AP Biology 2.1–2.2: Atoms, Bonds, Water & pH

94 focused flashcards covering atoms, isotopes, bonding, water's properties, acids, bases, pH, and buffers.

What is matter?

Anything that occupies space and has mass.

What is an element?

A unique form of matter that cannot be broken down by ordinary chemical reactions.

Which four elements make up most living matter?

Carbon, hydrogen, oxygen, and nitrogen (CHON).

What is an atom?

The smallest unit of matter that retains an element's chemical properties.

What are the charges and locations of protons, neutrons, and electrons?

Protons are +1 in the nucleus; neutrons are 0 in the nucleus; electrons are -1 in orbitals around the nucleus.

What is true of protons and electrons in a neutral atom?

The numbers are equal, so their charges cancel.

What is atomic number?

The number of protons; it identifies the element.

What is mass number?

The total number of protons plus neutrons.

How do you calculate the number of neutrons?

Mass number minus atomic number.

What is atomic mass or atomic weight?

The weighted average mass of an element's naturally occurring isotopes, so it is often a decimal.

What are isotopes?

Forms of the same element with the same number of protons but different numbers of neutrons.

What is a radioisotope?

An unstable isotope that releases radiation as it becomes more stable.

What is radioactive decay?

The release of radiation and energy from an unstable atomic nucleus.

What is half-life?

The time required for half of a radioactive isotope to decay.

What is the half-life of carbon-14?

About 5,730 years.

How does carbon-14 dating work?

After death, carbon-14 intake stops; scientists compare remaining carbon-14 with carbon-12 to estimate time since death.

About how old can remains be for accurate carbon-14 dating?

Not much older than about 50,000 years.

How is the periodic table organized?

By atomic number and shared chemical and physical properties.

What determines most of an atom's chemical reactivity?

Its electrons, especially its valence electrons.

What are valence electrons?

Electrons in the outermost electron shell.

How many electrons can the first and second shells hold?

The first holds 2; the second holds 8.

What is the octet rule?

Atoms are generally most stable with 8 valence electrons; the first shell is stable with 2.

How do atoms with incomplete valence shells become stable?

They gain, lose, or share electrons.

What are typical valence-electron patterns for Groups 1, 17, and 18?

Group 1 has 1; Group 17 has 7; Group 18 has full shells and is generally unreactive.

Why can carbon form many covalent bonds?

It has 4 valence electrons and can form four bonds to complete its outer shell.

What is an orbital?

A region around the nucleus where an electron is most likely to be found.

What are the capacities of s and p subshells?

An s subshell has 1 orbital and holds 2 electrons; a p subshell has 3 orbitals and holds 6.

What is a molecule?

Two or more atoms chemically bonded together.

What is a compound?

A substance containing atoms of more than one element.

Why is O2 a molecule but not a compound?

Its atoms are bonded, but they are all the same element.

What is a chemical reaction?

A process in which chemical bonds form or break.

Where are reactants and products written in an equation?

Reactants are on the left; products are on the right.

Why must chemical equations be balanced?

The same number of each atom must appear on both sides because matter is conserved.

What is the difference between reversible and irreversible reactions?

Reversible reactions proceed in both directions; irreversible reactions mainly proceed in one direction.

What is chemical equilibrium?

A dynamic state in which forward and reverse reactions occur at equal rates, so relative amounts remain stable.

What is an ion?

An atom or molecule with a net charge because it gained or lost electrons.

How does a cation form?

It loses electrons and becomes positively charged.

How does an anion form?

It gains electrons and becomes negatively charged.

What is an ionic bond?

The attraction between oppositely charged ions after electrons are transferred.

How does NaCl form?

Sodium loses an electron to become Na+; chlorine gains it to become Cl-; the opposite charges attract.

What are electrolytes, and why are they important?

Physiologically important ions such as Na+, K+, and Ca2+ used in nerve impulses, muscle contraction, and water balance.

What is a covalent bond?

A bond formed when atoms share electrons.

How many electron pairs are shared in single, double, and triple bonds?

One, two, and three pairs, respectively.

How do single, double, and triple covalent bonds compare in strength?

Generally, triple bonds are stronger than double bonds, which are stronger than single bonds.

Why is atmospheric nitrogen difficult for organisms to use?

The two nitrogen atoms in N2 are held by a very strong triple bond.

What is electronegativity?

An atom's tendency to attract shared electrons toward itself.

What is a polar covalent bond?

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

What is a nonpolar covalent bond?

A covalent bond in which electrons are shared equally or nearly equally.

Why are O-H bonds polar?

Oxygen is more electronegative and pulls shared electrons closer, becoming δ- while hydrogen becomes δ+.

Why is H2O polar but CO2 nonpolar?

Water is bent, so charges do not cancel; CO2 is linear, so its bond dipoles cancel.

What is a hydrogen bond?

A weak attraction between a partially positive hydrogen and a partially negative electronegative atom, often O or N.

Why are hydrogen bonds biologically important?

Together they create water's properties and help stabilize protein structure and the DNA double helix.

What are van der Waals interactions?

Weak attractions caused by temporary fluctuations in electron distribution between nearby molecules.

Which are stronger: covalent bonds or hydrogen bonds?

Covalent bonds are generally much stronger.

What two features explain most of water's special properties?

Water is polar, and water molecules form hydrogen bonds.

What does hydrophilic mean?

Water-loving; describes ions and polar substances that interact with or dissolve in water.

What does hydrophobic mean?

Water-fearing; describes nonpolar substances such as oils that do not interact well with water.

What happens to hydrogen bonds in liquid water?

They constantly form and break.

Why is ice less dense than liquid water?

Hydrogen bonds hold water molecules in an organized crystal structure that spaces them farther apart.

Why is floating ice important to aquatic life?

It forms an insulating surface layer, allowing liquid water and organisms to remain below.

What is specific heat?

The heat required to raise 1 gram of a substance by 1°C.

Why does water have a high specific heat?

Hydrogen bonds absorb energy before molecular motion and temperature rise much.

Why is water's high specific heat biologically useful?

It stabilizes body and environmental temperatures and resists rapid temperature changes.

What is heat of vaporization?

The energy required to convert 1 gram of liquid to gas.

Why does water have a high heat of vaporization?

A large amount of energy is needed to break its hydrogen bonds.

How does evaporative cooling work?

The highest-energy molecules escape as vapor and carry heat away; this is why sweating cools the body.

Why is water an excellent solvent?

Its polarity lets it surround and separate ions and interact with polar molecules.

What is a hydration shell?

A layer of water molecules surrounding a dissolved ion or polar particle.

How do water molecules orient around Na+ and Cl-?

Oxygen's δ- side faces Na+; hydrogen's δ+ sides face Cl-.

What is dissociation?

The separation of a molecule or ionic compound into ions, such as NaCl becoming Na+ and Cl-.

What is cohesion?

Attraction between molecules of the same substance; in water, water is attracted to water.

What is adhesion?

Attraction between molecules of different substances; water sticks to another surface.

What is surface tension?

Resistance of a liquid surface to being broken or stretched, caused in water by cohesion and hydrogen bonding.

What is capillary action?

Movement of water through narrow spaces due to adhesion to surfaces and cohesion between water molecules.

How do cohesion and adhesion help plants?

They keep a continuous water column attached to plant tissues as transpiration pulls water and minerals from roots to leaves.

How can water dissociate?

H2O ⇌ H+ + OH-; H+ may associate with water to form hydronium.

What is pH?

A measure related to hydrogen-ion concentration: pH = -log10[H+].

Why is pure water pH 7?

Its H+ concentration is about 1 × 10^-7 mol/L, and -log(10^-7) = 7.

What is a mole?

6.02 × 10^23 particles of a substance.

What do acidic, neutral, and basic pH values mean?

Acidic is below 7, neutral is 7, and basic or alkaline is above 7.

How does H+ concentration relate to pH?

More H+ means lower pH and greater acidity; less H+ means higher pH and greater basicity.

Why is the pH scale logarithmic?

Each one-unit pH change represents a tenfold change in H+ concentration.

What is an acid?

A substance that increases H+ concentration, often by donating H+.

What is a base?

A substance that lowers H+ concentration by accepting H+ or providing OH- that combines with H+.

What is the difference between strong and weak acids or bases?

Strong acids or bases dissociate extensively; weak ones dissociate only partially.

What are typical pH values for cells, blood, and the stomach?

Cells are about 6.8, blood about 7.4, and the stomach about 1-2.

Why must biological pH remain within a narrow range?

Proteins, enzymes, and biochemical reactions work properly only under particular pH conditions.

What is a buffer?

A substance or system that minimizes pH changes by absorbing excess H+ or OH-.

How does bicarbonate buffer excess H+ in blood?

HCO3- combines with H+ to form H2CO3, which can become CO2 and water; CO2 is exhaled.

How can carbonic acid buffer excess OH-?

It reacts with OH-, helping prevent blood from becoming too basic.

How do antacids work?

They reduce the effects of excess stomach acid by interacting with hydrogen ions.

Why does limestone reduce the effects of acid rain?

Calcium carbonate provides carbonate, a weak base that buffers acids.

What chain explains water's life-supporting properties?

O is more electronegative → O-H bonds are polar → water is polar → hydrogen bonds form → water gains its special properties.

What is the key difference among ionic, covalent, and hydrogen bonds?

Ionic bonds involve electron transfer and ion attraction; covalent bonds share electrons; hydrogen bonds are weak attractions between partial charges.