Atoms, Molecules, and Properties of Water – Lecture 2.1–2.2

These flashcards review atomic structure, chemical bonding, molecular geometry, and the unique properties of water—including its role as a solvent, its thermal characteristics, and its involvement in acid–base chemistry. Use them to test your understanding of key terms, definitions, and conceptual relationships from Sections 2.1 and 2.2.

Which four elements account for roughly 96 % of the atoms in living organisms?

Hydrogen, carbon, nitrogen, and oxygen.

What subatomic particles are found in an atom’s nucleus?

Protons and neutrons.

What charge do protons carry?

+1 (positive).

What charge do electrons carry?

–1 (negative).

Why is a neutral atom electrically balanced?

It contains an equal number of protons and electrons, so the charges cancel.

What unit is used to express the mass of protons and neutrons?

The dalton (Da).

How is an element’s mass number calculated?

Number of protons plus number of neutrons.

What does an element’s atomic number tell you?

The number of protons in its nucleus.

What are isotopes?

Atoms of the same element with different numbers of neutrons (different mass numbers).

Give the neutron count for carbon-12, carbon-13, and carbon-14.

Carbon-12: 6 neutrons; carbon-13: 7 neutrons; carbon-14: 8 neutrons.

What are radioactive isotopes?

Unstable isotopes that decay over time, emitting radiation.

What regions of space do electrons occupy around the nucleus?

Orbitals.

How many electrons can one orbital hold?

Up to two electrons.

What is an electron shell?

A group of orbitals at the same energy level around the nucleus.

Where are valence electrons found?

In the atom’s outermost (valence) shell.

Define the valence of an atom.

The number of unpaired valence electrons it possesses (and therefore the number of covalent bonds it can form).

When are atoms most stable?

When their valence shells are full.

How can atoms fill their valence shells?

By forming chemical bonds—covalent or ionic.

What is a covalent bond?

A chemical bond formed when two atoms share pairs of valence electrons.

What is a molecule?

A group of atoms connected by covalent bonds.

Define electronegativity.

An atom’s ability to attract shared electrons in a covalent bond.

Describe the general trend of electronegativity on the periodic table.

It increases moving up and to the right.

List the relative electronegativities of O, N, S/C/H/P in decreasing order.

O > N > S, C, H, P.

What is a nonpolar covalent bond?

A covalent bond in which electrons are shared equally between the atoms.

What is a polar covalent bond?

A covalent bond in which electrons are shared unequally, creating partial charges on the atoms.

Explain how partial charges arise in a water molecule.

Electrons are drawn toward the more electronegative oxygen, giving O a partial negative (δ–) and each H a partial positive (δ+) charge.

What happens in an ionic bond?

One atom completely transfers an electron to another, creating oppositely charged ions that attract each other.

Differentiate cations and anions.

Cation: positively charged ion (lost an electron). Anion: negatively charged ion (gained an electron).

Arrange these bond types from equal to no electron sharing: ionic, polar covalent, nonpolar covalent.

Nonpolar covalent → Polar covalent → Ionic.

How many covalent bonds can oxygen, nitrogen, and carbon typically form based on their unpaired electrons?

Oxygen: 2, Nitrogen: 3, Carbon: 4.

Give an example of a molecule with single bonds only.

Methane (CH4).

Give an example of a molecule containing double bonds.

Carbon dioxide (CO2).

Which common diatomic molecule exhibits a triple bond?

Molecular nitrogen (N2).

Describe the molecular geometry of methane (CH4).

Tetrahedral.

Why is water (H2O) bent instead of linear?

Repulsion from two pairs of unshared electrons on oxygen bends the molecule.

What percentage of a typical cell is water?

Approximately 75 %.

Define solvent, solute, and solution.

Solvent: the dissolving agent; Solute: the substance dissolved; Solution: a homogeneous mixture of solute in solvent.

Why is water called an excellent solvent?

Its polarity and ability to form hydrogen bonds allow it to dissolve many charged or polar substances.

What types of molecules are hydrophilic?

Ions and polar molecules that interact with water’s partial charges.

What does hydrophobic mean?

Describing non-polar, uncharged molecules that do not dissolve in water.

Name the weak interactions that stabilize clustered hydrophobic molecules.

Hydrophobic interactions and van der Waals forces.

Define cohesion and adhesion in water.

Cohesion: attraction between water molecules; Adhesion: attraction between water molecules and other polar surfaces.

What property of water creates surface tension?

Cohesion among water molecules at the surface via hydrogen bonding.

Why does ice float on liquid water?

Hydrogen bonds in ice form an open lattice that makes ice less dense than liquid water.

What is specific heat?

The energy required to raise the temperature of 1 g of a substance by 1 °C.

Why does water have a high specific heat?

Many hydrogen bonds must be broken before water molecules can move faster, absorbing much energy.

Define heat of vaporization.

Energy needed to convert 1 g of a liquid into gas.

How does water’s high heat of vaporization make sweating effective?

Evaporation removes large amounts of heat from the body, cooling it.

What ions result from the dissociation of water?

Hydrogen ion (H+) and hydroxide ion (OH–).

How do acids affect hydronium ion concentration?

They donate protons, increasing [H3O+].

How do bases affect hydronium ion concentration?

They accept protons, decreasing [H3O+].

Define mole in chemistry.

6.022 × 10^23 particles of a substance.

What is molarity (M)?

Number of moles of solute per liter of solution.

At 25 °C pure water has what proton concentration and pH?

[H+] = 1 × 10⁻⁷ M and pH = 7 (neutral).

How much does proton concentration change with a one-unit change in pH?

By a factor of 10.

What is the primary function of biological buffers?

To minimize changes in pH and help maintain homeostasis.