Mass Number & Isotopes

Mass Number =

A

-sum of protons and neutrons

Atomic Number =

Z

-equal to number of protons

Relative Atomic Mass =

Ar

-mean mass of an atom of an element, divided by one twelfth of the mean mass of one atom of the carbon-12 isotope

Isotopes

atoms of the same element with the same atomic number, but different numbers of neutrons, resulting in a different mass number

Neutral Atoms of Isotopes =

react chemically in the same way (proton no. and same electron configuration)

different mass number = different physical properties

hydrogen

1 proton

0 neutrons

deuterium

1 proton

1 neutron

titrium

1 proton

2 neutrons

ions -

formed when atom loses/gains electrons, no longer neutral, has an overall charge

mass spectrometry

analystical technique - identifies different isotopes, finds overall relative atomic mass of an element

TOF Mass Spectrometry

-records time taken for ions of each isotope to reach a detector

-spectra produced, shows each isotope present

TOF: 1

Ionisation

-sample vapourised

-injected into mass spectrometer where high voltage passes over chamber

-causes electrons to be removed from atoms

-leaves 1+ charged ions

TOF: 2

Acceleration

-positively charged ions accelerated towards negative charged detection plate

TOF: 3

Ion Drift

-ions deflected by magnetic field into curved path

-radius of path = dependant on charge + mass of ion

TOF: 4

Detection

-positive ions hit negatively charged detection plate, gaining an electron + producing a flow of charge

-greater abundance = greater current produced

TOF: 5

Analysis

-current values + flight times produce spectra print-out with relative abundance of each isotope displayed

production of a 2+ charge ion =

-more affected by magnetic field

-produces a more curved path of smaller radius

-mass to charge ratio (m/z) is halved

Ar =

m/z x abundance / total abundance

Chlorine Spectra

characteristic pattern

-3:1 ratio for Cl⁺ ions

-3:6:9 ration for Cl_2⁺ ions

-one isotope more common than the other + chlorine molecule forms in different combinations