1.1 Atomic Structure Part 1 Flashcards
What is the mass of a proton?
1
What is the mass of a neutron?
1
What is the mass of an electron?
1/2000
What is the charge of a proton?
+1
What is the charge of a neutron?
O
What is the charge of an electron?
-1
What is the atomic number?
The number of protons in the nucleus of an atom
What is the mass number?
The number of protons and neutrons in the nucleus of an atom
What is an isotope?
Are atoms of the same element (have the same number of protons) with a different number of neutrons
Why do isotopes have the same chemical properties?
They have the same number and arrangement of outer shell electrons
When are posture ions formed?
When the atom loses electrons
When are negative ions formed?
When atoms gain electrons
What are the 4 steps in mass spectrometry?
- ionisation
- acceleration
- separation of ions
- detection
What are the two ways an atom can be ionised in step one of mass spec.?
Electron impact and electrospray ionisation
Describe electron impact ionisation
- Sample is injected into spectrometer and vaporised
- High energy electrons from an electron gun are fired at the sample
- The high energy electrons knock off an electron from each particle to form positive ions
Write the equation for the electron impact ionisation of magnesium
Na (g) -> Na+ (g) + e-
g
Explain electrospray ionisation
- sample is dissolved in polar solvent
- injected through a fine hypodermic needle to give a fine mist
- the tip of the needle is attached to the positive terminal of a high voltage supply
- particles gain a proton (H+ ion) from the solvent to form positive ions
What is the general equation for electrospray ionisation?
M(g) + H+ -> MH+ (g)
What is important about the mass of the result of electrospray ionisation?
Mass is one more than the sample
Why is a volatile solvent used in electrospray ionisation?
So it can be easily evaporated
What does the mass of the ion at the end of mass spec. depend on?
The type of ionisation
What is the mass of ion in electron impact ionisation?
Mass of element / compound
What is the mass of the ion at the end of electrospray ionisation?
(Mass of element / compound) + 1
How are positive ions accelerated in mass spec.?
An electric field is applied
What does applying an electric field do to the ions in mass spec.?
- accelerates them
- gives them the same kinetic energy
What does the velocity that an ion travels in mass spec. Depend on?
The mass
Which type of ions reach the detector first in mass spec.?
Lighter ions
Why is the whole spectrometer kept under vacuum?
- to stop unwanted reactions
- only analyse our sample
What is the detection part of mass spec.?
The detector records the different ions as they arrive
How can ions be detected in mass spec.?
The positive ion accepts an e-
A graph of the results from mass spec has been drawn. The relative atomic mass of Mg is 24.3. Why is there no peak at 24.3?
Because isotopes have whole mass numbers, 24.3 is the average
Why must the sample be ionised in mass spec.?
So it can be accelerated
So it can be detected
What is the isotopic mass?
The mass of an isotope relative to 1/12th the mass of an atom of carbon-12
What is relative atomic mass (Ar)?
The weighted average mass of all the isotopes relative to 1/12th the mass of an atom of carbon-12
How do you calculate relative atomic mass?
(Mass x abundance of each isotope) / total abundance
Why is there only two peaks on the graph of mass spectra obtained from electrospray ionisation?
It doesn’t cause fragmentation
What is electron impact ionisation used for?
Identifying unknowns
What is electrospray ionisation used for?
Finding the Mr of large, delicate molecules such as DNA
