MasteringBiology Chapter 2

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

2 electrons is the maximum count for the ________

This is the maximum number of electrons that can occupy the ________

Which of these relationships is true of an uncharged atom?

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

In this situation the number of positive charges is equal to the number of negative charges.

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

The number of electrons in the outermost electron shell.

An atom is least likely to participate in a reaction when its outermost shell is stable.

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

This is an atom of carbon, an element with 6 protons.

Atomic number is the number of protons in an atom.

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

These atoms are isotopes.

Atoms with the same atomic number and different atomic masses are referred to as isotopes.

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

Atomic mass is equal to the number of protons plus the number of neutrons (19-9 = 10).

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

5

The atomic number is equal to the number of protons.

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

Electron-sharing

The ________ are on the left side of a chemical equation and the ________ are on the right side.

Reactants (left side chemical equation)

Products (right side chemical equation)

In O₂, the type of bond that holds the two oxygen atoms together is a ________

Nonpolar Covalent Bond

The breaking and forming of chemical bonds that leads to a change in the composition of matter.

Chemical Reaction

In H₂O, the type of bond that holds one of the hydrogen atoms to the oxygen atom is a ________

Polar Covalent Bond

Substance consisting of two or more different elements combined in a fixed ratio

Compound

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

Nitrogen, Carbon, Hydrogen, & Oxygen

Compared to covalent bonds, ionic bonds ...

Rely much less on electron-sharing

Covalent bonds are entirely based on sharing electrons, whereas ionic bonds are attractions between ions that keep their electrons.

A positive ion with two units of charge has 10 neutrons and 8 protons. The ion also has ________.

6 electrons

True or false: Ions may form by transferring an atomic nucleus (with or without electrons) to another molecule.

True

For example, a hydrogen nucleus (H+) may move between two water molecules, making an H₃O+ ion and an OH- ion.

True or False: atoms tend to avoid having a partly filled valence shell

True

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

Are Different Ions.

Ions are charged Atoms.

________ are atoms of the same element with different atomic masses, but the same number of protons.

Isotope

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

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

Ionic

Sodium and chlorine ions are attracted to each other.

An ionic bond involves ________

An attraction between ions of opposite charge.

This is the definition of an ionic bond

A radioactive isotope is an isotope that ________

Decays

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 a/an ________.

Ion

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

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 blood

What type of bond is joining the two hydrogen atoms?

Covalent

A covalent bond is one in which electron pairs are shared.

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

Molecule

A molecule is defined as two or more atoms held together by covalent bonds.

An atom with four electrons in the valence shell can form up to ________ single covalent bonds

Four

There are four electrons in the outermost electron shell

When chemists and biologists want to show how atoms are bonded in a molecule, they usually use a ________

Structural Formula

This kind of model is designed to show which atoms are bound to one another, but it doesn't accurately picture the sizes of atoms.

Covalent bonds hold atoms together because they ...

Fill shells without giving atoms much charge

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.

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

4, 1, 2, 3

H: 1
O: 2
N: 3
C: 4

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.

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.

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

Electrons in two orbitals

A double bond involves two orbitals, each with a shared pair of electrons.

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

N has four pairs of electrons in the valence shell

Electrons repel one another

N has 7 protons in its nucleus

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.

Without making or breaking bonds, a 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.

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

a. The bonds orient like tripods or pyramids
b. The bonds orient in a tetrahedral fashion
c. All the atoms lie in a plane
d. All the atoms lie in a line
e. The groups rotate around the C=C bond

All the atoms lie in a plane

The double bond limits all the atoms to a plane.

Partial charges occur when ...

Atoms share electrons unequally

Covalent bond links atoms of two kinds

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.

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.

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.

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

A knowledge of bonding capacities makes it easier to understand the structure of molecules.

A polar covalent bond _____.

Results in an unequal sharing of electrons

Polar covalent bonds share electrons unequally, so that one part of the molecule has a negative pole and one part has a positive pole.

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

Four

Hydrogen

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.

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

Electronegativity

For H₂O, what type of bond is found between O & H?

Polar Covalent

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

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

Which bond weaker? Hydrogen or Covalent?

Oxygen is more ________ than hydrogen.

Electronegative

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.

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.

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.

Hydrogen is the source of ____ and ____ for chemical reactions in the cells

Hydrogen

Oxygen

Many reactions incorporate O and H from water into biological molecules. This happens when you digest starch and protein, for example.

Which is true of water's tensile strength?

It results from hydrogen bonding

It helps pull water through plants

It involves both cohesion and adhesion

Because of hydrogen bonding, water coheres to itself and adheres to cell walls. That makes it possible to pull water through plants without breaking the water column.

Water has surface tension because...

Hydrogen bonds between surface water molecules resist being stretched.

The hydrogen bonds between surface water molecules are normally slightly stretched. Like a stretched sheet of rubber, the surface tends to contract and resists being penetrated.

Why does water have 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.

What factors are important in making it possible to cool yourself by sweating?

Molecules collide with varied angels and degrees

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.

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.

Why is ice 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.

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.

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.

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

Two polar covalent bonds

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

Hydrogen bonds

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

Element

This is the definition of an element

The number of protons in an uncharged atom _____.

Equals the total number of electrons in the orbital shells

The atom has an equal number of protons and electrons, which gives it a neutral charge overall.

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

3 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.

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

The properties of the atom change

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

H-N

Polar Covalent Bond

Which answer correctly ranks the atoms in terms of decreasing electronegativity (the highest electronegativity first)?

O, N, C, H

Remember this, and you'll be able to predict polarity when molecules have O or N linked to C or H.

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

When two atoms of the same kind form a covalent bond, they share electrons equally because their electronegativity is the same.

Use your knowledge of electronegativity to pick out the most polar bond in the list below.

a. C-H
b. O-N
c. O-H
d. N-H
e. O-C

O-H

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

If you want a molecule that is highly polar, look for one that contains:

a. C-H
b. O-N
c. H-N
d. S-H
e. C-S

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.

Polar Attractions

Forces between atoms with partial charges

Weaker than covalent bonds

Important because they are numerous

Polar attractions occur between atoms with partial charges and are weaker than covalent bonds. Still, they're important because so many of them occur in living cells.

Hydrogen Bonds

Can form between H & N

Do not occur in water molecules

Do not share electrons between two bonded atoms

Are not covalent bonds

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

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

NC₂O

5

O and N can make hydrogen bonds, as can each H attached to O or N.

The brackets are indicating a(n) _____ bond.

δ+ .... δ-

Hydrogen

A hydrogen atom with a net positive charge is attracted to an oxygen atom with a net negative charge.

What name is given to the bond between water molecules?

Hydrogen Bond

A hydrogen atom with a net positive charge is attracted to an oxygen atom with a net negative charge.

A solution contains only glycerol and fructose. There is ten times as much glycerol as fructose. Therefore, ...

Glycerol is the solvent

In any solution, the most abundant substance is the solvent.

A solution is aqueous if ...

Water is the solvent

That's the definition of "aqueous." Water is the solvent if the solution contains more water molecules than any other kind of molecule.

A 1 M solution contains ...

One mole of solute per liter of solution

To make the solution, you dissolve the solute in some solvent and then add more solvent to bring the volume up to a liter.

he water molecule has a bent shape because ...

Oxygen has two unbonded pairs of valence electrons

Repulsion between electrons governs the angles between bonds. The two pairs of unbonded electrons repel the two pairs of bonding electrons, creating the bend in the molecule.

Which statement is true of water?

Which statement is true of water?
(a) The O atom in water has a partial positive charge.
(b) The H atoms in water have partial positive charges.
(c) Its polarity results from hydrogen's high electronegativity.
(d) About 50% of the average cell's mass consists of water.
All of the above except for (a).

The H atoms in water have partial positive charges

H is less electronegative than O, so the shared electrons spend most of their time near O.

Dr. Haxton told his class that a water molecule can make 4 hydrogen bonds, all of them in the same plane as the three atoms. What would a good student reply?

Wrong! Two hydrogen bonds are not in the plane of the atoms.

The array of hydrogen bonds is three-dimensional.

What is part of the explanation for water's high cohesion?

Oxygen has four valence orbitals

Oxygen is much more electronegative than hydrogen

The electronegativity difference creates charges that make hydrogen bonds possible, and the four outer orbitals permit water to make four hydrogen bonds. The result is a three-dimensional network of bonds between water molecules.

Despite its cohesion, water can flow because ...

Hydrogen bonds break and re-form rapidly

There are always enough hydrogen bonds to hold the water together, but the bonds break and re-form rapidly, allowing molecules to move relative to one another.

Which answer helps to explain how many hydrogen bonds a water molecule can form?

Oxygen makes two covalent bonds

A water molecule has four orbitals

The angle between H-O bonds in water is 180∘

Water can ionize, making hydronium and hydroxide ions

The oxygen atom has six protons

A water molecule has four orbitals

Each of those orbitals can engage in a hydrogen bond

In a neutral solution the concentration of _____.

Hydrogen ions is equal to the concentration of hydroxide ions

This is the definition of a neutral solution

How does the pH of the solution on the right compare with that of the solution on the left?

The solution on the right is basic relative to the solution on the left

The solution on the right has a lower concentration of hydrogen ions relative to hydroxide ions than does the solution on the left.

Which answer is a true statement about pH values?

Pure water has a pH of 0

A high pH corresponds to a high H+ concentration

pH is important in life mainly because it affects the cohesion of water

Stomach juice has a high pH

None of the above

None of the above is true about pH values

What conditions must be met in order for an aqueous solution to be called "neutral"?

The concentrations of hydronium and hydroxide ions are equal.

In a neutral solution the concentrations of hydroxide and hydronium ions must be equal.

Factory smoke has led to acid rain that lowered the pH of a lake to 5.0. What is the H+ concentration in the lake?

10⁻⁵ M

The pH is the negative exponent of the H+ concentration.

In a washing machine, the detergent raised the pH to 9.0. The concentration of OH- in the solution is ...

10⁻⁵ M

If you divide 10⁻¹⁴ by 10⁻⁹ M (the H+ concentration at pH 9), you get 10⁻⁵ M, the OH- concentration.

Your tank of swamp fish needs a pH of 5, and the pH is 7 at present. What should you do to the H+ concentration?

Raise it to 100 times its present value.

Your tank of alkali fish needs a pH of 8, and the pH is 6 at present. What should you do to the H+ concentration?

Reduce it to 1/100 of its present value

Each pH unit is a 10-fold change in [H+]

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

Acid

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

Neutral

The term for a solution with a higher pH number, such as ammonia or bleach is

Basic

An acid is a compound that donates __ to a solution

H+

The higher the pH number, the higher the concentration of __ in a solution

OH+

A substance that accepts H+ when they are in excess and donates H+ when their concentration drops is called a(n)

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