Atomic Structure
Fundamental Particles & Isotopes:
• Mass Number = number of protons + number of neutrons
• Atomic Number = number of protons
= number of electrons
• Number of neutrons = mass number – atomic number
Atoms form ions by losing or gaining electrons
Negative ions (anions) have gained electrons, positive ions
(cations) have lost electrons.
Isotopes are atoms of the same element with a different number of neutrons.
History of the Atomic Model:
• John Dalton = Atoms are solid spheres
• JJ Thompson = Plum pudding model, electrons in a sphere of positive charge
• Ernest Rutherford = Nuclear model – Dense positive centre, surrounded by cloud of
electrons.
Rutherford fired alpha particles at gold foil, most
particles passed through showing that atoms were
mostly empty space. Some were deflected,
showing that there was a dense positive charge in
the centre.
• Niels Bohr = Bohr Model – Dense positive nucleus, electrons contained in surrounding
shells.
• Further experiments by Rutherford and Chadwick found neutrons and protons located in
the nucleus.
Relative Mass:
• The relative atomic mass (Ar)is the average mass of an atom of an element on a scale in
which an atom of carbon-12 has a mass of 12.
• The relative molecular mass (Mr) is an average mass of a molecule on a scale in which an
atom of carbon-12 has a mass of 12.
, • The relative isotopic mass is the mass of an atom of an isotope on a scale in which an
atom of carbon-12 has a mass of 12.
Mass Spectrometry:
The Ar can be calculated by using a mass spectrometer. It separates atoms based on their
mass, and then gives the relative abundances of each component.
Mass Spectrometry can be used to identify isotopes and find their relative abundances.
Calculating RAM:
1. Multiply the m/z value by the relative abundance for each peak.
2. Add results together.
3. Divide by total relative abundance.
Time of Flight Mass Spectrometry:
1. Ionisation
2. Acceleration
3. Ion Drift
4. Ion Detection
• Ionisation can be done in two ways: electrospray ionisation and electron gun impact.
Electrospray ionisation = sample is dissolved in a volatile solvent and passed through a
positively charged needle where the particles gain a proton.
.X(g) + H+ -> XH+(g)
Electron gun impact = High energy electrons are fired at the gaseous sample, which
ionise the particles in the sample by knocking off an electron.
X(g) -> X+(g) + e-
• The ions are accelerated so that they all have the same kinetic energy.
• Then, the ions are left to drift in the flight tube. Lighter particles travel faster and so will
have a shorter time of flight.
Fundamental Particles & Isotopes:
• Mass Number = number of protons + number of neutrons
• Atomic Number = number of protons
= number of electrons
• Number of neutrons = mass number – atomic number
Atoms form ions by losing or gaining electrons
Negative ions (anions) have gained electrons, positive ions
(cations) have lost electrons.
Isotopes are atoms of the same element with a different number of neutrons.
History of the Atomic Model:
• John Dalton = Atoms are solid spheres
• JJ Thompson = Plum pudding model, electrons in a sphere of positive charge
• Ernest Rutherford = Nuclear model – Dense positive centre, surrounded by cloud of
electrons.
Rutherford fired alpha particles at gold foil, most
particles passed through showing that atoms were
mostly empty space. Some were deflected,
showing that there was a dense positive charge in
the centre.
• Niels Bohr = Bohr Model – Dense positive nucleus, electrons contained in surrounding
shells.
• Further experiments by Rutherford and Chadwick found neutrons and protons located in
the nucleus.
Relative Mass:
• The relative atomic mass (Ar)is the average mass of an atom of an element on a scale in
which an atom of carbon-12 has a mass of 12.
• The relative molecular mass (Mr) is an average mass of a molecule on a scale in which an
atom of carbon-12 has a mass of 12.
, • The relative isotopic mass is the mass of an atom of an isotope on a scale in which an
atom of carbon-12 has a mass of 12.
Mass Spectrometry:
The Ar can be calculated by using a mass spectrometer. It separates atoms based on their
mass, and then gives the relative abundances of each component.
Mass Spectrometry can be used to identify isotopes and find their relative abundances.
Calculating RAM:
1. Multiply the m/z value by the relative abundance for each peak.
2. Add results together.
3. Divide by total relative abundance.
Time of Flight Mass Spectrometry:
1. Ionisation
2. Acceleration
3. Ion Drift
4. Ion Detection
• Ionisation can be done in two ways: electrospray ionisation and electron gun impact.
Electrospray ionisation = sample is dissolved in a volatile solvent and passed through a
positively charged needle where the particles gain a proton.
.X(g) + H+ -> XH+(g)
Electron gun impact = High energy electrons are fired at the gaseous sample, which
ionise the particles in the sample by knocking off an electron.
X(g) -> X+(g) + e-
• The ions are accelerated so that they all have the same kinetic energy.
• Then, the ions are left to drift in the flight tube. Lighter particles travel faster and so will
have a shorter time of flight.
