3.1.1 Atomic Structure Flashcards
Purpose of a mass spectrometer:
To measure the mass of atoms and molecules very accurately
What did Robert Boyle do?
Put forward the idea that certain substances could not be broken down further (1661)
What did John Dalton do?
Suggested that elements were atoms which could not be broken down.
Atoms of the same element had the same mass but different to atoms of other elements.
(1803)
What did Henri Bequerel discover?
Radioactivity (1896)
What did JJ Thomson do?
Discovered the electron and showed it was negatively charged (1897)
What did Ernest Rutherford do?
Showed that most of the mass and positive charge was in the nucleus (1911)
What did Niels Bohr do?
Suggested that atoms consisted of of a tiny positive nucleus orbited by negatively charged electrons (1913)
Who was Schrödinger?
Developed the Quantum Theory which could be used to predict the behaviour of subatomic particles. (1932)
Relative mass of an electron?
1 / 1840
Symbol for mass number:
A
Symbol for atomic number:
Z
Mass number:
Total number of protons and neutrons in the nucleus of an atom
What are isotopes?
Atoms of the same element with the same number of protons but different numbers of neutrons (different mass number)
Relative atomic mass:
The average mass of an atom of an element on a scale where an atom of Carbon-12 has a mass of exactly 12
Uses of mass spectrometer:
-Gives accurate information about relative atomic mass
-Gives accurate information about the relative abundance of isotopes
-Used to identify elements
Determines relative molecular masses
Time of Flight Mass Spectrometer
-Particles are ionised to form 1+ ions
-Ions are accelerated so they all have the same kinetic energy
-Time taken to travel a fixed distance is used to find each ion’s mass
Electron impact
-Sample is vaporised
-High energy electrons fired at sample from an electron gun
-Results in loss of an electron, forming a 1+ ion
-Ions are then attracted to a negative electric plate where they are accelerated
When is electron impact used
Elements with low formula masses
If a molecule is ionised in this way, its called the molecular ion
Electrospray ionisation
-Sample is dissolved in a volatile polar solvent
-Sample is injected through a fine hypodermic needle at high voltage to produce a fine mist
-Particles are ionised by gaining a proton from the solvent as they leave the needle producing XH+ ions
-Solvent evaporates away while ions are attracted to negative plate and accelerated
-Positive ions travel through a hole in the negatively charged plate into a tube, velocity is calculated
-Positive ions are discharged by gaining electrons from the negative plate
-This generates a movement of electrons - size of current measures number of ions hitting the plate
When is electrospray ionisation used?
For substances with higher molecular masses (eg biological molecules such as proteins)
Advantage of electrospray ionisation
Fragmentation rarely takes place
Equation for kinetic energy:
1/2 x m x v^2
Velocity equation for particles in mass spectrometer
V = root (2KE / m)
Interpreting a mass spectrum
For elements, each line represents a different isotope of the element
Name the sub shells
S
P
D
F
Describe sub shell s
1 orbital
2 electrons
Describe sub shell p
3 orbitals
6 electrons
Describe sub shell d
5 orbitals
10 electrons
Describe sub shell f
7 orbitals
14 electrons
Rule when electrons are lost to form ions:
Lost from the 4s orbital before the 3d
Ionisation energy
The amount of energy required to remove one mole of electrons from one mole of atoms in the gaseous phase
What does a high ionisation energy mean
That there is a strong attraction between the electron and the nucleus so more energy is needed to remove the electron
Why does nuclear charge affect ionisation energy
The more protons the nucleus contains, the stronger the positive charge and the stronger the attraction to the electrons
Why does distance from the nucleus affect ionisation energies
The greater the distance from the nucleus of the electron, the weaker the attraction and the lower the activation energy
What is the shielding effect
As number of electron shells increases, outer shell electrons are less attracted to nucleus
This decreases ionisation energy
Ionisation trends down group 2
Going down , each element has one more electron shell than the one above, so gets bigger
This extra shell shields the outer electrons from nucleus, and is not compensated for by an increase in the number of protons in the nucleus
Therefore attraction is weaker and so ionisation energy values decrease going down the group
THIS PROVIDES EVIDENCE THAT ELECTRON SHELLS EXIST
Ionisation trends across period 3
General trend is that IE increases due to increase in number of protons and similar shielding
EXCEPT Mg to Al and P to S
Aluminium:
Outer shell electron is in 3p orbital , easier to remove
Sulfur:
Electron is being removed from a double occupied orbital, more repulsion, less energy needed
THUS, provides evidence for the existence of sub shells
