Atomic Structure and�Interatomic Bonding

Importance of considering bonding

Atomic structure

• atom – electrons – 9.11 x 10^-31 kg � protons � neutrons �atomic number = # of protons in nucleus of atom = # of electrons in neutral species
•  atomic mass unit = amu = 1/12 mass of ¹²C� A = Atomic wt = wt of 6.022 x 10²³ molecules or atoms: 1 amu/atom = 1 g/mol
• Some of the following properties are determined by an atom's electronic structure:
• Chemical
• Electrical
• Thermal
• Optical

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} 1.67 x 10^-27 kg

Quantum numbers

• Electrons have wave-like and particle-like characteristics.
• Two wave-like characteristics are
• Electron position in terms of probability density
• shape, size, orientation of probability density determined by quantum numbers� 
• Quantum # Designation/Values

n = principal (shell) K, L, M, N, O (1, 2, 3, 4, etc.)

l = azimuthal (subshell) s, p, d, f (0, 1, 2, 3,…, n-1)

m_l = magnetic (no. of orbitals) 1, 3, 5, 7 (-l to +l)

m_s = spin ½, -½

Electronic configuration

• Why not stable? Valence (outer) shell usually not � completely filled.

• Most elements: Electron configurations not stable.

• Valence electrons – those in outer unfilled shells
• Filled shells are more stable – require more energy to gain or lose electrons
• Valence electrons available for bonding and tend to determine an atom’s chemical properties: C (atomic number = 6) 1s² 2s² 2p²

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Periodic Table

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• Elements in each column: Similar valence electron structure

Electronegativity

• Ranges from 0.7 to 4.0

Large values: tendency to acquire electrons.

 Dissimilar electronegativities  

ex: MgO (Mg: 1s² 2s² 2p⁶ 3s²; O: 1s² 2s² 2p⁴)

Mg²⁺ 1s² 2s² 2p⁶; O^2- 1s² 2s² 2p⁶ -

BONDING FORCES AND ENERGIES

• Melting Temperature, T_m

The larger E_o, the higher T_m

Properties Related to Bonding: CTE

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• Coefficient of thermal expansion, α_l

• Increase in bond length is due to asymmetry of the E vs. r curve. This results in an increase in a_l.

• As E₀ increases this asymmetry decreases.

The smaller E_o, the larger α_l.

Ionic Bonding

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Ionic Bonding

Covalent Bonding

Silicate networks

Bond Hybridization

Bond Hybridization

Metallic Bonding

SECONDARY BONDING OR VAN DER WAALS BONDING

• In spite of the small energies associated with secondary bonds, they nevertheless are involved in a number of natural phenomena and many products that we use on a daily basis.
• Adhesives—van der Waals bonds form between two surfaces so that they adhere to one another
• Surfactants—compounds that lower the surface tension of a liquid and are found in soaps, detergents, and foaming agents
• Emulsifiers—substances that, when added to two immiscible materials (usually liquids), allow particles of one material to be suspended in another (common emulsions include sunscreens, salad dressings, milk, and mayonnaise)
• Desiccants—materials that form hydrogen bonds with water molecules (and remove moisture from closed containers—e.g., small packets that are often found in cartons of packaged goods)

Electric dipoles

• An electric dipole exists whenever there is some separation of positive and negative portions of an atom or molecule.
• The bonding results from the coulombic attraction between the positive end of one dipole and the negative region of an adjacent one.
• Dipole interactions occur between induced dipoles, between induced dipoles and polar molecules (which have permanent dipoles), and between polar molecules.
• A dipole may be created or induced in an atom or molecule that is normally electrically symmetric i.e. the overall spatial distribution of the electrons is symmetric with respect to the positively charged nucleus.

Induced dipoles

• All atoms are experiencing constant vibrational motion that can cause instantaneous and short-lived distortions in the electrical symmetry for some of the atoms or molecules, and the creation of small electric dipoles.
• One of these dipoles can in turn produce a displacement of the electron distribution of an adjacent molecule or atom, which induces the second one also to become a dipole that is then weakly attracted or bonded to the first.
• These attractive forces may exist between large numbers of atoms or molecules, which forces are temporary and fluctuate with time.

Polar molecule dipoles

• Permanent dipole moments exist in some molecules by virtue of an asymmetrical arrangement of positively and negatively charged regions; such molecules are termed polar molecules.
• Hydrogen Chloride molecule: A permanent dipole moment arises from net positive and negative charges that are respectively associated with the hydrogen and chlorine ends of the HCl molecule.
• Polar molecules can also induce dipoles in adjacent nonpolar molecules, and a bond will form as a result of attractive forces between the two molecules.
• The magnitude of this bond will be greater than for fluctuating induced dipoles.

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Permanent Dipole Bonds or Hydrogen Bonds

MIXED BONDING

• Mixed ionic-covalent: electronegativity difference
• Mixed metallic-covalent: Metalloids
• Mixed metallic-ionic: alloy vs. compounds