1.1 Atomic Structure Flashcards
Define mass number.
The sum of the protons and neutrons in an atom. It is always a WHOLE NUMBER.
Define atomic number.
The number of protons in an atom.
What makes up the nucleus?
Protons and neutrons (the nucleons) are held together by nuclear forces.
Define relative atomic mass.
The average mass of one atom relative to the mass of 1/12th of one atom of Carbon 12.
What is mass spectrometry?
The mass spectrometer gives accurate information about relative isotopic mass and also about the relative abundance of isotopes.
It can be used to identify elements and to determine relative molecular mass.
Why does the mass spectrometer need to be under a vacuum?
If it was not then air particles would ionise and register on the detector.
What happens during the first stage of TOF MS?
Electron Impact Ionisation
1- Ionisation
Electron Impact Ionisation is used for atoms with a low formula mass as it can cause larger organic molecules to fragment. A vaporised sample is injected at low pressure. An electron gun (hot wire filament which emits electrons as a current runs through it) fires high energy electrons at the sample. This knocks an outer electron, forming a positive ion with a 1+ charge.
What happens during the first stage of TOF MS?
Electrospray Ionisation
Electrospray Ionisation is used for larger organic molecules as ‘softer’ conditions of this technique mean fragmentation does not occur. The sample is dissolved in a volatile, polar solvent. It is then injected through a fine needle giving a fine mist or aerosol. The tip of the needle has a high voltage, and this is where the sample molecule gains a proton/hydrogen ion from the solvent.
What happens during the second stage of TOF MS?
2-Acceleration
Positive ions are accelerated towards a negatively charged plate by an electric field to a constant kinetic energy. Since all 1+ ions have the same KE, their velocity will depend on their mass. Ions with a lower m/z ratio move faster.
What happens during the third stage of TOF MS?
3-Ion Drift
The 1+ ions pass through the flight tube, where their time of flight depends on their velocity. Heavier particles take longer to move through the drift area.
What happens during the fourth stage of TOF MS?
4- Detection
The ions reach the detector and generate a current, which is fed to a computer for analysis. The current is produced by electrons transferring from the detector to the positive ions, and the size of it is proportional to the abundance of the species.
What is the equation for calculating relative atomic mass?
(mass of isotope A X relative abundance of A) + (mass of isotope B X relative abundance of B) / 100 = RAM
Define the 1st ionisation energy.
The energy required to remove 1 mole of electrons from 1 mole of gaseous atoms to make 1 mole of gaseous 1+ ions.
Define the 2nd ionisation energy.
The energy required to remove 1 mole of electrons from 1 mole of gaseous 1+ ions to form 1 mole of gaseous 2+ ions.
Electrons are removed sequentially.
What is the pattern for removing electrons from an atom?
Electrons further from the positive nucleus require less energy to remove whereas electrons closer to the positive nucleus require more energy to remove.
Evidence for the existence of orbitals:
Describe the overall pattern of first ionisation results across a period.
The first ionisation energies increase across a period because
- same number number of shells/energy levels and shielding (electrons in inner shells shield the nuclear charge)
-nuclear charge/number of protons increases
Evidence for the existence of orbitals:
First ionisation energy
Why is Aluminium lower than Magnesium?
-The electron is removed from 3p and not 3s
-3p is further from the nucleus than 3s (3p is a higher energy orbital)
Evidence for the existence of orbitals:
First ionisation energy
Why is Sulfur lower than Phosphorous?
-In Sulfur, an electron is removed from 3p4
-A pair of electrons are repelling each other
What is the rule for first ionisation energies down a group?
-They decrease down a group
-More shells and more shielding
-Outer electrons are further from the nucleus
On a graph there is a curve not a straight line because the orbitals are getting closer together.
What is the rule for the electron configuration of ions?
4s electrons are lost before the 3d electrons when ions are formed.
Other than D block elements, atoms lose an electrons from the outer orbital first.
