Atomic & Molecular Bonding

Ionic Covalent Metallic Weak Forces Types of Bonding: strong bonds (“primary”) (“secondary”)

Valence Electrons Those electrons that participate in bonding between atoms – usually the electrons with the highest energies.

Atomic Structure – planetary model Hydrogen Helium Oxygen

Orbital The quantum-mechanical description of an electron, which includes the electron’s energy and its time-averaged distribution in space.

z y x Quantum mechanical view: gives time averaged probability of an electron’s position. z there’s a higher probability this electron is nearer the atom’s nucleus. y there’s a lower probability that this electron is farther from the atom’s nucleus. x

z y x Quantum mechanical result: some valence electrons have a time-averaged probability that is stronger in some directions than in others. z Valence electrons are much more likely to be found in these directions. y x

Ionic bonding: A primary bond due to electrostatic attraction between oppositely charged ions.

Ionic bonding – electronegativity: Transfer of valence electrons Na Cl

Ionic bonding – electronegativity: Na+ Cl-

Ionic bonding – electronegativity:

Ionic bonding – electronegativity:

Ionic bonding – electronegativity: Electronegativity: The relative tendency of atoms to accept electrons.

Ionic bonding – charge balance: Cl - Na+ Attraction of ions with unlike charges Integrity of the ensemble depends on alternating charge Moving an ion will disrupt bonding

A primary bond that arises when two atoms share valence electrons. Covalent bonds: A primary bond that arises when two atoms share valence electrons.

Si O Si O

O O Si O O

Metallic bonds: A primary bond that can be thought of as the electrostatic attraction between an electron gas of valence electrons and positively charged ion cores.

Metallic bonding – the electron gas: free valence electrons not attached to a given atom conceived as an “electron gas” + + electron gas + + +

Metallic bonding – the electron gas: average electron midway between positive ions + bond is “omni-directional” - ions can move out of position without the bond breaking + + + virtual charge balance keeps the ensemble together + electron gas

Metallic bonding: bonds not dependent on direction – atoms can be displaced + + - + + electron gas + + + + + - + + + + + + + + + + +

Metallic: non-directional Covalent: often highly directional Bond directionality Metallic: non-directional Covalent: often highly directional Ionic: directional in the sense that bonding depends on a precise charge balance

Covalent bonding: strong in particular directions, break if atom displaced from that direction

Metallic: can be ductile Covalent: brittle Ionic: brittle Bond directionality Metallic: can be ductile Covalent: brittle Ionic: brittle

Bonding Substance KJ/mol Melting Pt. oC Ionic NaCl 640 801 MgO 1000 2800 Covalent Si 450 1410 C (diamond) 713 > 3550 Metallic Hg 68 - 39 Al 324 660 Fe 406 1538 W 849 3410

Bonding Substance KJ/mol Melting Pt. oC van der Waals Ar 7.7 - 189 Cl 2 31 - 101 Hydrogen NH3 35 - 78 H2O 51

Secondary Bonds – Intermolecular Forces: van der Waals dipole-dipole forces London forces Hydrogen Bonds

van der Waals forces: dipole-dipole forces H + - Cl Here molecules are permanent dipoles with net negative charge at one end and net positive charge at the other.

van der Waals forces: induced dipole + - + - Molecules with strong dipole moments can induce dipoles in nearby atoms and molecules + - - +

van der Waals forces: London forces + - + - Average Charge Distribution but in Quantum mechanics the above has a small probability of occurring London forces help the weak attraction between atoms and molecules that have no permanent dipole moment. The forces that arise between molecules from with such randomly appearing dipoles are called “London forces.”

Hydrogen Bonding: Oxygen - + H H hydrogen bonds hydrogen bonds The single electron around a hydrogen atom participates in bonding leaving behind the positively charged proton. The concentration of positive charge around the proton will be attracted toward areas of negative charge. In this sketch hydrogen bonds are causing water molecules to align.

Forces between polymer molecules: role of hydrogen bonds Particularly strong hydrogen bond arise between H+ and, strongly and relatively small electronegative atoms such as Cl, O, and F. PVC - polyvinylchloride

Forces between polymer molecules: role of hydrogen bonds weak bonding allows molecules to slide with respect to on another C l H x x x x C l H broken bond PVC - polyvinylchloride

The End