Atomic Structure
Basic Structure of an Atom
Nucleus consists of protons and neutrons
Electrons are in shells/energy levels surrounding the nucleus
Proton – +1 relative charge and 1 relative mass
Neutron – 0 relative charge and 1 relative mass
Electron – -1 relative charge and 1/1840 relative mass
Mass number – Sum of protons and neutrons in an atom
Atomic number – Number of protons in an atom
Relative atomic mass – Average mass of an atom of an element relative to one twelth of the
mass of a carbon-12 atom
Isotopes – Atoms of the same element with the same atomic number but different mass
number
o React chemically in the same way as atomic number and electron configuration is
the same but have different physical properties
Mass Spectrometry
Used to identify different isotopes and find the relative atomic mass of an element
1. Ionisation
o Electron impact
Sample of an element is vapourised and injected into the mass
spectrometer, where an electron gun fires high-energy electrons at the
sample, which knocks off an electron from each atom, forming a 1+ ion
o Electrospray ionisation
Sample is dissolved in a polar volatile solution
Injected through needle as fine spray into vacuum in ionisation chamber
High voltage is applied and atoms gain a proton (H+) and form gaseous
positive ions
2. Acceleration
o Ions are accelerated by an electric field – all have the same kinetic energy
3. Ion Drift
o Occurs in the flight tube
o Ions are separated as ions with different masses have different times of flight
o Lighter ions travel faster and take less time – lower times of flight
4. Detection
o Detector is negatively charged plate
o Positive ions gain an electron each and produce a current when they hit the plate
5. Data Analysis
o Produce a mass spectrum – relative abundance
o Size of current proportional to abundance of ion
Mass Spectra
Elements
Shows mass and abundance of each isotope – can be used to calculate relative atomic mass
,
Ar = ∑(mass of isotope x relative abundance)/∑(relative abundance)
Example:
o (20*90.9)+(21*0.26)+(22*8.8)/(90.0+0.26+8.8)=20.2
Molecules
Cl-35 = 75%
Cl-37 = 25%
Cl2-70 = Cl-35 + Cl-35 (3/4 x 3/4 = 9/16)
Cl2-72 = Cl-35 + Cl-37 OR Cl-37 + Cl-35 ((3/4 x 1/4) + (1/4 x 3/4) = 6/16)
Cl2-74 = Cl-37 + Cl-37 (1/4 x 1/4 = 1/16)
Time of Flight Calculations
KE – J
m = kg
v = m/s
d = length of flight tube (m)
Electron Configuration
Electrons are arranged in energy levels, which themselves have sub-levels
Each sub-level consists of electron orbitals (regions of space where electrons are most likely
to be) which can hold 2 electrons with opposite spins (
S sub-level – 1 orbital
Basic Structure of an Atom
Nucleus consists of protons and neutrons
Electrons are in shells/energy levels surrounding the nucleus
Proton – +1 relative charge and 1 relative mass
Neutron – 0 relative charge and 1 relative mass
Electron – -1 relative charge and 1/1840 relative mass
Mass number – Sum of protons and neutrons in an atom
Atomic number – Number of protons in an atom
Relative atomic mass – Average mass of an atom of an element relative to one twelth of the
mass of a carbon-12 atom
Isotopes – Atoms of the same element with the same atomic number but different mass
number
o React chemically in the same way as atomic number and electron configuration is
the same but have different physical properties
Mass Spectrometry
Used to identify different isotopes and find the relative atomic mass of an element
1. Ionisation
o Electron impact
Sample of an element is vapourised and injected into the mass
spectrometer, where an electron gun fires high-energy electrons at the
sample, which knocks off an electron from each atom, forming a 1+ ion
o Electrospray ionisation
Sample is dissolved in a polar volatile solution
Injected through needle as fine spray into vacuum in ionisation chamber
High voltage is applied and atoms gain a proton (H+) and form gaseous
positive ions
2. Acceleration
o Ions are accelerated by an electric field – all have the same kinetic energy
3. Ion Drift
o Occurs in the flight tube
o Ions are separated as ions with different masses have different times of flight
o Lighter ions travel faster and take less time – lower times of flight
4. Detection
o Detector is negatively charged plate
o Positive ions gain an electron each and produce a current when they hit the plate
5. Data Analysis
o Produce a mass spectrum – relative abundance
o Size of current proportional to abundance of ion
Mass Spectra
Elements
Shows mass and abundance of each isotope – can be used to calculate relative atomic mass
,
Ar = ∑(mass of isotope x relative abundance)/∑(relative abundance)
Example:
o (20*90.9)+(21*0.26)+(22*8.8)/(90.0+0.26+8.8)=20.2
Molecules
Cl-35 = 75%
Cl-37 = 25%
Cl2-70 = Cl-35 + Cl-35 (3/4 x 3/4 = 9/16)
Cl2-72 = Cl-35 + Cl-37 OR Cl-37 + Cl-35 ((3/4 x 1/4) + (1/4 x 3/4) = 6/16)
Cl2-74 = Cl-37 + Cl-37 (1/4 x 1/4 = 1/16)
Time of Flight Calculations
KE – J
m = kg
v = m/s
d = length of flight tube (m)
Electron Configuration
Electrons are arranged in energy levels, which themselves have sub-levels
Each sub-level consists of electron orbitals (regions of space where electrons are most likely
to be) which can hold 2 electrons with opposite spins (
S sub-level – 1 orbital
