Chemistry Y11 ATAR Bonding

Metallic bonding definition

A lattice of positive metal ions surrounded by a sea of delocalised (mobile) valence electrons, held together by electrostatic attraction between the ions and electrons

Why are metals malleable and ductile?

Layers of positive ions can slide past each other without repulsion because delocalised electrons move with them, maintaining electrostatic attraction

Why do metals conduct electricity?

Delocalised valence electrons are free to move throughout the lattice, carrying charge

Why do metals conduct heat?

Delocalised valence electrons are free to move and transfer kinetic energy through the lattice

Why do metals have high melting points?

Strong electrostatic attraction between positive ions and delocalised electrons requires large amounts of energy to overcome

Why does melting point decrease down Group 1?

Atomic radius increases down the group, so the nucleus is further from delocalised electrons, weakening the metallic bond strength

Why does Al have a higher melting point than Mg which is higher than Na?

Al contributes 3 delocalised electrons per atom, Mg contributes 2, Na contributes 1. More delocalised electrons = stronger metallic bonding = higher melting point

Ionic bonding definition

Electrostatic attraction between oppositely charged ions arranged in a 3D lattice, formed by transfer of electrons from metal to non-metal

Why do ionic compounds have high melting points?

Strong electrostatic attractions between oppositely charged ions act in all directions; large amounts of energy required to separate the ions

Why are ionic compounds brittle?

If one layer slides past another, like-charged ions align and repel, shattering the lattice

Why don't ionic compounds conduct in the solid state?

Ions are held in fixed positions in the lattice and cannot move; conduction requires mobile charged particles

Why do ionic compounds conduct when molten?

The lattice breaks down, releasing ions that are free to move and carry charge

Why do ionic compounds conduct when dissolved in water?

The lattice breaks down as ions separate into solution and are free to move

Covalent bonding definition

Two non-metal atoms share a pair of electrons; both nuclei are attracted to the shared pair, holding the atoms together

What is a bonding pair?

A pair of electrons shared between two atoms in a covalent bond

What is a lone pair?

A non-bonding pair of electrons belonging to one atom only, not involved in covalent bonding

Why do covalent molecular substances have low melting points?

Only weak intermolecular forces (dispersion forces) between molecules need to be overcome, not the strong covalent bonds within each molecule

Why don't covalent molecular substances conduct electricity?

Molecules are neutral and all electrons are tightly held; no mobile charged particles

Why is SiO₂'s melting point much higher than CO₂?

SiO₂ is a covalent network solid — strong covalent bonds throughout the entire lattice must all be broken. CO₂ is molecular — only weak dispersion forces between molecules

Properties of covalent network substances

Very high melting point, extremely hard and brittle, non-conductors in solid and liquid states (except graphite)

Examples of covalent network substances

Diamond, graphite, silicon (Si), silicon dioxide (SiO₂), silicon carbide (SiC)

Why does graphite conduct electricity?

Each carbon forms only 3 covalent bonds, leaving one delocalised electron per atom free to move between layers

Why is graphite soft and slippery?

Carbon atoms are bonded in layers of hexagonal rings with only weak forces between layers, so layers slide easily past each other

Why does graphite have a high melting point?

Strong covalent bonds within each layer must be broken to melt it

What are allotropes?

Different structural forms of the same element, e.g. diamond and graphite are both allotropes of carbon

How do allotropes differ from isotopes?

Allotropes differ in atomic arrangement/structure. Isotopes have the same proton number but different numbers of neutrons (different mass number)

Why does Si have a much higher melting point than P, S, Cl, Ar in Period 3?

Si is a covalent network solid with strong covalent bonds throughout. P, S, Cl and Ar are covalent molecular with only weak dispersion forces between molecules

Explain the melting point order S₈ > P₄ > Cl₂ > Ar

All are covalent molecular; dispersion force strength depends on total electrons per molecule. S₈ = 128e, P₄ = 60e, Cl₂ = 34e, Ar = 18e. More electrons = stronger dispersion forces = higher melting point

Why does melting point increase down Group 17?

Molecules get larger with more electrons, so dispersion forces between molecules increase, requiring more energy to melt

Why does melting point increase down Group 18?

Atoms get larger with more electrons, so dispersion forces between atoms increase, requiring more energy to melt

Trend in atomic radius across a period

Decreases from left to right

Explain why atomic radius decreases across a period

Same number of electron shells but increasing nuclear charge (more protons); greater electrostatic attraction pulls valence electrons closer to the nucleus

Trend in atomic radius down a group

Increases going down a group

Explain why atomic radius increases down a group

Each successive element has an additional electron shell placing valence electrons further from the nucleus; inner shells also shield valence electrons from the increasing nuclear charge

What is core charge (effective nuclear charge)?

The net positive charge experienced by valence electrons after accounting for shielding by inner electrons. Core charge = protons − inner electrons

Core charge of Na (Z=11)

11 protons − 10 inner electrons = +1

Core charge of Mg (Z=12)

12 protons − 10 inner electrons = +2

Core charge of Al (Z=13)

13 protons − 10 inner electrons = +3

Core charge of Si (Z=14)

14 protons − 10 inner electrons = +4

Core charge of F (Z=9)

9 protons − 2 inner electrons = +7

Core charge of K (Z=19)

19 protons − 18 inner electrons = +1

Atomic number definition

Number of protons in the nucleus; represented by Z

Mass number definition

Total number of protons and neutrons in the nucleus; represented by A

Definition of isotopes

Atoms of the same element with the same number of protons but different numbers of neutrons; same chemical properties, different physical properties

Electron configuration of Na

2, 8, 1

Electron configuration of Cl (Z=17)

2, 8, 7

Electron configuration of Ar (Z=18)

2, 8, 8

Electron configuration of Ca (Z=20)

2, 8, 8, 2

Electron configuration of oxide ion O²⁻

2, 8 (oxygen gains 2 electrons)

How is a cation formed?

An atom loses one or more valence electrons, forming a positive ion

How is an anion formed?

An atom gains one or more electrons, forming a negative ion

Electron dot diagram for an ionic compound

Draw each ion in square brackets with the charge shown outside. Cations show no valence electrons; anions show a full octet

Electron dot diagram for Cl₂

Each Cl has 3 lone pairs and shares 1 bonding pair: :Cl—Cl: with 3 lone pairs on each Cl

Electron dot diagram for CO₂

O=C=O with 2 lone pairs on each oxygen; carbon has no lone pairs; two double bonds

Electron dot diagram for NH₃

N at centre with 1 lone pair, bonded to 3 H atoms; each H has no lone pairs

Electron dot diagram for H₂O

O at centre with 2 lone pairs, bonded to 2 H atoms

Electron dot diagram for CS₂

S=C=S with 2 lone pairs on each S; analogous to CO₂

Electron dot diagram for PCl₃

P at centre with 1 lone pair, single bonds to 3 Cl atoms, each Cl has 3 lone pairs

Electron dot diagram for SO₂

S double-bonded to one O, single-bonded to the other; S has 1 lone pair; each O has lone pairs (3 bonding pairs, 6 non-bonding pairs total — or 4 bonding, 5 non-bonding with expanded octet)

How many emission lines from an electron promoted to energy level n?

Lines = n(n−1)/2. E.g. promoted to n=4: 4×3/2 = 6 lines. Promoted to n=9: 9×8/2 = 36 lines

Which hydrogen transition gives the red emission line?

n=3 → n=2 (lowest energy visible transition; longest wavelength = red)

What happens during electron emission?

An electron falls from a higher to a lower energy level, releasing energy as a photon of light

What happens during electron absorption?

An electron absorbs energy and is promoted from a lower to a higher energy level

Ionisation stage of mass spectrometer

Sample is bombarded by high-energy electrons, knocking electrons from atoms/molecules to form positive ions

Acceleration stage of mass spectrometer

Positive ions are accelerated toward a negatively charged plate; negative ions are repelled and not detected

Why can't negative ions be detected in a mass spectrometer?

They are repelled by the negatively charged accelerating plate and never reach the detector

Two factors governing deflection in a mass spectrometer (besides field strength)

Mass of the ion and charge of the ion (the m/z ratio); lighter and more highly charged ions are deflected more

How to calculate RAM from mass spectrum data

RAM = Σ(mass × relative intensity) ÷ total relative intensity

RAM calculation example: m/z 84 (2.8), 86 (49.3), 87 (35.0), 88 (412.9) — what is the RAM of strontium?

(84×2.8 + 86×49.3 + 87×35.0 + 88×412.9) ÷ (2.8+49.3+35.0+412.9) = 87.56 ≈ 87.6

How to find percentage abundance given RAM and two isotopes

Let x = abundance of isotope 1 (as decimal); (1−x) = abundance of isotope 2. Solve: mass₁(x) + mass₂(1−x) = RAM

Percentage abundance worked example: Mg RAM = 24.452, 10% Mg-25. Find % Mg-24 and Mg-26

Let a = % Mg-24, b = % Mg-26. a + b = 90. 24a + 26b = 24.452×100 − 25×10 = 2195.2. Solving: b = 22.6%, a = 67.4%

What ions are present when Cl₂ is introduced into a mass spectrometer?

Cl⁺, Cl²⁺, Cl₂⁺ — all positive. Cl⁻ is NOT present (negative ions are not accelerated)

4 classes of substance and their structure

Metallic (ion lattice + delocalised electrons), Ionic (+ and − ion lattice), Covalent molecular (discrete molecules), Covalent network (atoms covalently bonded throughout a lattice)

How to identify bonding type from properties

Conducts solid+liquid = metallic. Conducts only when molten/dissolved = ionic. Low MP, never conducts = covalent molecular. Very high MP, never conducts = covalent network (except graphite)

Which substance conducts at 800°C: NaBr (MP 755°C), BaCl₂ (MP 957°C), CCl₄ (MP −23°C), NH₃ (MP −78°C)?

NaBr — it melts at 755°C so at 800°C it is molten; ionic compounds conduct when molten

Ionic vs covalent: how to predict from position in periodic table

Metal (LHS) + non-metal (RHS) = ionic. Non-metal + non-metal (both RHS) = covalent

What type of bonding does a yellow solid melting at 119°C with no electrical conductivity have?

Covalent molecular (sulfur, S₈)

What type of bonding does a silvery solid melting at 98°C that conducts in solid and liquid state have?

Metallic (sodium)

What type of bonding does a dark shiny solid melting at 772°C, non-conductor as solid but conductor as liquid have?

Ionic (e.g. NaCl)

Correct increasing melting point order for CO₂, Hg, I₂, CaO

CO₂ < Hg < I₂ < CaO (molecular < liquid metal < molecular solid < ionic)

Why is Hg a liquid metal at room temperature?

Filled d-orbitals make metallic bonding in Hg unusually weak; melting point is −39°C

Which has discrete molecules in the solid state: sodium sulfate, chlorine, silicon, titanium?

Chlorine (Cl₂) — a simple covalent molecular substance

Compound formula rule for X (config 2,8,6) and Y (config 2,8,3)

X needs 2 electrons (Group 16 → X²⁻), Y loses 3 electrons (Group 13 → Y³⁺). Formula: Y₂X₃ (ionic)

NHBrI electron dot diagram

N at centre bonded to H, Br and I; N has 1 lone pair; each halogen has 3 lone pairs

Mg(NO₃)₂ electron dot diagram

[Mg]²⁺ and 2× [NO₃]⁻; in nitrate, N double-bonded to one O and single-bonded to two O with lone pairs; charge −1 in brackets

What is the gold(III) ion with 123 neutrons?

Mass number = 79 + 123 = 202; symbol ²⁰²Au³⁺ with Z=79

Formula of complex: one Au³⁺ ion and two CN⁻ ions

[Au(CN)₂]⁺ — Au is +3, two CN⁻ gives −2, net charge = +1

Cyanide ion CN⁻ bonding

Triple bond between C and N; C has 1 lone pair, N has 1 lone pair; total charge −1