Atomic Structure

Question 1

What is Mass number (A) (1)

Answer 1

⇒ Σ(proton+neutron)

Question 2

What is an Isotope? (2)

Answer 2

⇒ Atoms of the same element that have the same number of protons (same atomic number) (1)
⇒ But a different number of neutrons (different mass number) (1)

Question 3

Define Relative Molecular Mass (Mr). (2)

Answer 3

⇒ Mean mass of a molecule/(1/12th) mass of an atom of 12C (2)

Question 4

Define Relative Atomic Mass (Ar). (2)

Answer 4

⇒ (Mean mass of 1 atom)/(1/12th) mass of 1 atom of 12C (2)

Question 5

What is the use of mass spectrometry? (3)

Answer 5

⇒ Identify elements (1)
⇒ determining the relative molecular mass (Mr) of compounds (1)
⇒ gives information about the relative isotopic mass and relative abundance of isotopes. (1)

Question 6

Why is it important that the instrument is in a vacuum? (1)

Answer 6

⇒ To prevent collision of ions with gas molecules. (1)

Question 7

How are samples of gases, liquids, and solids inserted into the instrument? (3)

Answer 7

⇒ Gases and liquids must be volatile, and then inserted directly (2)
⇒ Solids must be vapourised into their gaseous form. (1)

Question 8

What does Electron impact/gun involve? (3)

Answer 8

⇒ A vaporised sample is injected at a low pressure (1)
⇒ An electron gun fires high energy electrons at a sample (1)
⇒ Which knocks off an electron from the atom thus creating a positive ion (1)

Question 9

Describe how molecules are ionised using electrospray ionisation. (3)

Answer 9

⇒ Sample is dissolved in a volatile solvent (1)
⇒ Injected through needle at high voltage (1)
⇒ Each molecule/particle gains a proton/H+ (1)

Question 10

How would you work out the mass of an isotope/element, given the Avogadro’s constant? (1)

Answer 10

⇒ MASS = Mass No1 / Avogadro’s constant
e.g: ⁷⁹Br⁺
Mass = 79/6.022 x 10²³ = 1.31 x 10⁻²² grams
Kg = 1.31 x 10⁻²²/1000 = 1.31 x 10⁻²⁵ kg

Question 11

What is the first step of mass spectrometry? (2)

Answer 11

⇒ IONISATION (1)

⇒ The sample loses an electron via electrospray or by High-speed electrons via electron gun. (1)

Question 12

What is the second step In mass spectrometry, name and explain the process (3)

Answer 12

⇒ Acceleration (1)
⇒ Positive ions are accelerated by an electric field (1)
⇒ Giving all the ions the same kinetic energy (1)

Question 13

What is the third step of mass spectrometry; name and explain the process (2)

Answer 13

⇒ Deflection (1)

⇒ a magnetic field will separate ions according to its M/Z ratio (1)

Question 14

What is the fourth step of mass spectrometry; name and explain the process (3)

Answer 14

⇒ Detection (1)
⇒ ions will collect at a detector and generator a current (1)
⇒ the current ∝ abundance of the isotope (1)

Question 15

Fifth extra step in mass spectrometry; state and explain the process. (2)

Answer 15

⇒ Data analysis (1)

⇒ The M/Z value and % abundance will be calculated and peaks will be displayed (1)

Question 16

Explain how ions are detected and relative abundance is measured in a TOF mass spectrometer. (2)

Answer 16

⇒ ion hits the detector (1)

⇒ (relative) abundance is proportional to (the size of) the current (1)

Question 17

How do we calculate the RAM from a graph? (1)

Answer 17

Σ(Μ/Ζ x relative abundance)/ Σ(relative abundance) (1)

Question 18

A time of flight (TOF) mass spectrum was obtained for a sample of barium that contains the isotopes 136Ba, 137Ba and 138Ba
The sample of barium was ionised by electron impact.
Identify the ion with the longest time of flight. (1)

Answer 18

⇒ 138Ba (1)

Question 19

Why is the process of ionisation important? (3)

Answer 19

⇒ to allow the ions to be accelerated via electric field (1)
⇒ to allow the ions to be deflected via the magnetic field (1)
⇒ for the ion to produce and current and be detected at the detector (1)

Question 20

State the difference, if any, in the chemical properties of isotopes of the same element. Explain answer (2)

Answer 20

⇒ No difference (1)

⇒ same electronic arrangement

Question 21

Explain why it would be difficult to distinguish between 48Ti(2+) and 24Mg(+). (2)

Answer 21

⇒ Both ions have the same M/Z value of 24 (1)

⇒ deflected equally (1)

Question 22

Define ionisation energy. (2)

Answer 22

⇒ is the energy required to remove 1 electron (1)

⇒ from a gaseous atom (1)

Question 23

What is ionisation energy measured in? (1)

Answer 23

⇒ Kj mol^-1 (1)

Question 24

What are the three factors that affect the ionisation energy of an element? (3)

Answer 24

⇒ Nuclear charge (1)
⇒ Distance from the nucleus (1)
⇒ shielding (1)

Question 25

State the three factors that affect ionisation and their affect (6)

Answer 25

⇒ Nuclear charge (1), the greater the number of protons, the stronger positive charge attracting electrons (1)
⇒ Distance from nucleus (1), The further away the shell is from the nucleus, the weaker the attractive for of the nucleus to the electrons (1)
⇒ Shielding (1), As the number of electrons between the outer electron and the nucleus increases, the outer electron feels less attraction towards the nuclear charge. (1)

Question 26

What are the four types of orbitals and how many electrons can they hold? (4)

Answer 26

⇒ S: 2e
⇒ P: 6e
⇒ D: 10e
⇒ F: 14e
goes up in 4's after S

Question 27

Electrons in orbitals are represented by an up and down arrow, what property do the arrows represent? (1)

Answer 27

⇒ spin (1)

Question 28

suggest why electrons which occupy the 2p sub-levels have a higher energy than electrons in the 2s sub-level (1)

Answer 28

⇒ further away from the nucleus (1)

Question 29

what are the two general trends in ionisation energy (2)

Answer 29

⇒ IE decreases as you go down a group (1)

⇒ IE generally increases across a period (1)

Question 30

State and explain the general trends of IE going down a group (3)

Answer 30

⇒ IE decreases as you go down a group (1),
⇒ Going down a group, the atoms increase in size, thus more shell so there are more electrons that can shield the outer electron from the nucleus’s attractive forces (1)
⇒ extra shells create a larger distance from the outer electron to the nuclear, decreasing the nuclear charge (1)

Question 31

State and explain the general trend of IE going across a period (3)

Answer 31

⇒ General trend: IE energy increases across a period (1)
⇒ the number of protons increases, ergo a stronger nuclear attraction (1)
⇒ for the same shielding power (1).

Question 32

How do Al and S deviate from the general trend? (1)

Answer 32

⇒ Too low relative to trend (1)

Question 33

How does Al deviate from the general trend? (2)

Answer 33

⇒ An electron is removed from the 3p orbital (1),

⇒ Which is better shielded thus requiring less energy for the electron to be lost (1)

Question 34

How does S deviate from the general trend (2)

Answer 34

⇒ An electron is removed from the 3p orbital that contains a lone pair.
⇒ The repulsion from the two electrons reduces the IE energy (1)

Question 35

State two features of the current model that are not shown in the Rutherford model. (2)

Answer 35

⇒ Neutrons AND protons (1)

⇒ Electrons in energy levels (1)