Textbook Ch.7: Covalent Bonding Flashcards
What did Lewis suggest about nonmetal atoms?
Lewis suggested that nonmetal atoms, by sharing electrons to form an electron-pair bond, can acquire a stable noble-gas structure
Valence electrons
electrons in the outermost principal energy level
- Shown as dot symbols around the element
In Lewis structures of a molecule or polyatomic ion, valence electrons ordinarily ____
occur in pairs
Covalent bond
a pair of electrons shared between two atoms
- Shown by a straight line between bonded atoms in a Lewis structure
Unshared pair / Lone pair
a pair of electrons owned entirely by one atom
- Shown as a pair of dots on that atom in a Lewis structure
Single bonds
a single electron pair shared between two bonded atoms
- 1 line between two atoms on a Lewis structure
Double bonds
when bonded atoms share two electron pairs
- 2 lines between two atoms on a Lewis structure
Triple bonds
when bonded atoms share three electron pairs
- 3 lines between two atoms on a Lewis structure
Octet rule
atoms in covalently bonded species tend to have noble-gas electronic structures
- Nonmetals (except H) achieve a noble-gas structure by sharing an octet of electrons (eight)
- Hydrogen atoms, in molecules or polyatomic ions, are surrounded by a duet of electrons (two)
List the steps for writing Lewis Structures
- Count the number of valence electrons. (the last digit of the group number)
- Draw a skeleton structure for the species, joining atoms by single bonds
- The central atom is usually the one written first in the formula, put this in the center. Terminal atoms are often H, O, or a
halogen - Determine the number of valence electrons still available for distribution
- Determine the number of valence electrons required to fill out an octet for each atom (except H)
If after drawing the skeleton of a Lewis structure, the number of electrons available is equal to the number required, you…
distribute the available electrons as unshared pairs, satisfying the octet rule for each atom
If after drawing the skeleton of a Lewis structure, the number of electrons available is less than the number required, you…
must modify the skeletal structure by changing single bonds to multiple bonds
If after drawing the skeleton of a Lewis structure, the number of electrons available is greater than the number required you…
distribute leftover electrons around the central atom
Multiple bond formation is limited to which elements?
C, N, O, and S
Which atoms never form double bonds?
hydrogen and halogen atoms
Resonance forms
two structures separated by a double-headed arrow when the single Lewis structure does not adequately reflect the properties
of the substance
- Ex: in SO₂, you don’t know which oxygen the sulfur will be double-bonded to, it could be double-bonded to either
Resonance forms differ only in:
the distribution of electrons, not n the arrangement of atoms
Formal charge
a way to choose the more plausible of two resonance structures. It is the difference between the number of valence electrons in the free atom and the number of valence electrons assigned to the atom in the Lewis structure. The assigned electrons include
- All the unshared electrons owned by that atom
- One half of the bonding electrons shared by that atom
Equation of formal charge (Cբ):
Cբ = X-(Y + Z/2)
X= number of valence electrons in the free atom in the Lewis structure
Y=the number of bonding electrons shared by the atom in the Lewis structure
Z=the number of bonding electrons shared by the atom in the Lewis structure
Ordinarily, the more likely Lewis structure is the one in which:
- The formal charges are as close to zero as possible
- Any negative formal charge is located on the most strongly electronegative atom
What are some molecules that do not follow the octet rule?
- NO
- NO₂
- Species in which the central atom is surrounded by more than 4 pairs of valence electrons
Bond angles
the angles between bonds. Must be considered when dealing with molecules with three or more atoms
Valence-shell-electron-pair repulsion (VSEPR) model
a model in which the valence electron pairs surrounding an atom repel one another. Consequently the orbitals containing those electron pairs are oriented to be as far apart as possible
Electron-pair geometry
helps in determining the arrangement of various electron groups
Molecular geometry
helps in determining the entire atom and its configuration, it’s 3D
When describing the species type in molecular geometry, what does X represent?
number of terminal (bonded) atoms
When describing the species type in molecular geometry, what does E represent?
the number of unshared pairs
Polar
as a result of an asymmetrical distribution of electrons, the bond or molecule contains a positive and negative pole and is therefore a dipole
Nonpolar
a symmetrical distribution of electrons which leads to a bond or molecule with no positive or negative poles
Nonpolar bonds
formed whenever atoms joined are identical, H₂, F₂, etc
Polar bonds
bonds in which the electron density is unsymmetrical
The extent of polarity is related to the difference in ____
electronegativity
- If it is large, the bond is strongly polar
- If small, the bond is slightly polar
How do polar molecules align themselves in the presence of an electric field?
- Positive pole aligns with the negative charge
- Negative pole aligns with positive charge
How do nonpolar molecules behave in the presence of an electric field?
they show no preferred orientation
Dipole moment
a measure of the extent to which molecule tend to orient themselves in an electric field
- Polar molecules have dipole moment
- Nonpolar molecules have no dipole moment
When working with a diatomic molecule, how do you find the polarity?
Polarity of the molecule = polarity of the bond
If the molecule has more than two atoms, how do you find the polarity?
Use bond polarity AND molecular geometry (if bonds are symmetrically distributed around central atom or not)
If the polar A–X bonds in a molecule AXE are arranged symmetrically around the central atom, A, the molecule is nonpolar
Atomic model / valence bond model
a covalent bond consists of a pair of electrons of opposed spin within an orbital
Ex: an H atom forms a covalent bond by accepting electrons from another atom to complete it’s orbit
For an atom to form a covalent bond, it MUST have…
an unpaired electron
- The number of bonds formed by an atom should be determined by the number of unpaired electrons
- Noble-gas atoms DO NOT form covalent bon
Hybrid orbital
Used to modify simple valence bond theory. It involves mixing atomic orbitals to form new hybrid orbitals suitable for pairing electrons
What kind of electron pairs can be located in hybrid orbitals?
unshared as well as shared electron pairs
The number of hybrid orbitals formed is always equal to:
the number of atomic orbitals mixed
How are the geometries of hybrid orbitals predicted?
by the VSEPR theory
What happens to multiple bonds in hybrid orbitals?
The extra electron pairs in a multiple bond (one pair in a double bond, two pairs in a triple bond) are not located in hybrid
orbitals
Sigma (σ) bonding orbital
consists of a single lobe in which the electron density is concentrated in the region directly between two bonded atoms
Sigma (σ) bond
consists of an electron pair occupying a sigma bonding orbital
- All sigma bonds are single bonds
Pi (π) bonding orbitals
consist of two lobes one above the bonding axis, one below it. Along the bonding axis, the electron density is 0
Pi (π) bond
electron pair occupying the pi orbital
- One of the electron pairs in a multiple bond is a sigma bond, the others are pi bonds
