Atomic Structure

Question 1

Give the relative masses and charges of protons, neutrons and electrons

Answer 1

Protons:
Relative mass = 1
Relative charge = +1

Neutrons:
Relative mass = 1
Relative charge = 0

Electrons:
Relative mass = 1/1836
Relative charge = -1

Protons and neutrons have a very similar mass so each is assigned a relative mass of 1 whereas electrons are 1836 times smaller than a proton and neutron

Question 2

Why do subatomic particles not have units

Answer 2

They are too small that they can only be measured in comparison in comparison to one another, hence the importance of relative masses and relative charges

Question 3

What are protons and neutrons also refered to as?

Answer 3

Nucleons

Question 4

Explain the difference between atoms and ions

Answer 4

An atom is neutral and has no overall charge
Ions on the other hand are formed when atoms either gain or lose electrons, causing them to become charged

Question 5

Explain the existence of isotopes

Answer 5

-Isotopes are atoms of the same element that contain the same number of protons and electrons but a different number of neutrons
-The symbol for an isotope is the chemical symbol (or word) followed by a dash and then the mass number
-E.g. carbon-12 and carbon-14 are isotopes of carbon containing 6 and 8 neutrons respectively

Isotopes have the same chemical properties but different physical properties
Chemical properties
-Isotopes of the same element display the same chemical characteristics
-This is because they have the same number of electrons in their outer shells
-Electrons take part in chemical reactions and therefore determine the chemistry of an atom
Physical properties
-The only difference between isotopes is the number of neutrons
-Since these are neutral subatomic particles, they only add mass to the atom
-As a result of this, isotopes have different physical properties such as small differences in their mass and density

Question 6

Explain the principles of a simple time of flight (TOF) mass spectrometer, limited to ionisation, acceleration to give all ions constant kinetic energy, ion drift, ion detection, data analysis.

Answer 6

The mass spectrometer gives accurate information about relative isotopic mass and also about the relative abundance of isotopes.

Mass spectrometry can be used to identify elements.

Mass spectrometry can be used to determine relative molecular mass.

Question 7

What are relative abundance values and give the equation

Answer 7

The relative atomic mass of an element can be calculated by using the relative abundance values
The relative abundance of an isotope is either given or can be read off the mass spectrum

(relative abundance of isotope 1 x mass of isotope 1) + (relative abundance of isotope 2 x mass of isotope 2) all divided by 100

Question 8

What is mass spectrometry?

Answer 8

-useful for accurate determination of relative atomic mass of an element, based on the abundance and mass of each of its isotopes
-also used to find the relative molecular mass of molecules

-as sample passes through mass spectrometer, a spectrum is produced of mass / charge ratio against abundance
-spectrum can be used to find relative isotopic abundance, atomic and molecular mass and the structure of a compound
-peak with the highest mass is the molecular ion peak, M+, and the peak which has the largest abundance (tallest peak) is called the base peak
-several types of mass spectrometer, but all of them are based on an ionised sample being accelerated through the mass spectrum, and being separated based on the ratio of their charge to their mass

Question 9

What is Time of Flight (TOF) mass spectrometry?

Answer 9

-all particles of the sample to be analysed are ionised to form 1+ ions
-which are then accelerated to high speeds, deflected through the spectrometer and then arrive at the detector
-as they hit the detector, the mass spectrum graph is produced
-apparatus is kept under a high vacuum to prevent any ions that are produced from colliding with molecules in the air

Question 10

What is stage 1 of (TOF) mass spectrometry

Answer 10

Stage 1: Ionisation
-sample can be ionised by electron impact or electrospray ionisation

Electron Impact Ionisation:
-used for substances with a lower molecular mass
-sample is vaporised and then bombarded with high energy electrons
-electrons are emitted from electron gun
-as sample is bombarded by these electrons, an electron is knocked off each particle, forming a 1+ ion
X (g) → X+ (g) + e-
-1+ ions which have been formed are called molecular ions, or M+ ions

Electrospray Ionisation
-used for substances which have a higher molecular mass
-fragmentation is unlikely to happen
-sample is dissolved in a volatile solvent
-solvent is injected into the mass spectrometer using a hypodermic needle
-produces a fine mist or aerosol
-needle is attached to a high voltage power supply, so as the sample is injected, the particles are ionised by gaining a proton from the solvent
X (g) + H+ → XH+ (g)
-solvent evaporates and the XH+ ions remain

Question 11

What is stage 2 of (TOF) mass spectrometry

Answer 11

Stage 2: Acceleration
-the 1+ ions formed are then attracted towards a negatively charged plate
-accelerates them through the mass spectrometer to have the same kinetic energy
-as all 1+ ions will have the same kinetic energy, their velocity will depend on their mass
-lighter ions will move faster and heavier ions will move slower

The molecular ion can be broken down further, or fragmented
The fragments are also accelerated through the sample and hit the detector, causing different peaks to show on the mass spectrum which is produced

Question 12

What is stage 3 of (TOF) mass spectrometry

Answer 12

Stage 3: Ion Drift (in the flight tube)
-1+ ions will pass through a hole in the negatively charged plate and move into a flight tube
-time of flight of each 1+ ion in this tube depends on their velocity

Question 13

What is stage 4 of (TOF) mass spectrometry

Answer 13

Stage 4: Detection
-once they pass through the mass spectrometer, the 1+ ions will hit a negatively charged ‘detector’ plate
-as they hit this electric plate, they gain an electron
-by gaining an electron, the ion is discharged, causing a current to be produced
-size of the current is proportional to the abundance of those ions hitting the plate and gaining an electron
-the detector plate is connected to a computer, which produces the mass spectrum

Question 14

Key equations/extra info for the mass spectrum

Answer 14

KE = 1/2 MV^2
v = √(2KE)/m

t = d/v
t = d√m/ (2KE)

where

KE = kinetic energy of the particles (J)

m = mass of the particles (kg)

v = velocity of the particles (ms-1)

t = time of flight of the particles (s)

d = the length of the flight tube (m)

All particles in the mass spectrometer are accelerated to the same kinetic energy.

The time of flight is proportional to the square root of the mass of the ions, showing that the lighter the ion the faster it will pass through and the quicker it will hit the detector.

The heavier the ion, the slower it will travel and the longer it will take to hit the detector.

Question 15

What is electronic structure?

Answer 15

The arrangement of electrons in an atom

Question 16

Explain what principal quantum numbers are

Answer 16

Principal quantum numbers (n) are used to number the energy levels or quantum shells
The lower the principal quantum number, the closer the shell is to the nucleus
So, the first shell which is the closest to the nucleus is n = 1
The higher the principal quantum number, the greater the energy of the shell and the further away from the nucleus
Each principal quantum number has a fixed number of electrons it can hold
n = 1 : up to 2 electrons
n = 2 : up to 8 electrons
n = 3 : up to 18 electrons
n = 4 : up to 32 electrons

Question 17

What are subshells and orbitals

Answer 17

The principal quantum shells are split into subshells which are given the letters s, p and d
Elements with more than 57 electrons also have an f shell
The energy of the electrons in the subshells increases in the order s < p < d
The order of subshells appear to overlap for the higher principal quantum shells as seen in the diagram below:

Subshells contain one or more atomic orbitals
Orbitals exist at specific energy levels and electrons can only be found at these specific levels, not in between them
Each atomic orbital can be occupied by a maximum of two electrons
This means that the number of orbitals in each subshell is as follows:
s : one orbital (1 x 2 = total of 2 electrons)
p : three orbitals ( 3 x 2 = total of 6 electrons)
d : five orbitals (5 x 2 = total of 10 electrons)
f : seven orbitals (7 x 2 = total of 14 electrons)
The orbitals have specific 3-D shapes

s orbital shape
The s orbitals are spherical in shape
The size of the s orbitals increases with increasing shell number
E.g. the s orbital of the third quantum shell (n = 3) is bigger than the s orbital of the first quantum shell (n = 1)
p orbital shape
The p orbitals have a dumbbell shape
Every shell has three p orbitals except for the first one (n = 1)
The p orbitals occupy the x, y and z axes and point at right angles to each other, so are oriented perpendicular to one another
The lobes of the p orbitals become larger and longer with increasing shell number

The principal quantum shells increase in energy with increasing principal quantum number
E.g. n = 4 is higher in energy than n = 2
The subshells increase in energy as follows: s ＜ p ＜ d ＜ f
The only exception to these rules is the 3d orbital which has slightly higher energy than the 4s orbital
Because of this, the 4s orbital is filled before the 3d orbital

Question 18

What is ground state

Answer 18

The ground state is the most stable electronic configuration of an atom which has the lowest amount of energy
This is achieved by filling the sub shells of energy with the lowest energy first (1s)
The order of the sub shells in terms of increasing energy does not follow a regular pattern at n = 3 and higher

Question 19

How is the Periodic table arranged in terms of electronic configuration?

Answer 19

The Periodic Table is split up into four main blocks depending on their electron configuration
Elements can be classified as an s-block element, p-block element and so on, based on the position of the outermost electron:
s block elements
Have their valence electron(s) in an s orbital
p block elements
Have their valence electron(s) in a p orbital
d block elements
Have their valence electron(s) in a d orbital
f block elements
Have their valence electron(s) in an f orbital

Question 20

Define ionisation energy and explain its trends in the Periodic table

Answer 20

-the ionisation energy (IE) of an element is the amount of energy required for 1 mole of electrons to be removed from 1 mole of gaseous atoms to form 1 mole of gaseous ions
-ionisation energy is measured under standard conditions (298K and 101kPa)
-units of IE are kilojoules per mole (kJ mol-1)

The ionisation energy across a period generally increases due to the following factors:
-Across a period the nuclear charge increases
-This causes the atomic radius of the atoms to decrease, as the outer shell is pulled closer to the nucleus, so the distance between the nucleus and the outer electrons decreases
-The shielding by inner shell electrons remain reasonably constant as electrons are being added to the same shell
-It becomes harder to remove an electron as you move across a period; more energy is needed
-So, the ionisation energy increases

There is a large decrease in ionisation energy between the last element in one period, and the first element in the next period. This is because:
-There is increased distance between the nucleus and the outer electrons as you have added a new shell
-There is increased shielding by inner electrons because of the added shell
These two factors outweigh the increased nuclear charge

The ionisation energy down a group decreases due to the following factors:
-The number of protons in the atom is increased, so the nuclear charge increases
-But, the atomic radius of the atoms increases as you are adding more shells of electrons, making the atoms bigger
-So, the distance between the nucleus and outer electron increases as you descend the group
-The shielding by inner shell electrons increases as there are more shells of electrons
-These factors outweigh the increased nuclear charge, meaning it becomes easier to remove the outer electron as you descend a group
-So, the ionisation energy decreases

Question 21

Define and describe the trends and exceptions to the trends of first ionisation energy

Answer 21

the energy required to remove one mole of electrons from one mole of atoms of an element to form one mole of 1+ ions
E.g. the first ionisation energy of gaseous calcium:
Ca(g) → Ca+ (g) + e- IE1 = +590 kJ mol-1

The size of the first ionisation energy is affected by four factors:
-Size of the nuclear charge
-Distance of outer electrons from the nucleus
-Shielding effect of inner electrons
-Spin-pair repulsion
First ionisation energy increases across a period and decreases down a group

There is a slight decrease in IE1 between beryllium and boron as the fifth electron in boron is in the 2p subshell, which is further away from the nucleus than the 2s subshell of beryllium
-Beryllium has a first ionisation energy of 900 kJ mol-1 as its electron configuration is 1s2 2s2
-Boron has a first ionisation energy of 800 kJ mol-1 as its electron configuration is 1s2 2s2 2px1

Question 22

What are successive ionisation energies

Answer 22

-occur when further electrons are removed
-usually requires more energy as electrons are removed by electrostatic forces of attraction between the positive nucleus and the negative valence electrons increases
-more energy is therefore needed to overcome this attraction, hence why ionisation energy increases
-when successive ionisation energies are plotted on a graph, a large sudden increase indicates a change in energy level, as the electron is being removed from an orbital closer to the nucleus, hence requiring more energy to do so
-large energy increase provides supporting evidence for the atomic orbital theory

Question 23

explain how first and successive ionisation energies in Period 3 (Na–Ar) and in Group 2 (Be–Ba) give evidence for electron configuration in sub-shells and in shells

Question 24

what are mononuclear ions

Answer 24

ions which are formed from a single atom

Question 25

What are smaller and significantly lighter peaks due to?

Answer 25

Fragmentation

Question 26

During acceleration, all molecules are accelerates so they have the same….

Answer 26

Kinetic energy