Bond Types Unit 6 Chapter 12
Type Cast • When we look at various compounds, we see some very basic differences • Different substances fall into three categories: • Ionic • Covalent • Metallic
Ionic Substances • Form when two or more ions combine. • All ionic compounds are called salts. • They have very high melting points • (CaCl2 = 772°C) • All of them are solid at room temperature – the only exceptions are really complex organic molecules!
Ionic Substances • Will dissociate in water (if they dissolve) • Will not dissolve in Oil • Conductivity • As a solid, ionic compounds are insulators • When molten (liquid) & aqueous, they are conductors • Examples: NaBr, MgF2, AlCl3, CsOH
Covalent Substances • Usually form between a nonmetal and another nonmetal. • They have low melting points. • They can be solids, liquids, or gases at room temperature. • They may dissolve in water or oil – but usually not both
Covalent Substances • Two different types of covalent compounds: Polar and Nonpolar • They will not dissociate in water. • Will not conduct electricity • (though there are a few exceptions) • Examples: H2O, N2, CCl4, CH4, C8H18, SF6, CO2, SiO2, S8
Metallic Substances • Form from metals • Can be pure metals or alloys • Usually solid, but a few liquids at or near room temperature. • They will not dissolve in Water or Oil
Metallic Substances • May react with water • (forming H2 & a salt) • All metals conduct electricity at least a little bit. • Examples: Fe, Cu3Zn2 (Brass), Na, Mg, Ag29Cu4 (Sterling Silver)
It’s all about the Electrons • Bonds form between atoms due to the interactions of electrons. • Core Electrons • Electrons closest to the nucleus • Unable to participate in bonding • Shielded by electrons farther away • Valence Electrons • Electrons farthest from the nucleus • Determine the chemical reactivity of the atom
What’s the Difference? • Ionic Bonds form when electrons are transferred from one atom to another. • Bonding = Electrostatic attraction b/w positively and negatively charged ions – no physical connection! • Covalent Bonds form when electrons are shared between two atoms. • Bonding = Physical connection via overlap of shared electron’s wavefunction! • Metallic Bonds form when electrons between atoms are delocalized and are free to move around in an electron sea. • Bonding = Electrostatic attraction b/w positively charged metal atoms (not ions) and negatively charged electron sea! • electrical conductivity & malleability.
How Do We Know Bond Type? • Metallic bonds form between metals • There is no distinct line between covalent and ionic bonds • Remember, Ionic bonds transfer electrons • Covalent bonds share electrons K Na Na Cl Cl +1 K Na -1
The Tug of War • Ionic or Covalent – it depends on who pulls stronger! • Recall: Electronegativity is a measure of how much an atom wants another electron. • Think of it as how strong an atom is in a tug of war!
The Tug of War • Elements with high electronegativity will pull electrons away from those with low electronegativity. • Atoms with similar electronegativity will share electrons. • Few bonds are strictly ionic or covalent. • Ionic Character is the difference in electronegativity b/w bonding atoms.
Defining Ionic Character • If the difference is 0.4, bond is covalent • The e- is equally shared. • If the difference is 1.6 = Polar Covalent • Unequal Sharing – e- pulled more to one side. • If the difference is > 1.6 = Ionic • e- is transferred from one atom to the other.
Exceptions…Of Course Some compounds behave ionically even though their ENeg difference is < 1.6 HCl ionizes completely in water (ionic) Some compounds behave covalently even though their ENeg difference is > 1.6 HF is a gaseous molecule.
Comparison of Bond Types Ca F F +2 -1 -1 Ionic Bond O C O Covalent Bond I I Polar Covalent Bonds
