5. Atomic Structure

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CSS GSA — Paper 7, Part-I, Section I, Sub-topic 5: Atomic Structure

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1. History of the Atom

• 1913 — Niels Bohr
  • Studied under Rutherford at Victoria University, Manchester
  • Refined Rutherford's model: electrons orbit the nucleus in fixed orbits (shells), like planets around the Sun
  • Each orbit holds a set maximum number of electrons

1.1 📜 Development of Atomic Models

Rutherford's gold-foil experiment — mechanism

• Method: Fire alpha particles at a very thin gold foil and detect their scattering.
• Observation 1: Most passed straight through → most atomic volume is empty space.
• Observation 2: Some deflected through large angles → positive charge is concentrated in a small region.
• Observation 3: A tiny fraction bounced almost backward → the nucleus is dense and massive.
• Conclusion: The atom has a very small, positively charged nucleus surrounded by electrons.

Bohr's electron transitions

• Electrons occupy allowed energy levels in the simplified Bohr model.
• Absorption: an electron moves to a higher energy level after absorbing energy.
• Emission: returning to a lower level releases a photon; ΔE = hf.
• Important qualification: Shells/orbits in introductory diagrams are teaching models, not literal circular paths in the modern quantum-mechanical model.

1.2 ⚛️ Fundamental Particles and Nuclear Notation

• Atomic number (Z) = protons; mass number (A) = protons + neutrons; neutrons = A − Z.
• Ion charge: a cation loses electrons (+); an anion gains electrons (−).
• Example from the PDF: Uranium-238 and uranium-235 each have 92 protons; their neutron numbers are 146 and 143, respectively.
• Distinguish terms: an isotope has the same protons but different neutrons; an ion has unequal protons and electrons; atomic mass is not identical to integer mass number.

1.3 🧪 Duplet, Octet and Ammonia (NH₃)

• Duplet rule: small atoms such as hydrogen often achieve stability with two electrons in the first shell.
• Octet rule: many main-group atoms tend toward eight valence electrons when bonding; the rule has exceptions, so lower total energy is the more general explanation.
• Ammonia (NH₃), shown in PDF p. 17: nitrogen has five valence electrons, forms three N–H single covalent bonds, and retains one lone pair.
• Shape of NH₃: trigonal pyramidal; approximate H–N–H angle 107°. It differs from H₂O (bent, 104.5°), which has two lone pairs.

1.4 ☢️ Ionizing vs Non-Ionizing Radiation

• Ionizing radiation: carries enough energy per photon or particle to remove electrons from matter. X-rays, gamma rays, alpha and beta particles are standard examples.
• Non-ionizing: radio waves, microwaves, infrared and visible light; ordinary solar UVA and UVB are normally grouped here.
• Nuance: sufficiently energetic extreme ultraviolet can ionize matter; UV classification depends on photon energy.
• Do not conflate: alpha and beta are particle radiation; gamma and X-rays are electromagnetic photons. Radiation types overlap in wavelength, so origin is often used to distinguish X-rays from gamma rays.

2. Structure and Electronic Configuration

• Electron shells (maximum electrons)
  • 1st shell: 2
  • 2nd shell: 8
  • 3rd shell: 18
• Electronic configuration: numbers of electrons per shell
  • Example: Nitrogen = 2, 5 (2 in the 1st shell, 5 in the 2nd), total 7 electrons, atomic number 7

2.1 🧮 Formula for Maximum Electrons in a Shell

$$\text{Maximum electrons in shell} = 2n^2$$

• n = principal shell number (K = 1, L = 2, M = 3, N = 4).
• Why? The shell with principal quantum number n contains n² orbitals in total, and each orbital accommodates at most two electrons (Pauli exclusion principle). Therefore the capacity is 2 × n².

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CSS MCQ trap: 2n² tells the maximum capacity of a shell, not necessarily the order in which electrons fill shells. For example, calcium (20 electrons) is 2, 8, 8, 2, not 2, 8, 10. The 4s subshell fills before 3d in the usual neutral-atom filling order.

2.2 ⚛️ Subshell Capacities and Filling Order

• Each shell contains subshells such as s, p, d, f.
• Each orbital holds up to 2 electrons.

• Usual filling order (first 20 elements): 1s → 2s → 2p → 3s → 3p → 4s.
• Example — nitrogen (Z = 7): 1s² 2s² 2p³ → shell distribution 2, 5.
• Example — sodium (Z = 11): 1s² 2s² 2p⁶ 3s¹ → 2, 8, 1.
• Example — calcium (Z = 20): 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² → 2, 8, 8, 2.
• Finding electrons: In a neutral atom, total electrons = atomic number (Z). In a positive ion, subtract the positive charge; in a negative ion, add the magnitude of its charge.

2.3 ✍️ Quick Practice

1. Maximum electrons in the M shell? n = 3 → 2(3²) = 18.
2. Maximum electrons in the N shell? n = 4 → 2(4²) = 32.
3. How many electrons does Mg²⁺ have (Z = 12)? 12 − 2 = 10.
4. Shell distribution of chlorine (Z = 17)? 2, 8, 7.

3. Chemical Bonding

• A chemical bond is the force of attraction holding atoms together, from interactions between their electrons
• Atoms bond so that each reaches a more stable (usually full outer-shell) configuration
• Three main types: ionic (transfer), covalent (sharing), metallic (delocalized electron 'sea')

3.1 Covalent bonds

• Single covalent bond: two atoms share one electron pair (e.g. H₂)
• Double covalent bond: two atoms share two pairs, 4 electrons (e.g. O₂)

3.2 Covalent vs Ionic bonding

4. The Periodic Table

• Elements arranged by increasing atomic number, with similar properties grouped together
• 118 elements, from Hydrogen (1) to Oganesson (118)
• Periods (7 horizontal rows): same number of electron shells
• Groups (18 vertical columns): similar chemical properties, because they have the same number of valence electrons

5. Natural Radioactivity

• Radioactivity: release of particles and energy from unstable nuclei until they become stable
• Discovery: Henri Becquerel (1896); explained by Marie Curie (1898), who won a Nobel Prize for it
• Uranium (atomic number 92) is the heaviest naturally occurring element

6. Electromagnetic (EM) Radiation

• EM waves are transverse waves of oscillating electric and magnetic fields; they need no medium
• All travel at the speed of light, c ≈ 3 × 10⁸ m/s, in a vacuum
• Energy increases as wavelength decreases (E = hf = hc/λ)
• Order (low → high energy): Radio → Microwave → Infrared → Visible → Ultraviolet → X-ray → Gamma

6.1 Gamma rays and applications

• Highest-energy, shortest-wavelength EM radiation; no mass, no charge, highly penetrating (blocked only by thick lead or concrete)
• Applications
  • Cancer treatment (radiotherapy)
  • Sterilizing medical equipment and food
  • Industrial radiography (finding flaws in welds)
  • Nuclear medicine tracing and imaging
  • Astronomy (gamma-ray bursts)

6.2 UV rays

• Range ≈ 10–400 nm, between visible light and X-rays
• UVA (longest; skin ageing, tanning), UVB (sunburn, DNA damage, main skin-cancer risk; mostly absorbed by ozone), UVC (shortest and most dangerous; almost fully absorbed by the atmosphere)
• Beneficial: vitamin D synthesis, sterilization (UV purifiers), forensics, curing inks
• Harmful: sunburn, premature ageing, skin cancer, cataracts

6.3 X-rays

• Range ≈ 0.01–10 nm; discovered by Wilhelm Röntgen (1895)
• Pass through soft tissue but are absorbed by bone and metal, which creates the image
• Uses: medical imaging (fractures, dental), airport security, industrial inspection, crystallography

6.4 Infrared rays

• Range ≈ 700 nm – 1 mm, just beyond red light
• Felt as heat: every warm object emits IR in proportion to its temperature
• Uses: thermal imaging (security, firefighting, medicine), remote controls, IR heaters, some fibre-optic links, astronomy, night vision

7. Water Molecule Shape (Molecular Orbital Theory)

• Electron regions on oxygen: 2 bonding pairs (with the two H atoms) + 2 lone pairs
• Electron geometry: tetrahedral (4 regions, to minimise repulsion; VSEPR theory)
• Molecular shape: bent/angular, bond angle ≈ 104.5°
  • Lone pairs repel more strongly than bonding pairs and squeeze the bonding pairs closer than the ideal 109.5°
• Orbital description
  • Oxygen's orbitals hybridize to roughly sp³
  • Two sp³ orbitals overlap with hydrogen 1s orbitals to form two O–H sigma (σ) bonds
  • The other two sp³ orbitals hold the lone pairs
• Simple diagram: O at the vertex, two O–H bonds at ≈ 104.5°, two lone-pair 'bulges' on the opposite side
• Result: a strongly polar molecule (O is more electronegative), which gives hydrogen bonding, high boiling point, and water's role as a universal solvent

8. Why the Sun Has Such Strong Gravity (Despite Being Gas)

• Gravity depends on mass, not on whether matter is solid, liquid or gas (F = Gm₁m₂/r²)
• The Sun holds ≈ 99.86% of the Solar System's mass (≈ 333,000 × Earth's mass)
• That huge mass in one place gives a huge pull, which also compresses the core enough for continuous fusion

☢️ Radioactive decay, half-life and displacement laws — CSS 2020/2022/2024

• Decay law: Each unstable nucleus has a fixed probability of decaying per unit time; for a large sample the number left is N = N₀ e^(−λt), where λ is decay constant and t is elapsed time.
• Activity: A = λN (decays/s); SI unit becquerel (Bq) = one decay per second.
• Half-life: t½ = ln(2)/λ ≈ 0.693/λ; remaining fraction after n half-lives (½)^n.
• Example: 80 g of a radioactive isotope with a 5-year half-life becomes 40 g after 5 years, 20 g after 10 years and 10 g after 15 years.
• Alpha emission: Mass number A decreases by 4 and atomic number Z decreases by 2.
• Beta-minus emission: A stays unchanged, Z increases by 1 (neutron → proton + electron + antineutrino).
• Gamma emission: A and Z both remain unchanged; excited nucleus releases electromagnetic energy.
• Natural vs artificial radioactivity: Decay of naturally occurring unstable isotopes vs radioactive isotopes produced by human activity, for example neutron activation in reactors.
• Examples: Uranium-238 and radium-226 are naturally radioactive; cobalt-60 can be produced artificially.

⚗️ Mole, isotope and common chemistry MCQ connections — CSS 2024

• Mole: Contains exactly 6.02214076 × 10²³ specified entities (Avogadro constant per mole).
• Counting atoms: 1 mol ethanol (C₂H₆O) contains 2 mol carbon atoms; 1 mol methanol (CH₄O) contains 1 mol carbon atoms.
• Isotopes: Same proton number Z, different neutron number; e.g., protium ¹H (0 neutrons) and deuterium ²H (1 neutron).
• Heavy water: D₂O contains deuterium and is used as moderator in some reactors.
• Alum (potassium alum): KAl(SO₄)₂·12H₂O; the dot indicates waters of crystallization.
• Refraction: Light changes direction when travelling between media of different refractive indices (except normal incidence); explains why an immersed pencil appears bent.
• Red signal: Red light has a relatively long visible wavelength and low frequency, so in some atmospheric scattering conditions it is less scattered than blue.
• Study distinction: VSEPR/hybridization explain water's ~104.5° bent geometry in the introductory account; this should not be labelled a full molecular-orbital-theory derivation.

9. MCQ Traps

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• Gravity depends on mass, not on solid/liquid/gas state
• EM energy rises as wavelength falls: gamma = shortest wavelength and highest energy; radio = longest wavelength and lowest energy
• UVB (not UVA) causes sunburn and is the main ozone-linked band
• Water is bent (≈ 104.5°), not linear
• Alpha = positive; beta = negative; gamma = no charge, no mass
• Atomic number = protons only; mass number = protons + neutrons
• Single covalent bond = one shared pair; double = two pairs (4 electrons)
• Radioactivity was discovered by Becquerel, but explained by Marie Curie

10. Self-Test Questions

1. State the maximum electron capacity of the first three atomic shells.
2. Differentiate between atomic number and mass number, and between isotopes.
3. Explain the difference between a single and a double covalent bond with an example each.
4. How are elements organized in the periodic table (periods vs groups)?
5. Name the three types of particles emitted in radioactivity and state the charge of each.
6. Explain the shape of the water molecule with the help of Molecular Orbital Theory, and draw its orbital diagram.
7. What are gamma rays? Explain their applications.
8. How can the Sun have such a strong gravitational field if it's made up of gases?
9. Explain the electromagnetic spectrum and its components in detail with ranges.
10. What do you know about UV rays?
11. What are X-rays? Explain their uses.
12. Write a note on infrared rays.
13. Write a brief note on chemical bonding.
14. Differentiate between covalent and ionic bonding.

11. 📝 Past-Paper MCQs (FPSC)

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Source: FPSC CSS papers previously supplied (2015–2025). These are worked answers, not an official FPSC answer key. Each MCQ has its options clearly separated and an explanation under a collapsible answer.

MCQ 1 — Chemical Formula of Alum

CSS 2024 · Q1.4

Question: The chemical formula for alum is:

• A. KAl(SO₄)₂·12H₂O
• B. KSi(SO₄)₂·12H₂O
• C. KAl(SO₄)₂·8H₂O
• D. None of these
• ✅ Answer: A — KAl(SO₄)₂·12H₂O
  • Explanation: Common potassium alum is potassium aluminium sulfate dodecahydrate. The 12H₂O represents twelve water-of-crystallization molecules per formula unit.
  • Exam trap: Do not confuse aluminium (Al) with silicon (Si), or change 12 waters to 8.

MCQ 2 — Isotopes

CSS 2024 · Q1.9

Question: The isotopes of any element contain equal number of:

• A. Protons
• B. Neutrons
• C. Both electrons and protons
• D. None of these
• ✅ Answer: A — Protons
  • Explanation: Isotopes have the same atomic number (same number of protons) but different mass numbers because their neutron numbers differ.
  • Example: Hydrogen-1 and hydrogen-2 both contain one proton; hydrogen-2 has one neutron.
  • Exam trap: Their electron counts are not necessarily identical if ions are included.

MCQ 3 — Carbon Atoms in One Mole

CSS 2024 · Q1.12

Question: Which of the following contains a greater number of carbon atoms in one mole of each compound?

• A. Chloroform
• B. Methanol
• C. Water
• D. Ethanol
• ✅ Answer: D — Ethanol
  • Explanation: One molecule of ethanol (C₂H₅OH) has two carbon atoms. Chloroform (CHCl₃) and methanol (CH₃OH) each have one; water (H₂O) has none.
  • Calculation: One mole of ethanol therefore contains 2 × Avogadro's number of carbon atoms.
  • Exam trap: Compare the number of carbon atoms per molecule, not the molar mass of each substance.

MCQ 4 — Pencil Appears Bent in Water

CSS 2024 · Q1.13

Question: A pencil dipped in water appears bent due to:

• A. Reflection
• B. Diffraction
• C. Refraction
• D. None of these
• ✅ Answer: C — Refraction
  • Explanation: Light changes direction as it passes between water and air because its speed changes in media with different refractive indices.
  • Example: The submerged part of a pencil appears displaced relative to its part above water.
  • Exam trap: Reflection is light bouncing off a surface; refraction is bending during passage between media.

MCQ 5 — Red Traffic Signal

CSS 2024 · Q1.18

Question: Why is red light used in a traffic signal for danger?

• A. Due to short wavelength
• B. Due to high frequency
• C. It has long wavelength and high frequency
• D. It has long wavelength and short frequency
• ✅ Answer: D — Long wavelength and low frequency (intended answer)
  • Explanation: Red has the longest wavelength and lowest frequency of ordinary visible light. In many scattering conditions, longer wavelengths scatter less than shorter wavelengths, aiding long-distance visibility.
  • Qualification: Choice D uses imprecise wording: the correct physical term is low frequency, not “short frequency.” Visibility also depends on brightness, atmosphere and human perception.
  • Exam trap: Long wavelength corresponds to low, not high, frequency because speed = frequency × wavelength.

MCQ 6 — Heavy Water

CSS 2024 · Q1.19

Question: The heavy water is:

• A. Denoted by D₂O and contains simple hydrogen
• B. Denoted by H₂O and contains deuterium
• C. Denoted by D₂O and contains deuterium
• D. Denoted by H₂O and contains simple hydrogen
• ✅ Answer: C — D₂O containing deuterium
  • Explanation: Heavy water contains deuterium (²H or D), a hydrogen isotope with one proton and one neutron, instead of ordinary protium (¹H).
  • Application: Heavy water can serve as a neutron moderator in certain nuclear reactors.
  • Exam trap: D₂O is heavy water; ordinary water is H₂O.

⚡ Rapid Revision — Answer Key

12. Past-Paper Long Questions with Answers (FPSC)

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Every question below appeared in the FPSC papers you shared; the year and question number are in each title. Answers are model outlines written by me, ready to expand.

• CSS 2025 Q2(b): Describe Electromagnetic Radiations. Give its types and explain their applications. (5)

  • EM radiation: energy travelling as coupled oscillating electric and magnetic fields; transverse; no medium needed; speed c ≈ 3 × 10⁸ m/s
  • Types and applications (low → high energy)
    • Radio waves: radio/TV broadcasting, mobile and wireless communication
    • Microwaves: cooking, radar, satellite links
    • Infrared: thermal imaging, night vision, remote controls
    • Visible light: vision, photography
    • Ultraviolet: sterilization, vitamin D synthesis, forensic analysis
    • X-rays: medical imaging, security screening
    • Gamma rays: cancer radiotherapy, sterilization
  • Rule: as wavelength decreases, frequency and energy increase

• CSS 2023 Q2(c): Write two applications of each: ultraviolet, infra-red, microwaves, radio waves, X-rays. (5)

  Radiation	Application 1	Application 2
  Ultraviolet	Sterilizing water, air and equipment	Forensics, detecting fake currency; vitamin D
  Infrared	Night vision and thermal cameras	TV remote controls; heaters
  Microwaves	Microwave ovens	Radar and satellite communication
  Radio waves	Radio and TV broadcasting	Mobile phones, Wi-Fi, Bluetooth
  X-rays	Medical imaging (bone fractures)	Airport baggage scanning

• CSS 2024 Q5(d): What are longitudinal waves, electromagnetic waves and gamma radiation? Discuss them. (5)

  • Longitudinal waves
    • Particles vibrate parallel to the direction of travel, making compressions and rarefactions
    • Need a medium; example: sound waves, P-waves of earthquakes
  • Electromagnetic waves
    • Transverse: electric and magnetic fields oscillate at right angles to each other and to the direction of travel
    • No medium needed; speed of light; together form the EM spectrum
  • Gamma radiation
    • Highest-energy, shortest-wavelength EM waves, emitted from the nucleus of unstable atoms
    • Very penetrating; used in radiotherapy and sterilization
  • Key contrast: sound is longitudinal and needs a medium; light and other EM waves are transverse and do not

• CSS 2016 Q8(b): What are gamma rays? Explain their applications. (5)

  • Gamma rays: highest-frequency EM radiation, emitted during radioactive decay and nuclear reactions; no mass, no charge; discovered by Paul Villard (1900)
  • Applications
    • Medicine: radiotherapy for cancer; nuclear-medicine imaging
    • Sterilization of surgical instruments and some foods (kills bacteria)
    • Industry: radiography to find cracks in welds and pipelines
    • Astronomy: gamma-ray bursts show extreme cosmic events
  • Hazard: damages living cells, so thick lead or concrete shielding is needed

• CSS 2016 Q8(a): Explain the shape of the water molecule with the help of Molecular Orbital Theory, also draw its orbital diagram. (5)

  • Oxygen's configuration: 2, 6 (six valence electrons)
  • Hybridization: the 2s and three 2p orbitals mix to form four sp³ hybrid orbitals
  • Bonds: two sp³ orbitals (each with one electron) overlap with the 1s orbitals of two H atoms, giving two O–H σ bonds
  • Lone pairs: the other two sp³ orbitals hold two lone pairs
  • Shape: tetrahedral electron arrangement, but bent molecular shape, bond angle ≈ 104.5° (lone pairs repel more than bond pairs)
  • Diagram to draw: O in the centre, two H attached at ≈ 104.5°, two lone-pair lobes on the other side
  • Consequence: water is polar, forms hydrogen bonds and is a universal solvent

• CSS 2024 Q2(d): What is the difference between ionic and covalent bonding? Give examples. (5)

  • Use the Covalent vs Ionic table in section 3.2
  • Ionic example: NaCl; Na gives one electron to Cl, forming Na⁺ and Cl⁻, held by electrostatic attraction
  • Covalent examples: H₂O, CO₂ and O₂ (atoms share electron pairs)
  • Add the properties: ionic compounds have high melting points and conduct when molten or dissolved; covalent compounds usually do not

• CSS 2019 Q5(d): Why do atoms form bonds? Name three major types of chemical bonds. (5)

  • Why: atoms are most stable with a full outer shell (2 or 8 electrons); bonding lets them reach it, and the bonded state has lower energy than the separate atoms
  • Three major types
    • Ionic bond: electron transfer from metal to non-metal (NaCl)
    • Covalent bond: sharing of electron pairs between non-metals (H₂O)
    • Metallic bond: positive metal ions in a 'sea' of delocalized electrons (copper, iron)

• CSS 2024 Q4(d), 2022 Q4(d), 2020 Q4(c): What is radioactivity? Discuss the laws of radioactivity. Name two radioactive elements. Differentiate between natural and artificial radioactivity. (5)

  • Radioactivity: spontaneous emission of particles/rays from unstable nuclei (Becquerel, 1896; Marie Curie)
  • Laws
    • Law of radioactive decay: the decay rate at any moment is proportional to the number of undecayed atoms (N = N₀e^(−λt))
    • Half-life: the fixed time for half the atoms to decay; unaffected by temperature, pressure or chemical form
    • Displacement laws: alpha decay lowers the mass number by 4 and atomic number by 2; beta decay raises the atomic number by 1
  • Two radioactive elements: Uranium (U-238) and Radium (Ra-226)

  Feature	Natural radioactivity	Artificial radioactivity
  Cause	Spontaneous decay of unstable isotopes	Bombarding stable nuclei with neutrons, alpha particles etc.
  Discovered by	Henri Becquerel (1896)	Irène and Frédéric Joliot-Curie (1934)
  Example	Uranium, radium	Medical isotopes made in reactors
  Control	Cannot be controlled	Made deliberately in reactors/accelerators

• CSS 2019 Q4(d): How can the Sun have such a strong gravitational field if it is made of gases? (5)

  • See section 8 above, and the Universe chapter (chapter 1) for the full answer
  • Points: gravity depends on mass; the Sun holds ≈ 99.86% of Solar System mass; gravity pulling inward balances gas pressure pushing outward

• CSS 2015 Q4(a), Q5(a): Working principle of a microwave oven and RADAR; what is night vision technology? (5 each)

  • Microwave oven
    • A magnetron produces microwaves of ≈ 2.45 GHz
    • Water, fat and sugar molecules are polar; the rapidly alternating field makes them rotate, and friction produces heat
    • Food heats from the inside; metal reflects microwaves
  • RADAR (Radio Detection And Ranging)
    • Transmits short pulses of radio/microwave energy; a target reflects part back
    • Distance = (speed of light × time delay) ÷ 2
    • The Doppler shift of the echo gives the target's speed
  • Night vision technology
    • Image intensification: a photocathode turns faint light into electrons, an electron multiplier amplifies them, and a screen shows a bright image
    • Thermal imaging: infrared cameras detect heat from objects and make a picture without any light
    • Uses: military, security, wildlife observation, firefighting

• CSS 2017 Q4(A): What is a mirage? Describe in detail the creation of a mirage. (5)

  • Mirage: an optical illusion in which distant objects or the sky appear displaced, inverted or as a pool of water, caused by the refraction of light in air layers of different temperature
  • Why it happens
    • Air density, and so refractive index, changes with temperature
    • Light bends (refracts) as it passes from one layer to another
  • Creation of a desert/road (inferior) mirage
    1. The ground is very hot, so the air just above it is hot and less dense (lower refractive index)
    2. Light from the sky travels downward into these hotter layers and bends away from the normal
    3. At a certain angle it reaches the critical angle and undergoes total internal reflection, curving back upward
    4. The observer's eye receives this light and sees an inverted image of the sky on the ground, which looks like water
    5. As the observer moves, the 'water' keeps receding, because it is only an image
  • Types: inferior mirage (image below the object, over hot ground) and superior mirage (image above the object, over cold sea or ice; ships seem to float)
  • Key point: a mirage is a real optical effect that a camera can record; it is not a hallucination

13. CSS Exam Perspective

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• Most repeated areas: EM spectrum and applications, ionic vs covalent bonding, radioactivity (natural vs artificial), water molecule shape, gamma rays
• Short-question style: 'Differentiate between…', 'Explain…', 'Write two applications of…'
• Tip: learn the EM spectrum in order with one use each, and keep the covalent vs ionic table ready