Chapter 9 Flashcards
what is the lewis theory?
a simple model of chemical bonding using diagrams that represent bonds between atoms as lines or pairs of dots. In this theory, atoms bond together to obtain stable octets
what is metallic bonding?
the type of bonding that occurs in metal crystals, in which metal atoms donate their electrons to an electron sea, delocalized over the entire crystal lattice
what is a chemical bond?
the sharing or transfer of electrons to attain stable electron configuration
what is the octet rule?
The tendency for most bonded atoms to possess or share eight electrons in their outer shel to obtain stable electron configurations and lower their potential energy
what is lattice energy?
the energy associated with forming a crystalline lattice from gaseous ions
what is the Born-Haber cycle?
A hypothetical series of steps based on Hess’s law that represents the formation of an ionic compound from its constituent elements
What are the steps in the Born-Haber cycle (example NaCl)?
- formation of gaseous sodium from solid sodium
- Formation of chlorine atoms from chlorine molecule
- ionization of sodium gas
- addition of electron to gaseous chlorine
- formation of crystalline solid from gaseous ions
Summarize the trends in lattice energies?
- lattice energies become less exothermic (less negative) with increasing ionic radii
- lattice energies become more exothermic (more negative) with increasing magnitude of ionic charge
what are bonding pairs?
a pair of electrons shared between two atoms
what are lone pairs (non-bonding electrons) ?
a pair of electrons associated with only one atom
what is a double bond?
the bond that forms when two pairs of electrons are shared between two atoms
what is a triple bond?
The bond that forms when three electron pairs are shared between two atoms
summarize the method for drawing lewis structures as follows
- Calculate the total number number of electrons for the Lewis structure by summing the valence electrons of each atom in the molecule
- write the correct skeletal structure for the molecule drawing a bond between each set of bonding atoms
- distribute the remaining unaccounted-for electrons, in pairs, among the atoms, giving octets (except for hydrogen) to as many atoms as possible
- If any non-hydrogen atoms lack an octet, form double, or triple bonds as necessary to give them octets
what does bond energy mean?
the energy required to break 1 mol of the bond in the gas phase
what is an exothermic reaction?
a reaction is exothermic when weak bonds break and strong bonds form ie) energy is released
what is an endothermic reaction?
a reaction is endothermic when strong bonds break and weak bonds form ie) energy is absorbed
what is bond length?
the average length of a bond between two particular atoms in a variety of compounds
bond lengths are what?
bond lengths are not static, the distance between two atoms is not the same all the time, the two atoms move towards and away from each other constantly by a spring
what is bond-stretching vibration?
the molecular motion of bonds stretching and contracting
for a bond vibration to take place, energy must be supplied to the molecule
what is Infrared Spectroscopy?
The absorption of Infrared radiation by a compound can be measured easily by irradiating the sample with an infrared source called a (glow bar) and measuring the intensity of radiation transmitted through the sample at each wavelength or frequency
what is a wavenumber?
the reciprocal of wavelength
what is a polar covalent bond?
A covalent bond between two atoms with significantly different electronegativities, resulting in an uneven distribution of electron density
what does electronegativity mean?
the ability of an atom to attract electrons to itself in a covalent bond
what are the periodic trends in electronegativity?
- electronegativity GENERALLY INCREASES across a period in the periodic table
- electronegativity GENERALLY DECREASES down a column in the periodic table
- Fluorine is the most electronegative element
- Francium is the least electronegative element (most electropositive)
The effect of electronegativity difference in bond type?
- EN = 0-0.4; covalent (non-polar); Cl2
- EN = 0.4-2.0; Polar covalent (slightly polar); HCl
- EN = 2.0+; Ionic (polar); NaCl
what is a dipole moment (u)?
a measure of the separation of positive and negative charge in a molecule
what is percent ionic character?
The ratio of a bond’s actual dipole moment, it would have if the electron were transferred completely from one atom to another and then multiplied by 100%
Percent Ionic Character = measured dipole moment of bond / dipole moment if electron were completely transferred * 100%
what are resonance structures?
two or more valid Lewis structures that are shown with double-headed arrows between them to indicate that the actual structure of the molecule is intermediate between them
what is a resonance hybrid?
The actual structure of a molecule that is intermediate between two or more resonance structures
what is formal charge?
The charge that an atom in a lewis structure would have if all the bonding electrons were shared equally between the bonded atoms
Formal charge = #of valence electrons - (#of bonding electrons + 0.5 #of bonding electrons)
the sum of formal charge must?
The sum of formal charge must equal the overall charge of the ion or molecule
what are the two guidelines to follow when drawing a lewis structure of a molecule?
- smaller formal charges on individual atoms are better that larger ones
- when formal charges cannot be avoided, negative formal charges reside on the most electronegative ion
what are free radicals?
a molecule or ion with an odd number of electrons in its lewis structure (not an octet, more than 8)
