Chapter 2 Review

What is an element?

A substance that cannot be broken down into another substance by chemical reactions.

What is a compound?

A substance with 2+ different elements in a fixed ratio.

Which 4 elements make up ~96% of living matter?

Carbon (C), Hydrogen (H), Oxygen (O), Nitrogen (N).

What are some elements that make up the remaining 4% of living matter?

Phosphorus (P), Sulfur (S), Calcium (Ca), Potassium (K), Sodium (Na), Chlorine (Cl).

What are trace elements?

Elements required in very small amounts (e.g., Fe, I, Mg, Zn).

What are the 3 main subatomic particles?

Protons (+), Neutrons (0), Electrons (–).

What is the atomic number?

The number of protons in an atom.

What is the mass number?

The total number of protons + neutrons.

What are isotopes?

Different forms of the same element with varying numbers of neutrons.

What is a radioactive isotope?

An isotope whose nucleus decays spontaneously, releasing energy/particles.

What is half-life?

The time it takes for 50% of a parent isotope to decay.

Where are electrons found?

In orbitals within electron shells around the nucleus.

What are valence electrons?

Electrons in the outermost shell that determine chemical behavior.

Why are noble gases unreactive?

They have full valence shells.

How does potential energy of an electron relate to distance from nucleus?

Further from nucleus = higher potential energy.

What are orbitals?

3D regions where electrons are found 90% of the time.

How do periodic table trends move?

• Electron affinity: ↑ up and right.

• Ionization energy: ↑ right.

• Atomic radius: ↑ down and left.

Why do atoms form chemical bonds?

To complete their valence shells and become stable.

What is a covalent bond?

Sharing of valence electrons between atoms.

What is the difference between a single and double covalent bond?

Single shares 1 pair of electrons; double shares 2 pairs.

What is electronegativity?

An atom’s attraction for electrons in a covalent bond.

What is a nonpolar covalent bond?

Equal sharing of electrons.

What is a polar covalent bond?

Unequal sharing of electrons → partial charges.

What is an ionic bond?

Attraction between oppositely charged ions after electron transfer.

What is a cation? An anion?

Cation = positively charged ion; Anion = negatively charged ion.

What is a hydrogen bond?

Weak attraction between hydrogen (δ+) and an electronegative atom (δ–).

What are Van der Waals interactions?

Weak attractions from fluctuating charges between nearby molecules.

What determines a molecule’s shape?

The arrangement of orbitals around bonded atoms.

Why is molecular shape important in biology?

It determines how molecules recognize and bind with specificity (e.g., enzyme-substrate).

What happens in a chemical reaction?

Bonds are made and broken, changing matter’s composition.

Is matter created or destroyed in a chemical reaction?

No, it is conserved; atoms are rearranged.

What is chemical equilibrium?

When forward and reverse reactions occur at the same rate.