3_Bonds.pdf

MBiol 112 Organic Molecules BOND FORMATION Development of Chemical Bonding Theory ⬥ 1858 - August Kekulé and Archibald Couper ⬦ independently proposed that, in all organic compounds, carbon is tetravalent—it always forms four bonds when it joins other elements to form stable comp...

MBiol 112 Organic Molecules BOND FORMATION Development of Chemical Bonding Theory ⬥ 1858 - August Kekulé and Archibald Couper ⬦ independently proposed that, in all organic compounds, carbon is tetravalent—it always forms four bonds when it joins other elements to form stable compounds ⬥ Kekulé ⬦ carbon atoms can bond to one another to form extended chains of linked atoms ⬦ (1865) suggested that carbon chains can double back on themselves to form rings of atoms 2 Development of Chemical Bonding Theory ⬥ 1874 - Jacobus van’t Hoff and Joseph Le Bel ⬦ proposed that the four bonds of carbon are not oriented randomly but have specific spatial directions ⬥ Van’t Hoff ⬦ the four atoms to which carbon is bonded sit at the corners of a regular tetrahedron, with carbon in the center 3 Development of Chemical Bonding Theory 4 ⬥ Why, though, do atoms bond together, and how can bonds be described electronically? 5 Bond Formation ⬥ atoms bond together because the compound that results is more stable and lower in energy than the separate atoms ⬦ Energy (heat)—always flows out of the chemical system when a bond forms; conversely, energy must be put into the chemical system to break a bond ⬩ Making bonds always releases energy, and breaking bonds always absorbs energy. 6 Bond Formation ⬥ 8 electrons (an electron octet) in an atom’s outermost shell, or valence shell, impart special stability to the noblegas elements in group 8A of the periodic table: Ne,Ar,Kr ⬥ main-group elements is governed by their tendency to take on the electron configuration of the nearest noble gas 7 8 Bond Formation 9  Bond Formation ⬥ Ex. The alkali metals in group 1A, achieve a noble-gas configuration by losing the single s electron from their valence shell to form a ⬥ cation, while the halogens in group 7A achieve a noble-gas configuration by ⬥ gaining a p electron to fill their valence shell and form an anion. 10 Octet Rule ⬥ Atoms will lose, gain or share electrons to achieve the electron configuration of the nearest noble gas (8 valence electrons except for He with 2) ⬥ Elements in the third and higher rows (e.g. Al, Si, P, S, Cl and above) can have more than eight electrons. 11 12 Types of Bonds 13 Types of Bonds ⬥ Ionic bonds ⬦ electrostatic attraction (cation and anion) ⬥ Covalent bonds ⬦ a shared-electron bond ⬦ The neutral collection of atoms held together by covalent bonds is called a molecule. 14 Covalent Bonds ⬥ formed when the electronegativity difference between the two atoms is too small (