ATOMIC STRUCTURE PART 1

mass number

total number of protons and neutrons in the nucleus of an atom

atomic number

number of protons in the nucleus of an atom

mass number

total number of protons and neutrons in the nucleus of an atom

atomic number

number of protons in the nucleus of an atom

ions

formed from atoms that gain or lose electrons

negative ions

gain electrons

positive ions

lose electrons

isotopes

elements with the same number of protons but different number of neutrons

the number and arrangement of electrons…

…decides the chemical properties of an element

isotopes have the same electronic configuration

therefore isotopes have the same chemical properties

isotopes have different numbers of neutrons…

…therefore different physical properties which depend more on the mass of the atom

john dalton

described atoms as solid spheres and said different spheres made up different elements

jj Thompson plumb pudding model…

…described atoms as solid positively charged spheres with embedded negatively charged particles

Rutherford model

• positively charged alpha particles fired at a thin sheet of gold

• plum pudding model suggested most alpha particles would be deflected

• however, most alpha particles passed straight through the gold atoms

• a very small number deflected backwards

rutherford described electrons as a cloud around an atom…

…but that means the electrons would quickly spiral down into the nucleus causing the atom to collapse

bohr model

• electrons exist in fixed orbits and nowhere between

• electrons move between shells when electromagnetic radiation is absorbed or emitted

• since the energy of shells is fixed, the radiation will have a fixed frequency

the frequencies of radiation emitted and absorbed by atoms where already known from experiments

relative atomic mass

the average mass of an atom of an element on a scale where an atom of carbon-12 is 12

relative isotopic mass

the mass of an atom of an isotope of an element on a scale where an atom of carbon-12 is 12

why is relative isotopic mass a whole number

because it is exact

why is relative atomic mass not usually a whole number

because it is an average

relative molecular mass

the average mass of a molecule on a scale where an atom of carbon-12 is 12.

to calculate the Mr just add up the relative atomic mass values of all atoms in the molecule

relative formula mass

the average mass of a formula unit on a scale where an atom of carbon-12 is 12. it is used for compounds that are ionic or giant covalent

to find the relative formula mass…

…add up the relative atomic masses of all the ions in the formula unit

mass spectrometer

gives information about the relative atomic mass of an element and the relative abundance of its isotopes or the relative molecular mass of a molecule

mass spectrometer stages

• ionisation

• acceleration

• ion drift

• detection

TOF ionisation

samples must be ionised before they enter the mass spectrometer

electrospray ionisation

sample is dissolved in a solvent and pushed through a nozzle at high pressure. a high voltage is applied causing each particle to GAIN a H⁺ . the solvent is removed leaving the gas of positive ions

disadvantage of electrospray ionisation

fragments

electron impact ionisation

sample is vapourised and an electron gun is used to fire high speed electrons at it knocking 1 electron off each particles so they become +1 ions

TOF acceleration

electric field accelerates positive ions, giving them all the same kinetic energy. the lighter ions experience greater acceleration that the heavier ions

TOF ion drift

ions enter a region with no electric field. lighter ions drift faster than heavier ions

TOF detection

lighter ions arrive at the detection plate faster than heavier ions. the detector detects the current created when the ions hit it and records how long they took to pass the spectrometer. the data is then used to calculate the mass/charge values needed to produce a mass spectrum

how are ions detected

voltage/ current created when hitting detection plate as each ion gains an electron

how is relative abundance destermined

the current is proportional to abundance

WHY is it important to ionise molecules when measuring their mass in a TOF mass spectrometer

• because only ions will interact with and be accelerated by the electric field

• only ions will create a current when hitting the detector

why is electrospray ionisation used for proteins

so proteins don’t fragment