity of each particle depends on their mass - lighter ions move towards the detector first, they have a higher velocity, heavier ions have a lower velocity

ion drift/flight tube

positive ions with smaller m/z (mass:charge/atomic mass) has the same kinetic energy as ions with larger m/z and will move faster
heavier ions take longer to move to the detector
ions are distinguished by flight times

detection

when positive ions hit the negatively charge detection plate, it gains an electron producing a flow of charge
greater abundance = greater current produced - current is proportional to abundance of species

Ar formula

(isotope mass x abundance) + (isotope mass x abundance)/total abundance

mass spectra of Cl2

when it is a diatomic molecule, sometimes the m/z would be of the diatomic molecule instead of a singular one

principle of electroneutrality

cations will always be accompanied by anions