Atomic Structure

Question 1

Which letter is used to represent the atomic number of an atom?

Answer 1

Z

Question 2

What letter represents the mass of an atom?

Answer 2

A

Question 3

How is atomic mass calculated?

Answer 3

Number of protons + number of neutrons (total number of nucleons)

Question 4

What is relative atomic mass?

Answer 4

The average mass of all isotopes of an element compared to 1/12th the mass of an atom of 12C.

Question 5

What is an isotope?

Answer 5

Two of the same element (same proton number) with a different number of neutrons, with the same chemical properties.

Question 6

What sub-shell comes after 3p?

Answer 6

4s, then 3d, as 4s has a lower energy

Question 7

What is an orbital?

Answer 7

A region around the nucleus that can hold up to 2 electrons

Question 8

What is the relationship between the 2 electrons in an orbital? (spin)

Answer 8

They spin in opposite directions as the negative charges repel

Question 9

What 2 elements don’t fit the trend for electronic configuration?

Answer 9

Chromium and Copper

Question 10

Why doesn’t Chromium fit the trend for electronic configuration?

Answer 10

It only has 1 electron in the 4s orbital before filling the 3d orbital with the remaining 5 electrons.

Question 11

Why doesn’t Copper fit the trend for electronic configuration?

Answer 11

It only has 1 electron in the 4s orbital before filling the 3d orbital with the remaining 10 electrons.

Question 12

What is mass spectrometry used for?

Answer 12

• identify unknown compounds by finding the mass
• see the different isotopes of an element

Question 13

What are the steps of mass spectrometry?

Answer 13

Ionisation → acceleration → ion drift → detection

Question 14

What are the types of ionisation?

Answer 14

Electron impact and electrospray ionisation

Question 15

What is electron impact?

Answer 15

High energy electrons are fired at the sample from an electron gun, knocking off an electron from each particle to form 1+ ions.

Question 16

What is electrospray ionisation?

Answer 16

• the sample is dissolved in a volatile solvent (e.g. methanol or water) and is injected through a fine hypodermic needle as a fine spray into a vacuum in the ionisation chamber
• high voltage applied to end of needle where spray emerges
• the particles gain a protons and become 1+ ions as a fine mist
  X(g) + H+(g) → XH+(g)
• the solvent evaporates, leaving 1+ ions

Question 17

When would you use each type of ionisation in mass spec?

Answer 17

Electron impact - low Mr compounds
Electrospray - high Mr protiens (e.g bio molecules)

Question 18

What is acceleration?

Answer 18

Ions are accelerated using an electric field so that all ions have the same kinetic energy.

Question 19

What is ion drift?

Answer 19

• the ions enter the flight tube
• ions with different nesses have a different time of flight
• lighter ions take less time to reach the detector

Question 20

What is the formula for ion drift?

Answer 20

t = d √(2KE/m)

Question 21

What is detection?

Answer 21

• negative charged plate produces a current when the ions his the plate
• the more ions hitting the detector plate, the bigger the current
• the mass of ions can be calculated using the time of flight
• the mass spectrum shows the number of particles of each mass that hit the detector

Question 22

What is the first ionisation energy?

Answer 22

The energy required to remove one moles electrons of 1 mole of gaseous atoms of an element to form 1 mole of gaseous 1+ ions.

Question 23

What factors affects first ionisation energy?

Answer 23

Atomic radius, nuclear charge and shielding.

Question 24

How does atomic radius/nuclear charge affect ionisation energy?

Answer 24

The higher the nuclear charge, the lower the atomic radius so the ionisation energy will be higher as the outer electrons are closer to the nucleus and are more attracted so require more energy to remove.

Question 25

How does shielding affect ionisation energy?

Answer 25

The outer shell of electrons is further from the nucleus and the there are more inner shells “blocking” the nuclear charge from getting to the outer electrons, so they require less energy to remove.

Question 26

What is effective nuclear charge?

Answer 26

The nuclear charge that hasn’t been blocked by shielding so reaches the outer electrons in the outer shell.

Question 27

What element has the highest first ionisation energy and why?

Answer 27

Helium
H and He have no shielding but He has a higher nuclear charge.

Question 28

Why does first ionisation energy drop from group 2 to 3? (Period 2/3)

Answer 28

The element in group 3 has one more proton, but it has one more inner sub shell, which shields the charge of two protons, leaving an effective nuclear charge which is less than that of the element in group 2

Question 29

Why does first ionisation energy drop from group 5 to 6?

Answer 29

The electron removed from group 6 is paired, so there is higher repulsion between electrons, making it easier to remove.

Question 30

What is the second ionisation energy?

Answer 30

The energy required to remove one electron from each of a mole of free gaseous unipositive ions.
M+(g) → M2+(g) + e

Question 31

What is the third ionisation energy?

Answer 31

The energy required to remove one electron from each of a mole of free gaseous bipositive ions.
M2+(g) → M3+(g) + e