Flashcards · 9701 · AS Topic 3

Chemical bonding (AS Level) flashcards

33 cards on AS Topic 3, subtopics 3.1 to 3.7. Pick a subtopic or study them all.

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3.1 Electronegativity and bonding

Electronegativity: The ability of an atom to attract the bonding electrons in a covalent bond.
Trends in electronegativity across a period and down a group: Increases across a period (greater nuclear charge, same shielding, smaller radius). Decreases down a group (more shells, more shielding, greater distance). Fluorine is the most electronegative element.
How does electronegativity difference relate to bond type?: Difference about 0: pure covalent. Small difference: polar covalent. Large difference: ionic. It is a continuum, not a switch.
Pauling values H 2.2, Cl 3.2, Na 0.9. Predict the bonding in HCl and in NaCl.: HCl: difference 1.0, polar covalent. NaCl: difference 2.3, ionic.

3.2 Ionic bonding

Ionic bonding: Electrostatic attraction between oppositely charged ions (formed when electrons transfer from a metal to a non-metal).
Electron transfer in forming MgO: Mg (2,8,2) transfers two electrons to O (2,6), giving Mg²⁺ (2,8) and O²⁻ (2,8).
Electron transfer in forming CaF₂: Ca (2,8,8,2) transfers one electron to each of two F atoms (2,7), giving Ca²⁺ (2,8,8) and two F⁻ (2,8).
Giant ionic lattice: A regular arrangement of oppositely charged ions held by electrostatic attraction acting in all directions, not confined to one ion pair. In NaCl each Na⁺ is surrounded by six Cl⁻.

3.3 Metallic bonding

Metallic bonding: Electrostatic attraction between a lattice of positive metal ions and a sea of delocalised electrons.
Three factors that increase the strength of metallic bonding: More delocalised electrons per atom, smaller ionic radius, and higher ionic charge.
Why does magnesium have a much higher melting point than sodium?: Mg contributes 2 delocalised electrons per atom (Na only 1) and Mg²⁺ is smaller and more highly charged, so the attraction between the ions and the electron sea is stronger.
Why do metals conduct and why are they malleable?: Conduct (solid and molten): the delocalised electrons are free to move and carry charge. Malleable: layers of ions slide past each other while the electron sea redistributes, so no specific bond has to break.

3.4 Covalent bonding and coordinate (dative covalent) bonding

Covalent bond: The electrostatic attraction between two nuclei and a shared pair of electrons.
Coordinate (dative covalent) bond: A shared pair of electrons in which both electrons come from the same atom, e.g. N–H⁺ in NH₄⁺. Once formed it is identical to an ordinary covalent bond.
How is Al₂Cl₆ held together?: A lone pair on a chlorine atom of each AlCl₃ unit is donated to the aluminium of the other unit, forming two coordinate bonds and two bridging Cl atoms.
Expanded octet: examples and which atoms can do it: Only Period 3 (and later) central atoms. SO₂: 10 electrons around S. PCl₅: 10 around P. SF₆: 12 around S. Period 2 atoms (C, N, O) never exceed eight.
Sigma (σ) and pi (π) bonds: σ: direct, head-on overlap of orbitals between the bonding atoms. π: sideways overlap of adjacent p orbitals, above and below the σ bond. A single bond is one σ; a double bond is one σ + one π.
Bond energy and bond length: Bond energy: energy needed to break one mole of a particular covalent bond in the gaseous state. Bond length: the internuclear distance of two covalently bonded atoms. Triple bonds are shorter and stronger than double, which are shorter and stronger than single.

3.5 Shapes of molecules

The VSEPR rule and the order of repulsion: Electron pairs around a central atom repel and get as far apart as possible. lone pair–lone pair > lone pair–bond pair > bond pair–bond pair.
Shapes and bond angles of CH₄, NH₃ and H₂O: CH₄: tetrahedral, 109.5°. NH₃: trigonal pyramidal, 107° (one lone pair). H₂O: bent, 104.5° (two lone pairs). Each lone pair repels more than a bond pair, closing the angle by about 2.5°.
Shapes and angles: 2, 3, 5 and 6 bond pairs with no lone pairs: 2: linear, 180° (e.g. CO₂). 3: trigonal planar, 120° (e.g. BF₃). 5: trigonal bipyramidal, 120° and 90° (PCl₅). 6: octahedral, 90° (SF₆).
Shape with 4 bond pairs and 2 lone pairs: Square planar, 90°.
Hybridisation of carbon: sp³, sp² and sp: sp³: four hybrids, tetrahedral (methane, ethane). sp²: three hybrids in a plane plus one p orbital for a π bond, trigonal planar (ethene). sp: two hybrids, linear, two p orbitals for two π bonds (ethyne).

3.6 Intermolecular forces, electronegativity and bond properties

The two types of van der Waals' force: Instantaneous dipole–induced dipole (id-id, London) forces: in all molecules, increase with the number of electrons. Permanent dipole–permanent dipole (pd-pd) forces: between polar molecules. Hydrogen bonding is a special case of pd-pd.
Two requirements for a hydrogen bond (9701): A hydrogen atom bonded directly to N or O, and a lone pair on an N or O atom of a neighbouring molecule to accept it. HCl does not hydrogen bond.
When is a molecule polar? Compare CO₂ with H₂O, and CCl₄ with CHCl₃.: Only if the bond dipoles do not cancel. CO₂ (linear) and CCl₄ (symmetrical tetrahedral): non-polar. H₂O (bent) and CHCl₃ (asymmetric): polar.
Why is ice less dense than liquid water?: Each H₂O forms four hydrogen bonds in a tetrahedral arrangement, giving an open lattice with holes. On melting the lattice partly collapses, so the liquid is denser.
Method for comparing boiling points of molecular substances: 1 Compare the strongest intermolecular force in each. 2 If the same type, more electrons → stronger id-id forces → higher b.p. 3 If still tied, more branching → less surface contact → lower b.p.
What is overcome when a simple molecular substance boils?: Only the intermolecular forces between molecules. The covalent bonds within the molecules stay intact.

3.7 Dot-and-cross diagrams

Conventions for a dot-and-cross diagram: Dots for one atom's outer electrons, crosses for the other's, so the origin of every electron is visible. Outer shells only. Ions are drawn separately in square brackets with their charge.
How is a coordinate bond shown in a dot-and-cross diagram?: As two symbols of the same kind (e.g. two dots from N in the fourth N–H bond of NH₄⁺), because both electrons come from the donor atom.
Dot-and-cross diagram for NO: what is special?: NO has 5 + 6 = 11 outer electrons. N=O is two shared pairs, O has two lone pairs, N has one lone pair plus one unpaired electron (only seven around N).
Dot-and-cross diagram for MgCl₂: [Mg]²⁺ and two separate [Cl]⁻ ions, each chloride with a full octet (seven crosses plus one dot), each in square brackets with its charge.