Atomic Structure And Mass Spectrometery

Question 1

What is the mass and charge of a proton?

Answer 1

mass = 1, charge = +1

Question 2

What is the mass and charge of a neutron?

Answer 2

mass = 1, charge = 0

Question 3

What is the mass and charge of an electron?

Answer 3

mass = 0 (1/2000), charge = -1

Question 4

Where is most of the mass of an atom located?

Answer 4

In the nucleus, which contains protons and neutrons and is dense and tiny.

Question 5

Where are electrons found in an atom?

Answer 5

Electrons orbit the nucleus in energy levels (shells).

Question 6

Ions

What is the difference between a cation and an anion?

Answer 6

Positive ions are cations with fewer electrons than protons.

Negative ions are anions with more electrons than protons.

Question 7

Define the term isotope

Answer 7

Isotope are atoms of the same element with the same number of protons but different number of neutrons.

Question 8

Explain why isotopes have identical chemical properties but different physical properties.

Answer 8

They have the same number of electrons, so the same chemical properties.
Their physical properties differ (e.g. density, diffusion) due to differences in mass.

Question 9

Define the term relative atomic mass (Aᵣ).

Answer 9

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

Question 10

Define relative molecular mass (Mᵣ)

Answer 10

Relative Molecular Mass (Mᵣ) is the Sum of relative atomic masses (Ar) of all atoms in a molecule.

Question 11

What does the modern quantum mechanical model suggest about electrons?

Answer 11

Electrons are in orbitals/sub-shells

Question 12

Mass Spectrometry (TOF)

Describe the four main stages of TOF mass spectrometry.

Answer 12

1. Ionisation
    • Electrospray: involves dissolving the sample in a volatile solvent and praying it through a charged needle, which adds H⁺ to each molecule
    • Electron impact: involves firing high energy electrons at the vaporised sample to knock off an electron from each molecule resulting in a +1 charge
   
   2. Acceleration: the positively charged ions are accelerated towards a negatively charged plate by an electric field. All ions given same kinetic energy.
   3. Ion Drift: Ions travel through vacuum region at speeds based on mass.
   4. Detection: ions hit the detector and gain an electron generating current. Lighter ions hit the detector first

Question 13

What are the axes of a mass spectrum?

Answer 13

X-axis: mass/charge (m/z)
Y-axis: relative abundance

Question 14

State the formula for calculating relative atomic mass (Aᵣ) from mass spectrum data.

Answer 14

Aᵣ = Σ (isotopic mass × isotopic abundance) ÷ total abundance of all isotopes

Question 15

Electron Configuration and Structure

How are electrons arranged in an atom?

Answer 15

Electrons are arranged in shells (energy levels) around the nucleus.
• Shells are divided into sub-shells (s, p, d, f).

Question 16

What principle determines the order of sub-shell filling?

Answer 16

Electrons fill from lowest to highest energy sub-shells.

Question 17

What does a mass spectrum plot and what does each axis represent?

Answer 17

A mass spectrum plots relative abundance (y-axis) against mass-to-charge ratio (m/z) (x-axis).

Question 18

What does each peak on a mass spectrum represent and how is the abundance shown?

Answer 18

Each peak represents a different isotope. The height of a peak shows the abundance on the y axis

Question 19

What would you expect to see on the mass spectrum of an element with only one stable isotope?

Answer 19

If there’s only one stable isotope, there will be one peak.

Question 20

How many electrons can each orbital hold?

Answer 20

2 electrons

Question 21

What is the molecular ion peak and what does its m/z value represent?

Answer 21

For molecules, the peak with the highest m/z is the molecular ion (M⁺).
The m/z of the molecular ion represents the relative molecular mass (Mᵣ).

Question 22

What causes the smaller peaks at lower m/z values in a molecular mass spectrum?

Answer 22

Lower m/z peaks come from fragmentation of the molecular ion.

Question 23

Time of Flight (TOF) Mass Spectrometry

Write the formula used to calculate Time of Flight in a TOF mass spectrometer and define all variables. And state the units

Answer 23

Time of flight (t) equation =
d √m/(2Ke)
• d = length of flight tube (m)
• m = mass of ion (kg)
• E_k = kinetic energy (J)

Question 24

Electron Arrangement and Configuration

What are the names of the energy sub-shells, and what are the number of orbitals and max electrons in each?

Answer 24

Sub-shell No. of Orbitals Max e⁻
S 1 2
P 3 6
D 5 10
F 7 14

Question 25

Describe three key rules governing electron configuration.

Answer 25

Fill lowest energy orbitals first. Each Orbitals hold max 2 electrons with opposite spins. Electrons are placed in orbitals singly before pairing.

Question 26

Why is the 4s sub-shell filled before the 3d sub-shell?

Answer 26

Because 4s is lower in energy than 3d in neutral atoms.

Question 27

When forming ions, which sub-shell loses electrons first: 4s or 3d?

Answer 27

Electrons are lost from the 4s orbital before the 3d.

Question 28

Special Cases in Transition Metals Give the electron configurations of chromium and copper, and explain why they are exceptions.

Answer 28

Configuration Cr [Ar] 3d⁵ 4s¹ Cu [Ar] 3d¹⁰ 4s¹ These are exceptions because half-filled (d⁵) and fully filled (d¹⁰) subshells are more stable.

Question 29

Define ‘time of flight’ in the context of TOF mass spectrometry.

Answer 29

The time of flight (t) is the time its takes for an ion to travel through the flight tube in a TOF mass spectrometer. It depends on the ion’s velocity and kinetic energy.

Question 30

Write down the equation that relates time, distance, and velocity. State what each variable represents.

Answer 30

The equation is: t = d/v • t = time of flight (s) • d = distance (length of flight tube, m) • v = velocity of the particle (m/s)

Question 31

State the equation for kinetic energy and define each term.

Answer 31

KE= ½m x v² Where: • KE = kinetic energy (J) • m = mass of the ion (kg) • v = velocity (m/s)

Question 32

How do you derive the TOF equation from the kinetic energy and time-distance equations? Give each step clearly.

Answer 32

Step 1: Rearranging Equation 1 to get velocity: v= d/t Step 2: Substituting into the KE equation: KE= ½m x v²