AS edexcel chemistry - bonding
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| What is the structure of ionic compounds? | Lattices of anions and cations held together by the strong electrostatic forces of attraction between oppositely-charged ions.
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| When do ionic bonds tend to form? | Between atoms of significantly different electronegativities - this is a measure of the tendency of an atom to draw a bonding pair of electrons towards it.
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| What factors increase the strength of an ionic bond? | Small ionic radius and high charges on ions
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| What are the patterns of ionic radii in the periodic table? | Increases down a group - more electrons and more principal energy levels. Decreases between group 5 in one period to group 3 in the next - these ions/ atoms are isoelectronic (same electron configuration) - radius decreases due to higher nuclear charge
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| What is some evidence for the existence of ions? | * high melting + boiling points - strong attraction * conduct electricity when molten/aqueous * colour changes in electrolysis * electron density maps * shatter when hit - ions held in regular lattice * dissolving not as endothermic as expected
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| What are electron density maps, how are they produced and how do they provide evidence for the existence of ions? | Show positions of ions and electrons, X-rays pass through ionic compound and produce diffraction pattern. Extent to which radiation is scattered: electron density. Evidence: localized electrons, high density around cations, 0 density between ions
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| What is enthalpy change? | The amount of heat energy absorbed or released in a reaction, measured in kJmol-1. Exothermic reactions are given a negative value, endothermic reactions are given a positive value
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| What do Born-Haber cycles show + generalised steps? | The formation of an ionic compound in separate stages. Enthalpy of atomisation of both elements, enthalpy of first ionisation energy of metal, enthalpy of first electron affinity of non-metal, lattice energy, enthalpy change of formation.
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| Why are ionic compounds most stable as solids? | Steps 1-4 are endothermic and unfavourable. Lattice energy is very exothermic and favourable, solids have lowest energy.
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| What is lattice energy? | The enthalpy change when one mole of an ionic solid is produced from its gaseous constituent ions that start an infinite distance apart - exothermic, favourable
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| Why do theoretical and experimental values for lattice energies differ? | Theoretical assume 100% ionic bonding. Experimental - often higher due to polarisation of anion, causing a varying degree of covalency, which is stronger bonding
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| What is polarisation and how is it maximised? | When an anion is drawn significantly towards the cation. A cation with a high charge density (small radius, high charge) has a higher polarising power. An anion with a large radius and high charge is more polarisable.
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| What factors increase lattice energy? | Higher charged on the ions - stronger bonding so lattice energy is more exothermic. Smaller radii means stronger bonding so lattice energy is more exothermic. Higher polarisation means more covalent character, so lattice energy is more exothermic
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| What is coordination number? | The number of oppositely-charged ions most closely packed around an ion in an ionic compound. Ions arrange themselves to maximise lattice energy, minimise repulsion, have lowest potential energy
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| What is the octet rule? | When an atom gains, loses or shares electrons, its electron configuration becomes the same as the noble gas of the same period or the previous period.
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| What is a covalent bond + dative? | A pair of valence electrons shared between two atoms, where each atom supplies one electron. A dative covalent bond is where one atom supplies both of the electrons - caused by the overlap of an empty orbital and on containing a lone pair of electrons
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| What are the types of bonds? | Sigma - head on overlap of two atomic orbitals, single bond. Pi bond - sideways overlap of two p orbitals, electrons lie above and below the plane of the atoms.
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| Is covalent bonding strong or weak - why and what it is determined by? | Strong due to electrostatic attraction between nucleus and electrons between nuclei. Due to: sum of atomic radii of bonded atoms - smaller atoms form stronger bonds, number of electrons being shared (triple bonds are strongest, single are weakest)
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| What are the properties of giant molecular covalent substances? | * insoluble in water * high melting and boiling points due to strong covalent bonds extending over many atoms in structure * poor electrical conductors apart from graphite
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| What is the structure of metals? | A regular arranegement of closely-packed metal cations in a sea of delocalised valence electrons.
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| What is a metallic bond? | The strong attraction between the cations and the delocalised electrons
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| What affects the strength of a metallic bond? | The charge of the cations - affects attraction between cations and electrons and electron density. Metallic radius - smaller cations cause stronger bonds. Type of coordination and close-packing.
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| What are some physical properties of metals? | Good electrical conductors, good thermal conductors, malleable. Melting point and density decrease down a group due to same charges on ions but increased metallic radius, increase along a period due to decreased metallic radius and higher charge on ions.
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| What effect does the number of valence electrons have on properties of metals? | Increases electrical conductivity - current is flow of delocalised electrons. Increases thermal conductivity - more electrons pass on kinetic energy. Increases malleability - more electrons bind cation layers together preventing fracturing and repulsion
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