Presentation on theme: "Bonding and Structure. Octet Rule Atoms and ions prefer 8 electrons in the valence shell. Hence, the unreactivity of the inert / noble gases. If atoms."— Presentation transcript:
Bonding and Structure
Octet Rule Atoms and ions prefer 8 electrons in the valence shell. Hence, the unreactivity of the inert / noble gases. If atoms cannot achieve a full octet, they will settle for a filled or half-filled subshell. Atoms can achieve an octet by gaining or losing electrons (ionic bonding) or sharing electrons (covalent bonding)
Ionic Bonding When metals lose electrons, they acquire a positive charge. When nonmetals gain electrons, they acquire a negative charge. Likely charges can be inferred from the periodic table group numbers. Atoms lose or gain electrons to achieve same electron configuration as nearest noble gas. Discussion based on NIST periodic table.
Ions Positive ions are smaller than original neutral atom. Negative ions are bigger than original neutral atom. Positively charged ions are primarily from groups IA, IIA, IIIA. Maximum charge is the group number. Transition metals (B-groups) have multiple possibilities for charges. Due to proximity of d-electrons to valence shell. Negatively charged ions are primarily from groups VA, VIA, VIIA. Their negative charges are given by -8 + group number.
Ionic Compounds Bonding is electrostatic and strong Characterized by ionic crystal lattice structures. Packing arrangement determined geometrically by ion size. High melting point Total positive charge must balance total negative charge. Formulae for ionic compounds represent ratios of atoms in lattice, not independent molecules.
Nomenclature – ionic compounds Positive ions with more than one charge state have charge included in the name as a Roman numeral. Otherwise, the names of atoms with only one charge state omit the charge in the name. Negative ions have only one charge state. Necessary to memorize charge states for common elements. Use periodic table for clues in case you forget.
Polyatomic ions Some ions are covalently bonded units but with net charge. The deviation from charge neutrality provides octets for all atoms in the unit. Most of these ions carry negative charge. They also form crystals in association with positive ions. Overall charge balance between positive and negative ions must be present. Nomenclature – must memorize formulae and names.
Covalent bonding Electrons are shared between atoms to achieve octets. This forms structured units with integrity. This is where Lewis dot structures are useful. Simplest – diatomic molecules – hydrogen, nitrogen, oxygen, halogens
Example Draw electron dot structures for the common diatomic molecules – hydrogen, nitrogen, chlorine.
Covalent Bonding Most covalent bonds are between non- metals and between metalloids and non- metals. Nomenclature rules – different from ionic compounds. Structure of name signifies type of bonding. Actual molecules are formed.
Example Electron dot structures for methane, water, ammonia, carbon dioxide, hydrogen sulfide, formaldehyde, methanol, hydrogen cyanide, ozone, sulfur dioxide, sulfate ion, carbonate ion, ammonium ion, nitrate ion.
Covalent Bonding Sharing is not necessarily equal in a bond. Electrons shifted to atom with greater electronegativity. We get a dipole – a bond with a positive end and a negative end. When electronegativities are close, sharing is nearly equal and bond becomes closer to being non polar.
Molecules and Geometry Covalent bonding schemes have geometrical motifs – fundamental bonding structures underlying what you see. Molecular function depends on molecular structure both as individual molecules and in aggregate. Common structural motifs (not exclusive list): tetrahedral, flat triangle, linear. Note that some geometries become truncated because of lone pairs occupying positions. Tetrahedral – has single bonds (sp 3 ) Flat triangle – often has double bond (sp 2 ) Linear – often has triple bond (but not always) (sp).
example Tetrahedral motif: methane, carbon tetrachloride, ammonia, water
example Flat triangle: ozone, sulfur dioxide, nitrate ion, formaldehyde.
example Linear: hydrogen cyanide, acetylene
Bond vs. molecular polarity Although bonds may be polar, the molecule itself may not be. Dipole moments are vector quantities that can be resolved by trigonometry. If the vector components cancel, then overall molecule is nonpolar. Look for a high degree of symmetry and balance as quick way to spot nonpolar molecules.
example Sketch carbon tetrachloride, chloroform, methylene chloride, methyl chloride, methane to show polarity of bonds and of the molecule.
Intermolecular forces (among covalent molecules) Dispersion – based on fleeting and instantaneous shifts in electron distribution. Common in nonpolar and weakly polar molecules. Weak. Polar interactions – among molecules with permanent polarity. Intermediate. Hydrogen bonds – between hydrogen attached to N,O,F in one molecule and N,O,F in another molecule. Important biologically. Makes water a liquid at room temperature while hydrogen sulfide is a gas.
example Identify intermolecular forces in methane, ammonia, methanol, nitrogen trichloride.