1. Atomic Structure Flashcards
What is the relative mass of a proton?
1
What is the relative mass of a neutron?
1
What is the relative mass of an electron?
1/1840
What is the relative charge of a proton?
+1
What is the relative charge of a neutron?
0
What is the relative charge of an electron?
-1
What is the location of protons in an atom?
In the nucleus
What is the location of neutrons in the atom?
In the nucleus
What is the location of electrons in the atom?
In the energy levels
Where is most of the mass of the atom concentrated?
In the nucleus
Where is most of the volume of the atom?
In the energy levels
What evidence is there for strong forces of attraction being in the nucleus?
The nucleus is high density and the protons overcome the forces of repulsion so there must be stronger forces of attraction
What is the atomic number?
The number of protons
What letter represents the atomic number?
Z
What is the mass number?
The number of protons and neutrons
What letter represents the mass number?
A
How do you calculate the number of neutrons in an atom?
Mass number - atomic number
How do you find the number of electrons in an atom?
Electron number = proton number
How are ions formed from atoms?
By losing or gaining an electron to gain an overall charge
What is an isotope of an element?
Atoms with the same number of protons but a different number of neutrons. Same proton number but a different mass number.
Why do isotopes have the same chemical properties?
Because they have the same electron configuration
Why do isotopes have different physical properties?
Because they have different mass numbers, so different masses.
What is relative atomic mass?
The mass of an atom of an element relative to 1/12 of an atom of carbon-12
Why is atomic mass measured relative to carbon-12?
Because the mass of carbon-12 is exactly 12.0000 and carbon-12 is a solid rather than a gas which is more pratical
What is relative isotopic mass?
The mass of an atom of an isotope compared with one-twelfth the mass of a carbon-12 atom.
What is natural abundance?
The percentage of an isotope occurring in a natural sample of an element
What is relative abundance?
The percentage of that particular isotope that occurs in nature.
What are the key ideas in John Dalton’s model of the atom?
An atom is a solid sphere with no spaces surrounded by heat
Diff spheres make up diff elements
All matter made up of elements
Atoms cannot be made or destroyed
What are the key ideas in John Jacob Berzelius’s model of the atom?
Atoms are spherical
Atoms are all the same size but diff weights
Atoms joined together in fixed proportions by an electrochemical reaction
What are the key ideas in Joseph John Thomson’s model of the atom?
The atom is like a plum pudding; electrons dotted about a positive sphere of charge.
Who discovered the electron?
Joseph John Thomson
Describe the process of Rutherford’s scattering experiment
Rutherford (and his assistants) wanted to test the plum pudding model so they devised an experiment. Alpha particles were shot from a source at a thin sheet of gold foil.
What did Rutherford expect to happen?
Based on the plum pudding model, Rutherford expected most of the a particles to be deflected slightly by the gold foil bc the a particles are +ve
What were the results of Rutherford’s scattering experiment?
A majority of the a particles passed straight through the foil
Some particles were deflected by less than one degree
Some particles straight back to the source
What conclusions could be drawn from the results of Rutherford’s scattering experiment?
Atom made up of mostly empty space (bc most particles went straight through)
Atom must have a +ve part (to repel the +ve a particles)
The +ve nucleus of the atom must have a high mass if it can stop a high velocity a particle and deflect it back
What were the key ideas in Ernest Rutherford’s model of the atom?
Small +vely charge nucleus which contained nearly all of the atom’s mass
Large empty space around the nucleus where atoms are found
Atom is overall neutral bc no of electrons = no of protons
What were the key ideas in Bohr’s model of the atom?
Electrons only exist in fixed shells and nowhere in between
Each shell has a fixed energy
When an electron moves between the shells EM radiation is emitted or absorbed
Bc the energy of the shells is fixed the radiation will have a fixed frequency
Why isn’t Bohr’s model of the atom completely correct?
Because the electrons in a shell do not all have the same energy so sub-shells were included
Why isn’t the most accurate model of the atom used?
It’s extremely complicated as it involves quantum mechanics so it’s hard to explain
What is mass spectrometry?
A method of chemical analysis which is used to find the abundance and mass of the isotopes of an elements so we can determine its relative atomic mass
What are the four main stages in time of flight mass spectrometry?
Ionisation, acceleration, flight tube and detection
What are the two methods of ionisation called?
Electrospray ionisation and electron impact
Explain electrospray ionisation
Sample is dissolved in a volatile solvent (water/methane) then injected into a fine hypodermic needle to produce a fine mist at high pressure. Tip of the needle attached to the +ve terminal which ionises sample X to form XH+ ions
Explain electron impact ionisation
Sample is vaporised then electrons are fired at the sample by an electron gun to knock off an electron giving the sample a 1+ charge so the sample can be attracted to the negative plate
What is an electron gun?
A hot wire filament with a current through it
When do you use electrospray ionisation?
For substances with a higher Mr
When do you use electron impact?
For substances with a lower Mr
In which method of ionisation does fragmentation occur?
Electron impact
What is fragmentation?
The process by which a positive ion is split into a positive fragment ion and a neutral species
Why is the ionisation stage necessary?
Because the particles need to be charged so they can be attracted to the negative plates to be accelerated
Explain how particles are accelerated in a mass spectrometer
The positive ions are attracted to a set of negative plates with a gap in between them. The positive ions travel through this gap into the flight tube
Explain what happens in the flight tube of a mass spectrometer
The ions travel into the flight tube with the same kinetic energy as each other but different masses. They travel at constant speeds but their speeds depend on their masses - the heavier the ions the slower they travel. They do not lose speed as they fly through.
Explain how ions are detected in a mass spectrometer.
The positive ions are attracted to the negative detector plate. When they hit the plate they gain an electron and are discharged. The gain of an electron causes a flow of electron which causes a current which can be measured by a computer.
What is a mass spectrum?
A series of peaks on a graph which have an x axis of mass/charge and a y axis of relative abundance
If the sample in a mass spec is an element, what does each line on the mass spectrum represent?
An isotope of the element
If the sample in a mass spec is an element, what do the values on the x and y axis tell us about the isotope?
The value on the x axis tells us the relative isotopic mass and the value on the y axis tells us the relative abundance of that isotope
If the sample in a mass spec is an element, what does the tallest line represent?
The most abundant isotope
If the sample in a mass spec is an element, what does the line furthest to the right represent?
The heaviest isotope
Explain how elements which exist as diatomic molecules such as chlorine are ionised in a mass spectrometer
An electron is knocked off the whole molecule to form Cl2+ which isn’t stable and may fall apart.
How is a diatomic molecule fragmented in a mass spectrometer?
Cl2+ —-> Cl+ + Cl. The Cl ion goes through the machine and depending on the isotope it will give lines at 35 or 37. If the Cl atom isn’t ionised then it just gets lost in the machine
What is the molecular ion peak?
The last major peak on a mass spectrum when the sample is a compound
How do you find the Mr of a compound from its mass spectrum?
Find the m/z value of its molecular ion peak
What is the relative molecular mass?
The mass of a molecule relative to 1/12 the mass of an atom of carbon-12
What is the base peak?
The peak with the greatest abundance
What is another word for energy shells?
Energy levels
What is the principal quantum number?
The shell which the electrons occupy
What happens to the distance between energy shells as you move outwards?
The energy shells get closer together
What is a sub-shell made up of?
An orbital or multiple orbitals
What is an orbital?
A 3D space that can hold up to 2 electrons
Why do electrons spin in opposite directions in an orbital?
To reduce repulsions
How many orbitals and electrons in an s subshell?
One orbital
Two electrons
How many orbitals and electrons in a p sub-shell?
Three orbitals
Six electrons
How many orbitals and electrons in a d sub-shell?
Five orbitals
10 electrons
How many orbitals and electrons in an f sub-shell?
Seven orbitals
14 electrons
What sub-shells are found in the n=1 energy level?
One s sub shell
What sub-shells are found in the n=2 energy level?
One s sub shell and one p sub shell
What sub-shells are found in the n=3 energy level?
One s, one p and one d
What is an electron’s ground state?
When electrons are in their lowest possible energy level
How do electrons fill up a p sub shell?
They’ll first go into empty orbitals before they go into an orbital which already has an electron in it (like people on a bus)
How are metal ions formed?
Metal atoms lose an electron to become +ve ions
How are non-metal ions formed?
Gain an electron to become -ve ions
How can hydrogen form ions?
Gain an electron to become a hydride ion H- or lose an electron to form H+
What is meant by two particles being isoelectronic?
Have the same electron configuration
What is ionisation?
The process of removing electrons from an atom to form an ion
What are ionisation energies measured in?
kJ/mol-1
Why are all ionisation energy values positive?
Because the reaction is endothermic because energy is taken in to remove the electrons
