Atoms
What defines the atomic number of an element?
The atomic number is the number of protons in the nucleus of an atom. It uniquely defines a chemical element (e.g., all carbon atoms have 6 protons, so their atomic number is 6).
Draw Mg2+
What would you have to change to make it Magnesium 2+
To make a neutral magnesium (Mg) atom into a Mg²⁺ ion, you would have to remove two electrons from it.
Is carbon dioxide (CO₂) a polar or nonpolar molecule?
Nonpolar. Although the C=O bonds are polar, the molecule is linear (O=C=O). The two bond dipoles are equal in magnitude but point in opposite directions, so they cancel each other out completely.
What is a hydrogen bond?
A hydrogen bond is a strong type of intermolecular force (an attraction between molecules), not a covalent bond within a molecule. It is a special dipole-dipole attraction between a hydrogen atom bonded to a highly electronegative atom (N, O, F) and a lone pair on another electronegative atom.
What are the three main types of strong chemical bonds?
Ionic bonds, covalent bonds, and metallic bonds.
What is the difference between the atomic number and the mass number?
The atomic number is the number of protons (which defines the element). The mass number is the total number of protons and neutrons in a specific atom's nucleus.
Draw Magnesium oxide
• What is the ratio of magnesium:oxygen? (Hint: how many electrons does
Mg want to get rid of? How many electrons does Oxygen want to gain?)
• Is this a covalent or ionic compound?
• Is it polar?
MgO is an ionic compound with a 1:1 ratio of magnesium to oxygen ions. It is not classified as polar.
Is water (H₂O) a polar or nonpolar molecule? Explain.
Polar Bonds: The O-H bonds are polar (O is more electronegative than H).
Shape: The water molecule has a bent shape. The bond dipoles do not cancel; they combine to create a net molecular dipole moment with a partial negative charge on the oxygen and a partial positive charge on the hydrogens.
What three atoms must be involved for a hydrogen bond to occur?
Hydrogen must be covalently bonded to either Nitrogen (N), Oxygen (O), or Fluorine (F). This hydrogen then attracts a lone pair on a N, O, or F atom in a different molecule (or sometimes a different part of the same molecule).
What type of bond involves the transfer of electrons from one atom to another?
Ionic bonding
An atom has 12 protons. What element is it?
Magnesium (Mg). The atomic number (number of protons) defines the element.
Draw Carbon
• How many bonds does carbon want to form? Why?
Carbon wants to form 4 bonds.
Is boron trifluoride (BF₃) a polar or nonpolar molecule?
Nonpolar. The B-F bonds are polar, but the molecule has a symmetrical trigonal planar shape. The bond dipoles are symmetrically arranged and cancel each other out.
Is a hydrogen bond an actual chemical bond (like ionic or covalent)?
No. It is a very strong intermolecular force, but it is about 10-20 times weaker than a covalent bond. It can be broken and reformed relatively easily.
What type of bond involves the sharing of electrons between atoms?
Covalent bonding
An atom of Nitrogen has a mass number of 15. How many neutrons does it have?
8 neutrons
Draw Fluorine
• Do you think Fluorine would have a high or low electronegativity? Why?
Fluorine has the highest possible electronegativity of all the elements.
Is formaldehyde (CH₂O) a polar or nonpolar molecule? Explain. (Hint: The central atom is Carbon, and it has a trigonal planar shape)
Polar.
Polar Bonds: The C=O bond is highly polar, and the C-H bonds are slightly polar.
Shape: The molecule is trigonal planar, but the bonds are not all identical. The strong dipole of the C=O bond is not canceled by the much weaker dipoles of the C-H bonds. This results in a significant net dipole moment.
Which of the following can form hydrogen bonds with other molecules of the same type?
a) CH₄ (Methane)
b) NH₃ (Ammonia)
c) HCl (Hydrogen chloride)
d) CH₃OH (Methanol)
e) CH₃OCH₃ (Dimethyl ether)
b) NH₃
d) CH₃OH
Explanation
a) CH₄: No. H is bonded to C, which is not electronegative enough (N, O, F).
b) NH₃: Yes. H is bonded to N.
c) HCl: No. H is bonded to Cl. Cl is electronegative, but it is not N, O, or F.
d) CH₃OH: Yes. The H bonded to the O (the -OH group) can form hydrogen bonds.
e) CH₃OCH₃: No. There are no H atoms bonded to N, O, or F. The H atoms are all bonded to C.
Describe the difference between a polar covalent bond and a nonpolar covalent bond.
Nonpolar Covalent: Electrons are shared equally between two atoms. This usually occurs when the atoms are identical (e.g., O₂, H₂) or have very similar electronegativities.
Polar Covalent: Electrons are shared unequally. One atom has a stronger pull on the electrons (higher electronegativity), creating partial positive (δ+) and partial negative (δ-) charges on the atoms.
What is the role of electrons in chemical bonding?
Chemical bonds are formed by the interaction of the valence electrons (the electrons in the outermost shell) of different atoms. Atoms bond to achieve a more stable electron configuration, often by sharing, gaining, or losing electrons.
Draw Carbon fluoride
• What is the ratio of carbon:fluorine?
• Is this covalent or ionic?
• Is it polar?
CF₄ is a covalent molecule with a 1:4 ratio of carbon to fluorine. It has polar bonds, but due to its symmetrical tetrahedral shape, it is a nonpolar molecule overall.
Compare ozone (O₃) and oxygen (O₂). Which is polar and why?
O₂ (Oxygen gas): Nonpolar. It is a diatomic molecule with two identical atoms, so the bond is purely nonpolar covalent. There is no dipole moment.
O₃ (Ozone): Polar. The molecule has a bent shape. The central atom is different from the terminal atoms (it is bonded to two atoms, while they are bonded to one), creating an unequal distribution of electrons and a net dipole moment.
hich of the following pure substances can exhibit hydrogen bonding with other molecules of the same kind? Select all that apply.
a) H₂S (Hydrogen sulfide)
b) CH₃NH₂ (Methylamine)
c) HI (Hydrogen iodide)
d) CH₃COOH (Acetic acid)
e) C₂H₆ (Ethane)
b) CH₃NH₂
d) CH₃COOH
Explanation
a) H₂S (Hydrogen sulfide): Cannot form hydrogen bonds.
Why? The hydrogen atoms are bonded to Sulfur (S). While S is electronegative, it is not N, O, or F. The intermolecular forces here are dipole-dipole forces.
b) CH₃NH₂ (Methylamine): Can form hydrogen bonds.
Why? It contains an N-H bond. The H atoms attached to the nitrogen are excellent hydrogen bond donors. The nitrogen atom also has a lone pair, making it an excellent hydrogen bond acceptor.
c) HI (Hydrogen iodide): Cannot form hydrogen bonds.
Why? The hydrogen atom is bonded to Iodine (I). I is electronegative but is not N, O, or F. This molecule exhibits dipole-dipole forces (and significant London dispersion forces due to its size).
d) CH₃COOH (Acetic acid): Can form hydrogen bonds.
Why? It contains an O-H bond (in the -COOH group). This H is a strong hydrogen bond donor. The oxygen atoms in the carbonyl (C=O) and hydroxyl (-OH) groups have lone pairs, making them strong hydrogen bond acceptors. Acetic acid even forms dimers through hydrogen bonding.
e) C₂H₆ (Ethane): Cannot form hydrogen bonds.
Why? All hydrogen atoms are bonded to Carbon (C). Carbon is not electronegative enough to create a sufficiently strong partial positive charge on the hydrogen. The only intermolecular forces in ethane are weak London dispersion forces.
What is electronegativity and how does it determine bond type?
Electronegativity is an atom's ability to attract electrons in a chemical bond.
Small difference (0.0 - 0.4): Nonpolar Covalent
Intermediate difference (0.5 - 1.7): Polar Covalent
Large difference (>1.7): Ionic