Periodic Trends & Reactivity
What happens to atomic size down a group?
Atomic size increases due to added energy levels.
Why are the elements in Group 18 usually unreactive?
They already have full valence shells, so they are stable and don’t need to gain or lose electrons.
Which has higher electronegativity: metals or nonmetals? What does this mean?
Nonmetals generally have higher electronegativity, meaning they more strongly attract electron in a bond.
What are reactants and products?
Reactants are starting substances; products are substances formed.
What does a Lewis Dot model show, and how is that helpful to scientists?
Valence electrons, showing bonding possibilities to help predict bonding.
Why does sodium react more strongly than lithium?
Sodium’s valence electron is farther from the nucleus and easier to remove (low ionization energy).
An element gains one electron when it forms a bond. Which group is it likely in?
Group 17, because halogens have seven valence electrons and tend to gain one to become stable.
Which bond is more polar: C–H or O–H, and why?
O–H is more polar because oxygen is much more electronegative than carbon and hydrogen, so it pulls the electrons away from hydrogen more strongly than carbon does.
Aluminum reacts with oxygen to form aluminum oxide. Identify the reactants, product, and bond type.
Reactants: aluminum and oxygen. Product: aluminum Oxide (Al2O3). Bond type: ionic.
What is one limitation of a Lewis Dot model?
It doesn’t show the 3-D shape of a molecule. It also doesn't show interactions at the bulk scale.
Fluorine vs bromine — which is more reactive and why? Use Bohr's models to explain your reasoning.
Fluorine is more reactive because it has a smaller radius and higher electronegativity.
(Bromine has a larger radius and more electron shielding.)
Two nonmetals share electrons unevenly. What bond forms and why?
A polar covalent bond forms because one atom has higher electronegativity and pulls electrons more strongly.
How can CO₂ have polar bonds but be nonpolar overall?
The molecule has symmetrical electron distribution due to its linear shape, so the bond dipoles cancel out.
Why does a chemical equation need to show the same atoms on both sides?
Conservation of matter, atoms are conserved and cannot be created or destroyed.
Two elements both have two valence electrons, but Element A is more stable than Element B and only Element B commonly forms ionic compounds. Use Lewis Dot Diagrams to predict what these elements could be and explain why they have these different behaviors.
Element A is likely helium because its two electrons are paired and fill its outer shell, making it stable. Element B is likely a Group 2 element with two unpaired valence electrons that can be lost to form ionic compounds.
Why does reactivity increase down Group 1 but decrease down Group 17?
Metals lose electrons more easily down a group, while nonmetals attract electrons less strongly as size increases.
An unknown element is in Group 16 and reacts with hydrogen. Predict how many bonds it will form with hydrogen and what type of bond it would be.
It will form two bonds because Group 16 elements have six valence electrons and need two more. The bond with hydrogen would be covalent because both are nonmetals.
A molecule contains several polar bonds but behaves as a nonpolar substance in experiments. Predict what this tells you about the molecule’s overall structure.
The molecule is likely symmetrical, so the polar bonds cancel out and there is no overall dipole.
A reaction produces bubbles. What evidence does this give about the product of the reaction?
The bubbles show a gas (likely a nonpolar molecule with covalent bonds) is forming.
Explain how a molecule can be thought of as a system made of interacting subparts.
Atoms interact through shared or transferred electrons hold the atoms together in bonds.
An element reacts quickly to form an ion but forms relatively weak ionic bonds in its compounds (as compared to other ionic solids). Predict how ionization energy and bonding explain this behavior.
The element has a low ionization energy, so it easily forms an ion. But if the ion has a low charge (+1) and is large, the attraction between ions is weaker (based on Coulomb's Law), leading to weaker ionic bonds.
An unknown metal forms a compound with the carbonate ion (CO₃²⁻) in a 2:3 ratio (metal : carbonate). Predict the charge of the metal ion and explain your reasoning.
Carbonate has a 2− charge, so three carbonate ions have a total charge of −6. Two metal ions must balance that with +6 total, meaning each metal ion has a 3+ charge.
Two students are discussing ammonia (NH₃): Student A says it is polar because the molecule is not symmetrical. Student B says it is nonpolar because all three N–H bonds have the same atoms. Who is correct, and how do you know? Use a model to support your thinking.
Student A is correct. Although all three N–H bonds are the same, nitrogen is the most electronegative atom with a lone pair in the center, so the molecular is not symmetrical with bond dipoles that do not cancel out.
A student says, “The overall mass decreased because gas escaped.” How would you help them revise this explanation?
The gas still has mass; it just left the system, so matter was not destroyed.
Why might two different models be needed to explain the same molecule?
One model might show electrons and bonding; while another shows shape, electron density, or polarity.