WVU Exam 4 Practice Chem 115

1. What does it mean when a molecule has resonance structures?

Resonance structures occur when a molecule can be drawn multiple ways with the same arrangement of atoms, but different locations for electrons. If all structures are equivalent (like in SO₃), it means the bonding is delocalized.

2. Which molecules typically have resonance?

Molecules with multiple bonds and lone pairs (like SO₃, NO₂⁻, SO₄²⁻) often have resonance. Molecules with all atoms forming single bonds usually don't (like SF₄).

3. What is the formula that relates wavelength and frequency?

λ=c/f

4. How do you calculate the energy of a photon?

E=hf

5. How is electromagnetic radiation ordered by increasing energy?

A: Radio < Microwave < Infrared < Visible < Ultraviolet < X-rays < Gamma rays

6. What is the Rydberg equation for hydrogen spectral lines?

1/λ​=RH​(1/n1^2​ - 1/n2^2​)

7. What kind of bond is shorter and stronger — single, double, or triple?

Triple bond is shortest and strongest; it has the highest bond order and smallest bond length.

8. How can you tell if a compound is ionic or covalent?

Ionic = metal + nonmetal (e.g., SrBr₂)

Covalent = nonmetal + nonmetal (e.g., SF₄)

9. What does it mean if two species are isoelectronic?

They have the same number of electrons. Example: K⁺ and Cl⁻ both have 18 electrons.

10. What is the formal charge formula?

Formal charge = Valence electrons − (lone pair electrons + ½ bonding electrons)

11. How do you calculate the de Broglie wavelength of a particle?

λ=h/mv​

12. How do you determine if a set of quantum numbers is valid?

n > 0

l = 0 to n−1

ml = −l to +l

ms = ±½

13. What are the four main quantum numbers and what do they represent?

n: Principal quantum number (energy level)

l: Angular momentum (orbital shape)

ml​: Magnetic quantum number (orientation)

ms​: Spin quantum number (±½)

14. What are Bohr's key postulates for the hydrogen atom?

Electrons orbit in fixed energy levels

Energy is quantized

Electrons absorb or emit photons when changing levels

15. What is the difference between paramagnetic and diamagnetic?

Paramagnetic = unpaired electrons → attracted to magnets

Diamagnetic = all electrons paired → weakly repelled

How do you calculate ΔH from bond dissociation energies?

ΔH=∑(bonds broken)−∑(bonds formed)

17. What is the correct electron configuration of Fe³⁺?

Start with Fe: [Ar] 4s² 3d⁶

Remove 4s first, then 3d → Fe³⁺ = [Ar] 3d⁵

18. How do you know if an atom has a high or low first ionization energy?

- Across a period: ionization energy increases

- Down a group: ionization energy decreases

- Example: Rb has the smallest first ionization energy of listed options.

19. What determines electron affinity, and which elements have high values?

- Electron affinity = energy change when an atom gains an electron

- Nonmetals (like Br, Cl, O) have more negative (higher) affinities.

- left to right goes up

- top to bottom goes down

20. What does it mean when the energy of electrons is quantized?

Electrons can only exist in specific energy levels, not in between. This explains atomic emission spectra.