MasteringBiology Chapter 2
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.
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 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.
A radioactive isotope is an isotope that ________
Decays
Oxygen is more ________ than hydrogen.
Electronegative
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
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
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.
Basic pH has higher or lower [H+]?
Lower Low pH has more [H+] (more basic)
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.
The term for a solution that has an equal concentration of H+ and OH- is
Neutral
The higher the pH number, the higher the concentration of __ in a solution
OH+
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.
Water is a versatile ________ .
Solvent
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.
True or False: atoms tend to avoid having a partly filled valence shell
True
A single water molecule (H − O − H) is held together by ..
Two polar covalent bonds
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 hydrogen atom has one electron. How many covalent bonds can hydrogen form?
1
When full, the innermost electron shell of argon contains ________ electrons, and the outermost shell contains ________ electrons.
2 & 8
An uncharged atom of boron has an atomic number of 5 and an atomic mass of 11. How many electrons does boron have?
5
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.
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 positive ion with two units of charge has 10 neutrons and 8 protons. The ion also has ________.
6 electrons
An element has 8 protons, 9 neutrons, and 8 electrons. Its atomic number and its mass number, respectively, are _____.
8 & 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.
Buffers work best when
About half of the buffer molecules are dissociated That arrangement gives the least change in pH when either acid or base is added
What is the main reason that the oceans are becoming more acidic?
Absorption by the oceans of carbon dioxide generated by burning fossil fuels
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.
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 ionic bond involves ________
An attraction between ions of opposite charge. This is the definition of an ionic bond
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. Ions are charged Atoms.
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.
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).
A substance that accepts H+ when they are in excess and donates H+ when their concentration drops is called a(n)
Buffer
Substance consisting of two or more different elements combined in a fixed ratio
Compound
What are consequences of ocean acidification?
Corals and coral reefs are not able to form their hard skeletons Clams, mussels, lobsters, and shrimp are not able to form strong enough shells
What type of bond is joining the two hydrogen atoms?
Covalent A covalent bond is one in which electron pairs are shared.
As pH goes up, [H+] goes ?
Down (basic)
An atom's reactivity is determined by its _____.
Electron configuration The fullness of the valence shell determines the pattern of covalent bonding or ionization.
The tendency of an atom to pull electrons toward itself is referred to as its _____.
Electronegativity
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.
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.
Thus, water absorbs or releases a great deal of ________ , helping to moderate temperatures.
Heat
Acidic pH has higher or lower [H+]?
Higher High pH has less [H+] (more acidic)
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.
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.
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.
When water warms or cools, ________ either break or form.
Hydrogen Bonds
Water's surface tension and heat storage capacity are accounted for by its _____.
Hydrogen bonds
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.
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.
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
Because ________ is less dense than liquid water, it floats and protects lakes and oceans from freezing solid.
Ice
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.
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
Which statement is true of the equation shown below? H₂O ⇌ H+ + OH−
It omits an important water molecule and it is a standard shorthand for water dissociation Chemists use this shorthand equation for simplicity and to emphasize H+, but in reality an H+ jumps from one water molecule to another, making hydronium and OH-.
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.
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.
To determine whether a base is weak or strong, ...
Look for undissociated molecules of base Strong bases dissociate completely in solution; weak bases don't.
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.
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.
The atoms in the molecule of water are held together by which type of bond?
Polar Covalent Bonds 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.
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.
The ________ of water molecules and the hydrogen bonds between water molecules explain most of water's life-supporting properties.
Polarity
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.
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)
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+]
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.
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.
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.
Blood and other biological fluids are aqueous solutions with a diversity of dissolved ________ .
Solutes
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.
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.
Generally, what determines the number and kinds of chemical bonds an element can make and the chemical reactivity of an element?
The number of electrons in its outer shell Only electrons are directly involved in the interactions between atoms, and the number of electrons in the outer shell determines the chemical behavior of the atom.
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 happens to an atom if the electrons in the outer shell are altered?
The properties of the atom change
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 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.
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
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.
As pH goes down, [H+] goes ?
Up (acidic)
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.
How much carbon dioxide from the atmosphere is absorbed by the oceans on a daily basis?
20 million metric tons
Silicon is an element that is found directly underneath carbon in the periodic table. How many electrons are in the valence shell of a silicon atom?
4 2 e- in first shell 8 e- in second shell
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
Ammonia reacts with water as shown below. Which statement best explains why ammonia is considered to be a base? H₃ + H₂O ⇌ NH₄+ + OH−
Bases are compounds that remove H+ from solutions In becoming NH₄+, ammonia takes on H+. Removing H+ from a solution is the hallmark of a base.
What is the chemical reaction responsible for the oceans becoming more acidic?
Carbon dioxide reacts with seawater, forming carbonic acid
The ________ of water molecules to each other helps transport water from the roots to the leaves in plants.
Cohesion
A compound is an acid if it ...
Donates H+ to water Acids are defined by the ability to donate H+ to water.
A phrase that applies to covalent bonding and not other kinds of bonds is ...
Electron-sharing
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 or gains electrons, its charge becomes positive or negative, respectively.
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.
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.
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.
Each water molecule is joined to _____ other water molecules by ____ bonds.
Four Hydrogen
An acid is a compound that donates __ to a solution
H+
Water Molecule partial charges and types of bonds
H+ O- H & O bonded by Polar Covalent Bond Water molecules bonded by Hydrogen Bonds
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.
A solution is very acidic if it
Has a very low pH value Has a high hydronium ion concentration Very acidic solutions have high hydronium concentrations, and as a result, have low pH values.
Nonpolar molecules are ________
Hydrophilic
Polar molecules are ________
Hydrophobic
In salt, what is the nature of the bond between sodium and chlorine?
Ionic Sodium and chlorine ions are attracted to each other.
The dissociation of water
Is quickly reversed, giving neutral water molecules Involves pairs of water molecules Water dissociates when H+ jumps from one water molecule to another. The H+ quickly jumps back.
________ 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.
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 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.
Which of the following can be considered strong bases?
NaOH This is the only base that dissociates completely in water.
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
In O₂, the type of bond that holds the two oxygen atoms together is a ________
Nonpolar 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.
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.
Compounds that release OH- are bases because ...
OH- combines with H+ and removes it from solution This behavior of OH- earns the title of "base" for compounds that release OH-.
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.
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.
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.
How are oyster larvae affected by ocean acidification?
Oyster larvae cannot grow shells in acidified ocean waters
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.
Because of ocean acidification, the price of which of the following foods is most likely to increase?
Shellfish
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
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.
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
The term for a solution with a low pH number, such as lemon juice or vinegar, is __
Acid
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.
The term for a solution with a higher pH number, such as ammonia or bleach is
Basic
The breaking and forming of chemical bonds that leads to a change in the composition of matter.
Chemical Reaction
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.
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.
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.
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.
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 ________
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.
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.
Which four elements are the primary components of important biological molecules, such as proteins, fats, and carbohydrates?
Nitrogen, Carbon, Hydrogen, & Oxygen
Below is the structure of leucine, an amino acid. What type of bond is the arrow pointing to? H-N
Polar Covalent Bond
In H₂O, the type of bond that holds one of the hydrogen atoms to the oxygen atom is a ________
Polar Covalent Bond
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.
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.
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 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.
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?
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.
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.
