Fundamentals of Medicinal and Pharmaceutical Chemistry PDF

Fundamentals of Medicinal and Pharmaceutical Chemistry FUNCHEM.3 The Periodic Table: Trends and Properties D r. D a r r e n G r i ffi t h  General Chemistry - The...

Fundamentals of Medicinal and Pharmaceutical Chemistry FUNCHEM.3 The Periodic Table: Trends and Properties D r. D a r r e n G r i ffi t h  General Chemistry - The Essential Concepts by Chang Recommended and Goldsby 7e reading  Sections 2.4, 8.3, 8.4, 8.5, 9.5 F U N C H E M. 3 T h e P e r i o d i c Ta b l e : Tr e n d s a n d P r o p e r t i e s 2 FUNCHEM.3 Learning Outcomes Recognise the arrangement of elements in the periodic table according to increasing atomic number. Differentiate between periods and groups, metals, non-metals and metalloids, alkali metals, alkaline earth metals, transition metals, halogens, inert gases, lanthanides and actinides in the periodic table. Define ‘effective nuclear charge’. Define ‘atomic radius’, ‘ionic radius’, ‘ionisation energy’, ‘electron affinity’ and ‘electronegativity’ and describe how each varies on going across a period and down a group in the Periodic Table. 3 The Periodic Table (The alphabet of chemistry) When it comes to studying the elements nature has handed us a nice simplification: - 118 elements - Grouped into a few families - Displayed by the Periodic Table 4 Mendeleev’s Periodic Table In 1869 Dmitri Mendeleev published the first arrangement of the elements 1834 - 1907 had no idea what atoms were made of or why they behaved as they did. organised the elements in order of increasing atomic mass. based on gaps in his table predicted the existence and properties of several undiscovered elements. 5 The Modern Elements Periodic Table arranged according to increasing atomic number Periods Groups Elements represented by symbols Elements within a group show similar chemical properties 6 The Modern Periodic Table Metals and non metals A metal is a good conductor of heat and electricity, whereas a nonmetal is usually a poor conductor of heat and electricity 7 The Modern Periodic Table Elements show great variety in their chemical properties There are similarities between the physical and chemical properties of elements within a group for example. Group I and Group VII elements are the most reactive elements 8 Valence electrons The outmost or valence shell electrons are very important because they participate in the sharing and exchange that is responsible for chemical reactions The attraction between the positive nucleus and the outermost negative electrons (valence electron) depends on: Shell Model of an 1. Number of protons in the nucleus Atom 2. Shielding effect of other e- closer to nucleus 2. Distance form the nucleus Nucleus 9 Effective Nuclear Charge, Zeff The effective nuclear charge (Zeff) is the nuclear charge felt by an electron when both the actual nuclear charge (Z) and the repulsive effects (shielding) of the other electrons are taken into account. Zeff = Z - s where σ (sigma) is called the shielding constant (also called the screening constant). The shielding constant is greater than zero but smaller than Z. - Moving from left to right, - Zeff increases, - Z increases, - the added electron is a valence electron and valence electrons do not shield each other well, - greater effective nuclear charge felt by the valence electrons. - Going Down - Zeff increases as calculated per Slaters rules - Slaters rules calculates σ by accounting for the effective shielding of electrons in each orbital "shell". 10 Effective Nuclear Charge, Zeff The effective nuclear charge (Zeff) is the nuclear charge felt by an electron when both the actual nuclear charge (Z) and the repulsive effects (shielding) of the other electrons are taken into account. Zeff = Z - s 11 Atomic Radius The atomic radius of an element is half the distance between the nuclei of two adjacent atoms How does the atomic radius vary - On going across a period? - On going down a group? 12 Atomic Radius On going across a period 7+ 8+ 7- 8- Increasing atomic number, Z, therefore increasing number of protons in nucleus Adding e- to same energy level Effective nuclear charge, Zeff, increases Bigger attraction Atomic radius decreases 13 Atomic Radius On going down a group Electrons enter another energy level A greater distance between the nucleus and outermost shell Atomic radius increases 14 Atomic Radius 15 Ionic Radius Cl atom Radius of a cation or an anion 17+ Formation of an anion (X-) 17- Additional electron(s) 99 pm Nuclear charge remains the same Greater repulsion amongst electrons Cl- ion Size increases i.e. ionic radius increases 17+ 18- 181 pm 16 Ionic Radius Na atom Radius of a cation or an anion 11+ Formation of a cation (X+) 11- Less electron(s) 186 pm Nuclear charge remains 1s2 2s2 2p6 3s1 the same Less repulsion amongst electrons Na+ ion 11+ Size decreases i.e. ionic radius decreases 10- 99 pm 17 1s2 2s2 2p6 Ionic Radius Radius of Dipositive Ion < Radius of Unipositive Ion Na+ Mg2+ 99 pm 65 pm Why? 11 Na Na + Mg Mg2+ 12 11+ 11+ 12+ 12+ 11- 10 - 12- 10 - Na+ and Mg2+ are isoelectronic Mg2+ has Bigger nuclear charge Bigger attraction Smaller size 18 Ionic Radius Which is larger? O2- 8+ O2- or F- 10 e- Radius of a dinegative ion > radius of a uninegative ion F- O2- and F- are isoelectronic 9+ F- has bigger nuclear charge 10 e- Bigger attraction Smaller size Watch the animation titled ‘Atomic and Ionic Radius’ on the VLE 19 Ionic Radius Ions move through tiny channels in cell membranes. Some channels allow Na+ through but not larger K+ 20 Ionisation Energy Electrons are attracted to the positive charge in the nucleus. Energy is needed to overcome this attraction. Ionization energy (IE) is the minimum energy (in kJ/mol) required to remove an electron from a gaseous atom in its ground state M(g) M+(g) + e- (removal of first electron) First Ionisation Energy (IE1) 21 Ionisation Energy Na Na+ 22 Ionisation Energy Second Ionisation Energy (IE2) M+(g) M2+(g) + e- Third Ionisation Energy (IE3) M2+(g) M3+(g) + e- 23 Ionisation Energy ON GOING ACROSS A PERIOD 7+ 8+ 7- 8- Nuclear Charge, Z, increases Adding e- to same energy level Effective nuclear charge, Zeff, increases Bigger attraction Ionisation energy increases 24 Ionisation Energy ON GOING DOWN A GROUP Electrons enter another energy level i.e. greater distance between nucleus and outermost shell and outermost e- The increased distance weakens the nuclear attraction to the outermost e- Ionisation energy decreases 25 Ionisation Energy ? ? 4 Be 1s 2 2s 2 5 B 1s 2 2s 2 2p 1 Exceptions 7N 1s 2 2s 2 2p 3 8O 1s 2 2s 2 2p 4 26 Ionisation Energy Alkali metals – lowest ionisation energy Noble gases – highest ionisation energy 27 Electron Affinity X(g) + e- X-(g) Electron affinity (EA) is the negative of the energy change that occurs when an electron is accepted by an atom in the gaseous state to form an anion. The more positive the electron affinity of an element, the greater is the affinity of an atom of the element to accept an electron. When H is negative, energy is released F(g) + e- F-(g) H = - 328 kJ mol-1 Electron affinity is + 328 KJ mol-1 28 Electronegativity H:F Electrons in a bond rarely get shared equally. H F 29 Electronegativity Electronegativity, the ability of an atom to attract toward itself the electrons in a chemical bond H : H Electrons in the bond shared equally H : F F more electronegative than H F pulls e- towards itself + H F - H F movement of e- towards F slight gain of e- Dipole 30 Why is F the most electronegative element? - High nuclear charge 9F - Little shielding Electronegativity is related to electron affinity and ionization energy. Fluorine has a high electron affinity (tends to pick up electrons easily) Fluorine has a high ionization energy (does not lose electrons easily) F > O > N > C ‘Front Office Never Closes’ 31 Electronegativity 32  Across a Period Atomic Radius 1st I.E. Electronegativity Down a Group Atomic Radius 1st I.E. Electronegativity 33 Thank you F O R M O R E I N F O R M AT I O N P L E A S E C O N TA N T D r. D a r r e n G r i ffi t h E M A I L : d g r i ffi t h @ R C S I. C O M 34