Unit 6: chemical bonds

Question 1

What is a chemical bond?

Answer 1

• Force that holds atoms together as a unit
• Bonds form if the energy of the unit is lower that that of the separate atoms (more efficient!)

Question 2

Optimum distance?

Answer 2

• Distance between atoms that involves a net potentiel energy that minimizes overall energy of the system.
• So, maximum net attractive forces with minimum energy
• They can’t be too close and repulse, but not to far and not attract!

Question 3

Covalent vs ionic bonds?

Answer 3

• Covalent: sharing of electrons
• Ionic: transferring of electrons

Question 4

Non polar vs polar covalent?

Answer 4

• Non polar are electronically symmetrical
• Polar have partial charges (they share unequally)

Question 5

Periodic trend for electronegativity?

Answer 5

• Across a row; increased electronegativity (with higher Zeff)
• Down a group: decreased electronegativity (because electrons are more shielded, thus less attractions)

Question 6

How to determine electronegativity for molecule?

Answer 6

• Compare the actual X-H bond energy with the expected H-X bond energy
• If EN of X = EN of H, then they share equally and it’s nonpolar covalent
• If EN of X > EN of H, then electrons are shared unequally and it’s a polar covalent molecule

Question 7

Which atom between lithuim and fluorine will attract more electrons?

Answer 7

Fluorine because higher Zeff (more on right)

Question 8

Types of bonds with electronegativity?

Answer 8

• Exprimed with differences in electronegativity values in bonding atoms
• < 0.5 - non polar covalent
• 0.5 - 1.9 = polar covalent
• > 1.9 - ionic

Question 9

Electrostatic potential maps?

Answer 9

• Used to visualize the extent of electron density
• Molecules can be represented as color-coded maps of electron density
• Blue = low electron densitu (low electronegativity)
• Red = high electron density (high electronegativity)

Question 10

Dipole moments?

Answer 10

• Charge distribution of a molecule with partial separation of charges
• Represented with center of positive charge and center of negative charge
• Dipole moment is shown with an arrow
• Arrow points towards negative charge center and tail the positive

Question 11

Example of net dipole?

Answer 11

• Water
• Ammonia
• Dipole moment wil align itself with an external electric field

Question 12

Do all molecules with polar bonds have a net dipole moment?

Answer 12

NO!

Question 13

Bond dissociation energy?

Answer 13

Energy needed to break the bond of a gaseous molecule (endothermic)

• Creating a molecule release energy
• Deconstructing a molecule requires energy
• Use average values

Question 14

Lewis structure?

Answer 14

Shows the arrangement of valence electrons amongst atoms in a molecule

• Build like a square around the atom
• Valence electrons represented as dots and bonds as lines

Question 15

Duet rule?

Answer 15

By sharing electrons, each atom can have a valence shell. For example, H2 molecule

Question 16

Octet rule?

Answer 16

• Elements form stable molecules when surrounded by 8 electrons
• Lone pairs: pairs of electrons not involved in bonding
• Bonding pairs: pairs of electrons shared between atoms

Question 17

Covalent bonds in lewis structure?

Answer 17

• Single covalent bond: 2 atoms share 1 pair of electrons
• Double covalent bond: 2 atoms share 2 pairs of electrons
• Triple covalent bond: 2 atoms share 3 pairs of electrons

Question 18

Steps for lewis structure?

Answer 18

1. Count valence electrons
2. Connect atoms
3. Assign electrons to terminal atoms
4. Assign electrons to the central atom
5. Multiple bonds (to fill with 8 electrons each atom)

Question 19

Expanded octet?

Answer 19

• Second row elements (C, N, O, F) always follow octet rule
• Elements in 3rd row and below can have expanded octet, because of empty d orbitals available in the valence sheel

Question 19

Tips for Lewis?

Answer 19

• Hydrogen atoms are always terminal atom (on extremum)
• Carbon atoms are always central atoms
• Central atoms are generally those with lowest electronegativity (can get more electrons)

Question 20

Electron deficient elements?

Answer 20

• B and Be often have incomplete octets!

Question 21

Resonance?

Answer 21

• Used to describe molecules in which no Lewis structure is consistent
• Hybrid model!
• Represented with a double-headed arrow
• Ex of resonance molecules: O3, [CO3]2-

Question 22

Formal charges?

Answer 22

• Compares the number of valence electrons in an isolated neutral atom with the number of valence electrons around this atom in a molecule
• Formal charge = group number - number of bonds - number of e- in lone pairs
  *Specific for each atom!