Topic 1: Atomic structure (physical chemistry) Flashcards
What do the chemical properties of elements depend on
electronic configuration
What does the arrangement of electrons in energy levels link to
the way elements are organised in the periodic table
How can chemists measure the mass of atoms and molecules to a high degree of accuracy
By using a mass spectrometer (time of flight)
What is the mass and charge of protons
Mass = 1
Charge = +1
What is the mass and charge of electrons
Mass = 1/1835
Charge = -1
What is the mass and charge of neutrons
Mass = 1
Charge = 0 (neutral)
What is the overall charge of the nucleus of an atom
positive (contains protons and neutrons)
What is the overall charge of an atom
Neutral (positive and negative charges cancel eachother out)
What is an isotope
isotopes are atoms of the same element with the same number of protons but a different number of neutrons
what does a mass spectrometer give accurate information about
relative isotopic mass and relative abundance of isotopes
What are the steps for time of flight
Ionisation, acceleration, ion drift, detection
What is ionisation energy
the amount of energy required to remove one mole of electrons from one mole of gaseous atoms of an element to form one mole of gaseous ions.
what are cations
positive ions
What are anions
negative ions
what does the period number show
the amount of energy levels
What does the group number show
the amount of electrons in the outer level
how do you calculate mr
the mean mass of an atom / 1/12 x mean mass of a c-12 isotope
how do cations and anions form
cations by losing an electron
anions by gaining an electron
what are polyatomic ions
ions that possess more than one atom but have one overall charge
what is an ionic compound
when a negative ion joins with a positive ion
How do you work out ionic formula
drop the charges then swap
Define relative atomic mass
the average mass of an atom of an element relative to 1/12 the mass of an atom of carbon-12
State two features of the current model that arent shown in Rutherford’s model
-the current model includes protons and neutrons
-the current model shows electrons in different energy levels
Which atom contains only two unpaired electrons
oxygen
State two differences between the plum pudding model and the model of atomic structure used today
-nucleus contains protons and neutrons
-electrons are now arranged in energy levels
State which of the elements magnesium and aluminium has the lower first ionisation energy
Aluminium has the lower first ionisation energy as it’s outer electron is in the 3rd energy level. This means that the outer electron is easily lost.
What does time of flight record
the time it takes for ions of each isotope to reach a detector
What occurs during the first step of TOF
ionisation –> a sample of an element is vapourised and injected into the mass spectrometer where a high voltage is passed over the chamber. This causes electrons to be removed from the atoms creating +1 ions. Or the sample is hit with high energy electrons that knock an electron off.
What is the second stage of TOF
Acceleration –> these positively charged ions are then accelerated to a negatively charged detection plate
What occurs during the third stage of TOF
Ion drift –> the ions are then deflected by a magnetic field into a curved path. The radius of their path is dependent on their charge and mass of the ion
What occurs during the final stage of TOF
Detection –> when the positive ions hit the negatively charged detection plate they gain an electron thus producing a current. The size of the current is proportional to the relative abundance of an isotopes
how do you calculate kinetic energy
Ke = 1/2mv^2
How do you calculate velocity
square root 2Ke / m (mass)
How do you calculate time
distance (length) / velocity
distance / square root 2Ke / m
What are the positive ions called
molecular ions
complete the sentence —> the heavier the ion
the slower it will accelerate and the longer it will take to hit the detector
What is time of flight proportional to
square root the mass of the ions
what are the two ways a sample can be ionised
either electron impact or electrospray ionisationw
what does abundance tell us
how common each isotope is
the size of current is proportional to the abundance
true or false –> all ions with the same charge have the same kinetic energy as eachother
true
what is m/z ratio
the ratio of the mass of each ion to its charge
define empirical formula
the simplest whole number ratio of atoms of each element in a compound
Explain how ions are accelerated, detected and abundanced in a mass spectrometer
-ions are accelerated by attraction to negatively charged plate
-the ions are detected by gaining an electron
-abundance is proportional to the size of current flowing in the detector
Describe how ions are formed in a mass spectrometer
1) electrospray ionisation –> a sample is dissolved in a volatile solvent. The sample is then passed through a positvley charged needle thus causing each molecule gaining a proton
2) electron impact –> high energy electrons are fired at the sample which then knocks off an electron thus forming a positive ion
What are the two types of ionisation
electron impact and electrospray ionisation
Why is it necessary to ionise a molecule when measuring their mass
-ions only will interact with and be accelerated by an electric field
-only ions would be attracted to the negatively charged plate
-only ions will created a current
What is the purpose of mass spectrometry
to identify the masses of unknown chemical compounds
How are we able to determine the identity of a compound by knowing its mass
each compound has its own relative formula mass. By knowing the mass we can work out the elements present.
What is a summary of how the mass spectrometer works
-a gas sample enters the spectrometer
-a filament in the spectrometer ionises the gas using high energy electrons
-the ions are accelerated with an electric field
-the lighter ions will travel faster
-ions are separated and deflected by mass using a magnetic field
what is the equation for electron impact
X(g) –> X+(g) + e-
What occurs during electron impact
an electron is knocked off thus forming a positive ion
what is the equation for electrospray ionisation
X (g) + H+ (g) —> XH+ (g)
why are positive ions accelerated
so they all have the same kinetic energy
what type of compounds can only be used in a mass spectrometer
gaseous compounds
what does the flight of each particle depend on
velocity
What occurs during detection
1) the positive ion hits a negatively charged electric plate. When they hit the detector plate the positive ions gain electrons from the plate
2) this generates an electric current. (note if there were not ions, this would not be possible)
3) the abundance of ions is proportional to the size of current
what is the m/z ratio
the mass of ions (mass to charge)
What does each peak on the spectrum represent
an isotope
How do you calculate relative isotopic mass
1) calculate percentage abundance of unknown isotope
2) input values into equation
3) rearrange
How many isotopes does chlorine have
2
35Cl-35Cl +
35Cl-37Cl+ 37Cl-35Cl+
37Cl-37Cl+
How many isotopes does bromine have
2
81Br and 79Br
what is the abundance of chlorine 35 and 37
35 = 75%
37 = 25%
Define diatomic
a molecule possessing two molecules that are identical
What is ionisation energy
ionisation energy is the amount of energy required to remove one mole of electrons from one mole of gaseous atoms of an element to form one mole of gaseous ions
What are the units of ionisation energy
KJ/mol
do group 1 elements have a lower or higher ionisation energy than nobel gases
lower
What is first ionisation energy affected by
-size of nuclear charge
-distance of outer electron from nucleus (atomic radius)
-shielding effect of inner electrons
-spin pair repulsion