Chapter 2 Mastering Biology

Which statement helps to explain why ice is less dense than liquid water?

--Water molecules make hydrogen bonds at definite angles. --Cold molecules move less than warm molecules. The ice lattice has open spaces because of the angles at which hydrogen bonds form. Heat energy can break water molecules free of the lattice so they move into the openings.

Polar attractions are ...

--forces between atoms with partial charges. --weaker than covalent bonds. --important because they are numerous.

Which statement is true of water's tensile strength?

-It results from hydrogen bonding. -It helps to pull water through plants. -It involves both cohesion and adhesion.

Which factor is important in making it possible to cool yourself by sweating?

-Molecules collide with varied angles and speeds. -Hydrogen bonds are relatively weak. Random collisions allow some molecules to accumulate more energy than other molecules. The weakness of hydrogen bonds lets those molecules escape, leaving the cooler molecules behind.

A hydrogen atom has one electron. How many covalent bonds can hydrogen form?

1

An electrically neutral molecule has the formula C3H4O2N. If the carbon atoms form the usual number of bonds, how many covalent bonds will each hydrogen atom have with other atoms in the molecule?

1

Fluorine's atomic number is 9 and its atomic mass is 19. How many neutrons does fluorine have?

10

The innermost electron shell of an atom can hold up to _____ electrons.

2

To fill the valence shell, an electrically neutral, unbonded atom with atomic number 8 must add ...

2 electrons. The neutral atom has 8 electrons. Two electrons fill the first shell, and 6 go into the second (valence) shell. Two more electrons would fill the valence shell.

When full, the innermost electron shell of argon contains ________ electrons, and the outermost shell contains ________ electrons.

2; 8

An atom's atomic number is 7. Its valence is most likely ...

3 The neutral atom has 7 electrons. Two electrons fill the first shell, and 5 go into the second (valence) shell. The atom needs 3 more electrons to fill the valence shell.

Silicon is an element that is found directly underneath carbon in the periodic table and thus has similar properties to carbon, including electron orbitals. How many electrons are in the valence shell of a silicon atom?

4

This atom can form up to _____ single covalent bond(s). (There are four electrons in the outermost electron shell.)

4

In molecules, C, H, O, and N atoms usually make __, __, __, and __ bonds respectively.

4, 1, 2, 3

An uncharged atom of boron has an atomic number of 5 and an atomic mass of 11. How many electrons does boron have?

5

An uncharged atom of boron has an atomic number of 5 and an atomic mass of 11. How many protons does boron have?

5

How many atoms in the pictured molecule can form hydrogen bonds with water molecules?

5

What is the atomic number of an atom that has 6 protons, 6 neutrons, and 6 electrons?

6

An element has 8 protons, 9 neutrons, and 8 electrons. Its atomic number and its mass number, respectively, are _____.

8 and 17 The number of protons, 8, gives the atomic number. The mass number is the sum of the number of protons and neutrons, or 8 plus 9, which equals 17.

A nutrition facts label from a fortified cereal states that in a single serving of cereal there is 8% of the percent daily value of the element phosphorous. The label also says that a single serving of cereal plus one-half cup of skim milk has 20% of the percent daily value of phosphorus. Which of the following statements is false?

A half-cup of skim milk contains twice as much phosphorus as a serving of cereal. True: --Eating five servings of cereal and two and a half cups of skim milk will give you 100% of the phosphorus that you need for one day. --The amount of phosphorus in six servings of cereal is equal to the amount of phosphorus in two cups of skim milk. --A half-cup of skim milk contains 12% of the percent daily value of phosphorus.

Which of the following helps most to explain why water has a high specific heat?

A water molecule can make 4 hydrogen bonds. When you heat water, much of the heat is used to break hydrogen bonds. Only the remaining heat can increase molecular motion, raising the temperature.

A buffer consists of undissociated acid (HA) and the ion made by dissociating the acid (A-). How does this system buffer a solution against decreases in pH?

A- reacts with H+ to become HA. A- binds most of the added H+, so not much H+ is left to lower the pH.

Which of the following statements regarding matter is false?

All matter exists in the form of compounds. True: -All matter is composed of elements. -All life is composed of matter. -All matter has mass.

Two C atoms form a double bond. Each C is bound to two H atoms. Which statement is true?

All the atoms lie in a plane. The double bond limits all the atoms to a plane.

Which answer helps to explain why all living cells need pH buffers?

Amino acid side chains have many carboxyl and amino groups. When these side chains ionize, they exert forces that affect the protein's folding. Thus, if pH isn't stabilized, the proteins will change their folding and may not function properly.

An acid is a compound that donates _____ to a solution.

H+

If you want a molecule that is highly polar, look for one that contains: C-S H-N O-N C-H S-H

H-N N and H differ in electronegativity more than any other bonded pair of atoms in the list, making the N-H bond most polar.

Which of these bonds is weakest?

Hydrogen bonds Hydrogen Bonds are weaker than covalent bonds because they do not involvesharing of electrons, and they are weaker than ionic bonds because they involvethe attraction of partial (not full) opposite charges.

How does the way a buffer stabilizes pH during addition of acid differ from the way the same buffer stabilizes pH during addition of base?

It's the same reaction running backward or forward. The buffer accepts H+ in one case; it gives off H+ in the other case.

The ammonia molecule in the diagram has the observed bond orientation because ...

N has four pairs of electrons in the valence shell. N has 7 protons in its nucleus. electrons repel one another. Since N has 7 protons, it must fill the second shell, giving it 4 pairs of electrons. The electrons form 3 bonds and 1 lone pair of electrons. Each pair of electrons repels the other pairs, so they are equally far apart.

Which answer correctly ranks the atoms in terms of decreasing electronegativity (the highest electronegativity first)? (N, O, H, C)

O, N, C, H

Use your knowledge of electronegativity to pick out the most polar bond in the list below. C-H O-H N-H O-N O-C

O-H Yes! O and H are at opposite ends of the electronegativity scale, so the polarity is very large.

The higher the pH number, the higher the concentration of ______ in a solution.

OH-

Which of these figures correctly illustrates the nature of the bonding of H2O?

Oxygen is more electronegative than hydrogen.

Which statement must be mentioned in explaining why amphipathic molecules line up at a water surface?

Polar groups attract one another. Polar water molecules pull together, holding the polar parts of the amphipathic molecules and squeezing out the nonpolar parts. That leaves the nonpolar parts waving in the air.

Why doesn't oil mix with water?

Polar molecules attract one another. Water molecules cling to one another and won't part to make room for uncharged (nonpolar) molecules. There's no repulsion.

By making two covalent bonds, an O atom (with 8 protons) fills its valence shell. Why does the atom's charge stay close to zero?

Shared electrons aren't always near oxygen. Oxygen keeps 6 electrons to itself. The 4 shared electrons migrate between O and the other atoms, thus contributing 4 half-charges to oxygen. This gives oxygen about 8 units of negative charge, balancing the atom's 8 protons.

Why isn't this insect drowning?

Surface tension.

Which atom in the pictured molecule will have the strongest partial positive charge?

The C that's in C=O. Good for you! This C atom is bound to the two most electronegative atoms in the molecule (O), both of which are pulling electrons from C.

What happens to an atom if the electrons in the outer shell are altered?

The atom acquires different properties.

The body uses atoms in different ways to accomplish different tasks. For example, one portion of the body's calcium supply strengthens bones, whereas another portion combines with proteins to stimulate blood clotting after tissue injury. Which of the following statements provides the most logical chemical explanation of calcium's ability to perform such different functions?

The bone contains calcium salts, which are less reactive than the calcium ions found in the blood.

Which of these relationships is true of an uncharged atom?

The number of protons is equal to the number of electrons.

Which answer helps to explain why carbon atoms tend to make 4 covalent bonds?

The valence shell needs 8 electrons. Without help, C can only hold 4 electrons in the valence shell. To fill the shell, 4 more electrons must be shared, making 4 covalent bonds.

Which of these refers to atoms with the same atomic number but different atomic masses?

These atoms are isotopes.

Which statement is true of pH buffers?

They consist of weak acids and weak bases. By using weak acids and bases, the buffer can absorb or give off H+, stabilizing the pH.

Dr. Haxton says the O-O bond is polar and the C-C bond is nonpolar. A good student would say ...

Wrong again, Ralph. Both bonds are nonpolar. Yes! When two atoms of the same kind form a covalent bond, they share electrons equally because their electronegativity is the same.

Partial charges occur when ...

a covalent bond links atoms of two kinds, and atoms share electrons unequally. Each kind of atom has a different attraction for electrons. Thus, atoms of different kinds share electrons unequally. That gives one atom a partial negative charge; the other atom a partial positive charge.

Dissolving is best described as ...

a mingling of molecules and/or ions. When sugar dissolves in water, the sugar molecules remain intact but mingle with water molecules. When salt (NaCl) dissolves, its Na+ and Cl- ions separately mingle with water.

Below is the structure of leucine, an amino acid. What type of bond is the arrow pointing to?

a polar covalent bond

Buffers work best when ...

about half of the buffer molecules are dissociated. That arrangement results in the least change in pH when either acid or base is added.

The term for a solution with a low pH number, such as lemon juice or vinegar, is ______.

acidic

An ionic bond involves _____.

an attraction between ions of opposite charge

The open spaces in water's crystal structure make it possible for ...

aquatic life to exist at the North Pole. The openings in the lattice make ice less dense than liquid water, so it floats. That causes water to freeze from the top down, so there's always liquid in which organisms can live.

Atoms with the same number of protons but with different electrical charges _____.

are different ions

Trace elements _____.

are required in very small amounts Elements such as iodine, iron, and copper are needed in amounts less than 0.01%. Despite the small amounts, these elements are required for vital functions. Deficiency in iodine, for example, can produce a goiter, a large swelling in the neck.

The term for a solution with a high pH number, such as ammonia or bleach, is _______.

basic

A substance that accepts H+ when they are in excess and donates H+ when their concentration drops is called a ______.

buffer

How do isotopes of an element differ?

by atomic mass number Atomic forms of an element with the same number of protons but different numbers of neutrons are isotopes.

Hydrogen bonds ...

can form between H and N. They can also form between H and other highly electronegative atoms, such as O, if the atoms have strong partial charges.

6 CO2 + 6 H2O → C6H12O6 + 6 O2 The equation shows a ______ --the breaking and forming of chemical bonds that leads to a change in the composition of matter.

chemical reaction

What type of bond is joining the two hydrogen atoms?

covalent

A radioactive isotope is an isotope that

decays.

A(n) _____ bond joins these two oxygen atoms. (The oxygen atoms are sharing two pairs of electrons.)

double covalent

A phrase that applies to covalent bonding and not other kinds of bonds is ...

electron-sharing

The tendency of an atom to pull electrons toward itself is referred to as its _____.

electronegativity

An atom and an ion of the same element differ in the number of _____.

electrons Electrons are negatively charged particles. In a neutral atom, the number of electrons is equal to the number of protons. When an atom loses electrons, its charge becomes positive, and when it gains electrons, its charge becomes negative.

In a double covalent bond, a carbon atom shares ...

electrons in two orbitals.

A substance that cannot be broken down into other substances by ordinary chemical procedures is known as a(n) _____.

element Compounds can be broken down chemically into two or more elements that are found in a fixed ratio.

6 CO2 + 6 H2O → C6H12O6 + 6 O2 The number of oxygen atoms on the left side of the equation is ______ the number of oxygen atoms on the right side.

equal to

The number of protons in an uncharged atom _____.

equals the total number of electrons in the orbital shells

What process exerts the pull on water molecules that is relayed from leaf to root via cohesion?

evaporation The evaporation of water from leaves moves water up from the roots via cohesion.

Covalent bonds hold atoms together because they ...

fill shells without giving atoms much charge, and they bring electrons closer to protons. Electron-sharing brings electrons closer to nuclei and it fills shells without giving the atoms much charge. Both factors stabilize the bond.

Each water molecule is joined to _____ other water molecules by ____ bonds.

four ... hydrogen

To make a buffer, you need to ...

have a weak acid or a weak base half ionized in water. When half of the molecules are ionized, there are plenty of molecules that can either absorb H+ or give off H+ when the pH is disturbed from outside.

What name is given to the bond between water molecules?

hydrogen

Water is a source of ______________ for chemical reactions in cells.

hydrogen atoms and oxygen atoms but not energy Many reactions incorporate O and H from water into biological molecules. This happens when you digest starch and protein, for example.

Water has surface tension because ...

hydrogen bonds between surface water molecules resist being stretched.

Water's surface tension and heat storage capacity are accounted for by its

hydrogen bonds.

In a neutral solution the concentration of _____.

hydrogen ions is equal to the concentration of hydroxide ions

In salt, what is the nature of the bond between sodium and chlorine?

ionic

Medicines are often administered in pill form. In many cases, the active ingredient of the pill (the drug) is joined to another substance by ________. This forms a(n) ________, which is stable in the dry environment of a pill bottle but dissociates under the wet conditions of the digestive system to release the drug to the body.

ionic bonds; salt

When the proton number and electron number are unequal, the atom or molecule _____.

is an ion When the proton number and electron number are unequal, the atom or molecule has a net positive or negative charge.

Though you add heat, the temperature of boiling water remains constant because ...

it takes energy to break hydrogen bonds. At boiling, all the added heat is used to break hydrogen bonds. Free of the water mass, the departing steam carries away all the added energy, with none left over to raise the temperature.

A(n) _____ refers to two or more atoms held together by covalent bonds.

molecule

The term for a solution that has an equal concentration of H+ and OH- is ______.

neutral

Which four elements are the primary components of important biological molecules, such as proteins, carbohydrates, and lipids?

nitrogen, carbon, hydrogen, and oxygen

6 CO2 + 6 H2O → C6H12O6 + 6 O2 In O2, the type of bond that holds the two oxygen atoms together is a ______.

nonpolar covalent bond

The unequal sharing of electrons within a water molecule makes the water molecule _____.

polar The electrons spend more time with the oxygen of the water molecule than with the hydrogens of water. Thus, the oxygen has a net negative charge and the hydrogens have a net positive charge.

In this molecule, what type of bond is found between the oxygen and hydrogens?

polar covalent While oxygen shares electrons with hydrogen, the electrons spend more time with the oxygen than with the hydrogens.

6 CO2 + 6 H2O → C6H12O6 + 6 O2 In H2O, the type of bond that holds one of the hydrogen atoms to the oxygen atom is a ______.

polar covalent bond

The atoms in the molecule of water shown below are held together by which type of bond?

polar covalent bond The hydrogen and oxygen atoms do not equally share their electrons, which results in the formation of electronegative oxygen atoms and electropositive hydrogen atoms. These are then able to interact with other water molecules to form hydrogen bonds.

6 CO2 + 6 H2O → C6H12O6 + 6 O2 In the equation, C6H12O6 is a ______.

product

6 CO2 + 6 H2O → C6H12O6 + 6 O2 In the equation, CO2 is a ______.

reactant

Without making or breaking bonds, the pictured molecule can change its shape because ...

rotation can occur around single bonds. Rotation allows groups of atoms to change position while preserving the usual bond angles around each atom.

What type of bond joins the carbon atom to each of the hydrogen atoms?

single (nonpolar) covalent

Which of the following substances is a compound? hydrogen gas liquid nitrogen sodium sugar

sugar

What determines the types of chemical reactions that an atom participates in?

the number of electrons in the outermost electron shell

A polar covalent bond results from _____.

the unequal sharing of electrons

Phosphorus-32 (radioactive) has _____ than phosphorus-35 (normal).

three fewer neutrons The numbers 32 and 35 refer to mass number (number of protons plus neutrons). All the atoms of the same element have the same number of protons, but different isotopes of the same element have different numbers of neutrons. Because 35 - 32 = 3, P-32 has three fewer neutrons than P-35.

A single water molecule (H − O − H) is held together by

two polar covalent bonds.