Atomic Structure

Question 1

Describe Dalton’s solid sphere model

Answer 1

• Proposed by John Dalton, this model suggested that atoms were indivisible, solid spheres.
• It also suggested that different elements were made up of different types of these spheres.

Question 2

Describe JJ Thomson’s plum pudding model

Answer 2

• The discovery of the electron by J.J. Thomson proved that atoms were divisible.
• His ‘plum pudding’ model depicted atoms as spheres of positive charge with negative electrons randomly embedded throughout, similar to the raisins in a plum pudding.

Question 3

What experiment did Rutherford do ? And what did it involve ?

Answer 3

• Involved firing alpha particles (which are positively charged) at a sheet of thin gold foil inside a vacuum.
• According to the plum pudding model, the positive ‘pudding’ would slightly deflect most of the particles.
• Contrary to expectations, most alpha particles passed straight through, with a few deflecting at large angles or even bouncing back.

Question 4

What was Rutherfords conclusion after his experiment ?

Answer 4

• The fact that most alpha particles passed through the gold foil undeflected indicated that atoms are mostly empty space.
• The small number of particles that deflected at large angles or bounced back suggested the presence of a tiny, dense, positively charged nucleus at the centre of the atom. This nucleus was responsible for the strong deflection of the positively charged alpha particles due to electrostatic repulsion.
• Rutherford proposed that negative electrons must orbit the nucleus in a ‘cloud’ to maintain the atom’s overall neutrality.

Question 5

What did Rutherfords discovery of the proton help the explanation of charges ?

Answer 5

The differences in nuclear charges among elements could now be explained by the presence of varying numbers of protons.

Question 6

Who discovered the neutron and how did it contribute to the nuclear model ?

Answer 6

James Chadwick later discovered the neutron, which provided the mass needed to account for the discrepancy in the nucleus’s weight.

Question 7

How did Neils Bohr contribute to the nuclear model ?

Answer 7

• Electrons orbit the nucleus in specific energy levels, or shells.
• They can jump between these shells by absorbing or emitting light of particular wavelengths.

Question 8

Define element

Answer 8

Substance made up of only one type of atom, where all the atoms of a substance have the same number of protons.

Question 9

Define isotope

Answer 9

Atoms of the same element that have the same number of protons but different numbers of neutrons.

Question 10

Chemical vs Physical properties of isotopes

Answer 10

Chemical properties:
- Isotopes have the same electron configuration.
- This means they have the same chemical properties (e.g. reactivity).

Physical properties:
- Isotopes have slightly different physical properties (e.g. mass and density).
- This is because physical properties depend on atomic mass.

Question 11

Define isotopic mass

Answer 11

The mass of an atom of an isotope compared to 1/12 the mass of a carbon-12 atom.

Question 12

Define atomic mass

Answer 12

The weighted mean mass of an atom of an element compared to 1/12 the mass of a carbon-12 atom.

Question 13

Define molecular mass

Answer 13

The average mass of a molecule compared to 1/12 the mass of a carbon-12 atom.

Question 14

Define formula mass

Answer 14

Used for ionic compounds. It is calculated by adding up the Ar values of all the ions in one formula unit.

Question 15

What is a Mass Spectrometer? (1)

Give the basic definition and type of mass spec used.

Answer 15

• A mass spectrometer is an analytical tool used to estimate the different isotopes in a substance.
• We use Time of Flight (TOF) mass spec machines.

Question 16

What are the Steps to Mass Spectrometry? (4)

Give all four stages.

Answer 16

• Ionisation
• Acceleration
• Ion Drift
• Detection

Question 17

Explain the Ionisation stage of Mass Spectrometry. (2)

Explain the process and give both methods.

Answer 17

• During ionisation, a sample is injected and vaporised, allowing for easier ionisation.
• Two methods of ionisation include electron bombardment and electrospray ionisation.

Question 18

Describe the Process of Electron Bombardment. (3)

Give the whole process.

Answer 18

• The sample is hit with electrons from an electron gun at high speeds.
• These electrons knock off other electrons from a sample, turning them into positive ions.
• This often causes the sample to fragment.

Question 19

Describe the Process of Electrospray Ionisation. (3)

Give the whole process.

Answer 19

• The sample is dissolved in a volatile solvent and sprayed through a charged needle connected to a high voltage.
• The high voltage rips a proton (H+ ion) off the solvent and attaches them to each molecule, creating 1+ ions.

Question 20

Give the Equations for both Ionisation methods. (2)

Including state symbols.

Answer 20

• Electron Impact -
  X(g) —> X+(g) + e-
• Electrospray Ionisation -
  X(g) + H+ —> XH+(g)

Question 21

When is each Ionisation Method Used? (2)

Give the type of molecule which each method is used for.

Answer 21

• Electrospray ionisation is used for large polymers or biological molecules (e.g. DNA) as this reduces fragmentation.
• The majority of other molecules (e.g. hydrocarbons) uses electron bombardment, where fragmentation is less of a problem.

Question 22

Explain the Acceleration stage of Mass Spectrometry. (2)

Explain the process.

Answer 22

• Molecules are accelerated by a negatively charged plate, which ensures they all have the same kinetic energy.
• As a result, lighter ions move faster than heavier ions.

Question 23

Explain the Ion Drift stage of Mass Spectrometry. (2)

Explain the process.

Answer 23

• Molecules pass through an ion drift region, where lighter ions have a higher velocity and so take less time to be detected.
• Time of flight= distance/velocity, and therefore Time of flight=distance x |m/2KE.

| = root

Question 24

Explain the Detection stage of Mass Spectrometry. (3)

Explain the process.

Answer 24

• Positive ions hit the negatively charged plate and gain electrons, and the flow of electrons causes a current to flow.
• Current is directly proportional to the abundance of ions, allowing this to be measured on a mass spectrum.

Question 25

How do you calculate relative atomic mass from mass spectra

Answer 25

- Multiply the relative isotopic mass by its relative abundance for each isotope. - Add these products together. - Divide the total by the sum of the relative abundances (100 if using percentages).

Question 26

What are electron shells?

Answer 26

Energy levels around the nucleus where electrons are likely to be found.

Question 27

What is the principal quantum number (n)?

Answer 27

A number that indicates the shell’s energy level and distance from the nucleus.

Question 28

How does energy change with shell number?

Answer 28

Energy increases as the distance from the nucleus increases.

Question 29

What does each shell contain?

Answer 29

Sub-shells labelled s, p, d, and f.

Question 30

What is an orbital?

Answer 30

A region around the nucleus where there is a high probability of finding an electron.

Question 31

How many electrons can each orbital hold?

Answer 31

Two, with opposite spins.

Question 32

What is an electron configuration?

Answer 32

The arrangement of electrons in atomic orbitals.

Question 33

What is sub-shell notation?

Answer 33

e.g. Neon = 1s² 2s² 2p⁶.

Question 34

What does electrons-in-boxes notation show?

Answer 34

Orbitals as boxes and electrons as arrows (↑↓) representing opposite spins.

Question 35

Why do electrons in the same orbital have opposite spins?

Answer 35

To minimise electron-electron repulsion (Pauli exclusion principle).

Question 36

State the Aufbau principle.

Answer 36

Electrons fill the lowest energy orbitals first.

Question 37

Which sub-shell fills first: 4s or 3d?

Answer 37

4s fills before 3d.

Question 38

State Hund’s rule.

Answer 38

Orbitals of equal energy are occupied singly before pairing.

Question 39

What happens when electrons pair up in an orbital?

Answer 39

They have opposite spins.

Question 40

Which sub-shell loses electrons first when forming ions?

Answer 40

The highest energy sub-shell — e.g. 4s before 3d.

Question 41

In what order do orbitals fill?

Answer 41

1s → 2s → 2p → 3s → 3p → 4s → 3d → 4p → 5s → 4d …

Question 42

What does the term “lowest energy arrangement” refer to?

Answer 42

The most stable electron configuration.

Question 43

How are electron configurations used in periodic trends?

Answer 43

To explain ionisation energies, atomic radius, and chemical reactivity.