1.1 Atomic Structure Flashcards
What is the symbol for atomic number?
Z
What is the symbol for mass number?
A
Define atomic number
The number of protons in an atom
Define mass number
The amount of protons plus neutrons in an atom
Define relative atomic mass
Average mass of an atom compared to 1/12th the mass of a carbon-12 atom
What is an isotope?
An atom of the same element with the same number of protons but a different number of neutrons
Why do isotopes have the same chemical properties?
Because they have the same electronic configurations
What is mass spectrometry used to determine?
Ar of elements and isotopes/relative abundance of isotopes/Mr of covalent compounds
What is ionisation?
High energy electrons fired from an electron gun knock off an electron from a gaseous atom, forming a positive ion
What is acceleration?
Positive ions are accelerated using negatively charged plates as opposite charges attract- positive ions are attracted and accelerated by these negatively charged plates
What is ion drift?
As all ions have a different mass but have the same kinetic energy so they have different velocities. ToF depends on the mass of the ions, and the lighter ions travel faster so reach the detector first
What is detection?
When positively charged ions hit the detector, electrons flow from the detector plate to the positive ions, providing a current. The greater the current, the greater the abundance of the ion
How do you know the Mr from a mass spectra graph?
The highest (furthest along the x-axis) m/z value is equal to the Mr (with electron impact ionisation)
Why is it necessary to ionise the atoms?
-particles need to have a positive charge to be accelerated with the negative plates
-neutral particles cannot generate a current in order to be detected at the detector plates
Why might the Ar be slightly different on the periodic table than to what you work out?
Because the relative abundance of isotopes varies/differs in different parts of the world
What will always generally happen when you have a peak of C-12 on a mass spectra graph?
There will be another very small peak of C-13 that is roughly 100x smaller
What are the two isotopes of Br and the relative ratio they’re found in?
79Br and 81Br, in a relative ratio of 1:1
What are the two isotopes of Cl and the relative ratio they’re found in?
35Cl and 37Cl, in a relative ratio of 3:1
What are the two types of ionisation and what are they each used for?
Electron impact ionisation - used for atoms/elements/small molecules
Electron spray ionisation - used for very large molecules like proteins/DNA
What are the steps to electron spray ionisation?
-sample is dissolved in a volatile solvent
-this is injected through a fine hypodermic needle giving a fine mist
-the tip of the needle has a high voltage
-each molecule gains a proton (H+)
What is the general formula for electron spray ionisation?
M + H^+ –> [MH^]+
How do you find out the Mr from a mass spectra graph of an electron spray ionisation?
Find the highest m/z value and -1 (to take into account the proton it gained during ionisation)