ACSI 2022 HL Chemical Bonding and Structure Core (Teacher) (1)

IBDP Chemistry HL/Chemical Bonding and Structure Page 1   Anglo − Chinese School (Independent) Year 5 (2022) IBDP Chemistry HL (IBDP syllabus Topic 4) 4.1 Ionic bonding and structure - Essential Idea: ionic compounds consist of ions held together in lattice structures by ionic bonds. 4.2 Covalent bonding - Essential Idea: covalent compounds form by the sharing of electrons. 4.3 Covalent structures - Essential Idea: Lewis (electron dot) structures show the electron domains in the valence shell and are used to predict molecular shape. 4.4 Intermolecular forces - Essential Idea: the physica l properties of molecular substances result from different types of forces between their molecules. 4.5 Metallic bonding - Essential Idea: metallic bonds involve a lattice of cations with delocalised electrons     TOPIC 4 CHEMICAL BONDING AND STRUCTURE TEACHER COPY – WITH SUGGESTED SOLUTIONS

IBDP Chemistry HL/Chemical Bonding and Structure Page 3   Introduction  A chemical bond is a force which holds two or more atoms or ions together.  All chemical reacti ons involve bond breaking followed by bond making. o The breaking of bonds requires (absorbs) energy – endothermic process. o The formation of bonds releases e nergy – exothermic process. The feasibility of a reaction depends on the energy transfers a ssociated with the breaking and making of bonds. Hence, by forming bonds with each other, atoms become part of a more stable system.  All types of bonding involve interactions between opposite cha rges and they are electrostatic in nature. o Ionic bond – attraction between some positively charged particles (cations) and negatively charged particles (anions) o Covalent bond – attraction between nuclei and shared electrons o Metallic bond – attraction between metal ions and delocalised electrons  The electrons involved in the chemical bond formation usually come from the outermost shells of the atom s. These electrons are called valence electrons. The nuclei of the atoms are unaffect ed. The valence electrons rearr ange to attain a state of minimum energy.  Usually, atoms achieve stabili ty by acquiring the nearest nobl e gas electronic configuration. This means possessing a completely filled valenc e shell with 8 outermost shell electrons. This is known as octet configuration . Helium is an exception because it has only two electrons.  Bonding affects the chemical and physical properties of compounds.

IBDP Chemistry HL/Chemical Bonding and Structure Page 4   HF, NH3, H2O, CH3COOH, CH3CH2OH, CH3CONH2 3 types CHEMICAL BONDING IONIC BONDING COVALENT BONDING (incl Coordinate Covalent Bonds) METALLIC BONDING Giant Ionic Structure Simple Molecular Structure Giant Covalent Structure (Macromolecular Structure) Giant Metallic Structure Hydrogen Bonding Polar molecules Non–polar molecules CO, HCl, CH3Cl, SO2 CCl4, CO2, H2, I2, PCl5, SF6 Dipole – Dipole Forces Instantaneous Induced Dipole – Induced Dipole Forces ‘London (Dispersion) Forces’ Yes No Between polar molecules that contain H directly bonded to N, O, or F

IBDP Chemistry HL/Chemical Bonding and Structure Page 5   4.1 Ionic Bonding and Structure Nature of science: Use theories to explain natural phenomena – molten ionic compou nds conduct electricity, but solid ionic compounds do not. The solubility and melting points of ionic compounds can be used to explain observations. (2.2) Understandings:  Positive ions (cations) form by metals losing valence electrons.  Negative ions (anions) form by non–metals gaining electrons.  The number of electrons lost or gained is determined by the el ectron configuration of the atom.  The ionic bond is due to electrostatic attraction between oppo sitely charged ions.  Under normal conditions, ionic compounds are usually solids with lattice structures. Applications and skills:  Deduction of the formula and name of an ionic compound from it s component ions, including polyatomic ions.  Explanation of the physical p roperties of ionic compounds (vol atility, electrical conductivity and solubility) in terms of their structure. Guidance:  Students should be familiar with the names of these polyatomic ions: NH4+, OH–, NO3–, HCO3–, CO32–, SO42– and PO43–. 4.1.1 Definition The ionic or electrovalent bond refers to the electrostatic attraction experienced between the electric charges of a cation (positive ion) and an anion (negative ion). Therefore,  when an atom loses electrons it becomes a positively charged ion (cation);  when an atom gains electrons i t becomes a negatively charged ion (anion);  these ions will usually have the nearest noble gas electronic configurations;  and these oppositely charged ions will have electrostatic attr action for one another. This is the ionic bonding. Ionic compounds are usually formed between two elements of very different electronegativities (usually a difference in electronegativity of greater than 1.8) . The highly electronegative element (non–metal) will gain el ectrons to form the anion while the element with low electronegativity (metal) will lose its valence electrons forming the cation.

IBDP Chemistry HL/Chemical Bonding and Structure Page 6   4.1.2 Dot and Cross Diagrams Example: NaCl Exercise 1: Draw dot and cross diagrams for the following compounds: (a) KF (b) MgC l 2 (c) Na 2O (d) A l2O3 4.1.3 Nature of Ionic Bonding Ionic bonds are the strong electrostatic attraction between ions of opposite charges. They are  Electrostatic in nature;  Usually strong, therefore ioni c compounds generally have high melting and boiling points;  Non–directional i.e. equally s trong in any direction. Na x +  Cl      [ ] Na+ Cl       x  x [ ] K+ F       x [ ] Mg2+ Cl      2 2 x x [ ] 2 Na+ O     3 2 x x [ ] 2 Al3+ O     Sodium atom 1s2 2s2 2p6 3s1 Chlorine atom 1s2 2s2 2p6 3s2 3p5 Chlorine ion 1s2 2s2 2p6 3s2 3p6 (electronic structure of argon) Sodium ion 1s2 2s2 2p6 (electronic structure of neon)

IBDP Chemistry HL/Chemical Bonding and Structure Page 7   4.1.4 Ionic Bond Strength and Lattice Energy The interactions between these charged particles give rise to t he electrostatic attraction. The strength of this electrostatic attraction is indicated by its lattice energy. Lattice energy of an ionic compound is defined as the enthalpy change when one mole of a solid ionic compound is separated into gaseous ions under standard conditions. Its magnitude is proportional to the charges on the ions (q + and q–) and inversely proportional to the distance separating them (r+ + r–) i.e. |Lattice Energy| ∝ ฬ qାqି rା ൅ rି ฬ Where q+ = charge of cation q– = charge of anion r + = radius of cation r – = radius of anion From the graph:  Electrostatic attraction between the two ions increases when r decreases, i.e., they come closer.  But the repulsion of the electron clouds prevents the inter–nu clear distance from being smaller than r.  At an inter–nuclear distance of r, the system reaches a minimu m potential energy and the ions experience the strongest electrostatic attraction. Generally, decreasing the size and increasing the charge of the ion will increase the strength of the ionic attraction; leading to higher melting and boiling points. But there are exceptions. For example, the melting point of Al2O3 is 2072 oC and this is lower than that of MgO which is 2852 oC. E.g. 1 Melting point of KF > KC l and MgO > Na2O E.g. 2 Melting point of MgO > Melting point of NaC l (Mg 2+O2