Mastering Biology: Chapter 2

In the term trace element, the adjective trace means that
the element is required in very small amounts.
Compared with 31P, the radioactive isotope 32P has
one more neutron.
The reactivity of an atom arises from
the existence of unpaired electrons in the valence shell.
Which statement is true of all atoms that are anions?
The atom has more electrons than protons.
Which of the following statements correctly describes any chemical reaction that has reached equilibrium?
The rates of the forward and reverse reactions are equal.
We can represent atoms by listing the number of protons, neutrons, and electrons-for example, 2p +, 2n 0, 2e – for helium. Which of the following represents the 18O isotope of oxygen?
8p +, 10n 0, 8e –
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The atomic number of sulfur is 16. Sulfur combines with hydrogen by covalent bonding to form a compound, hydrogen sulfide. Based on the number of valence electrons in a sulfur atom, predict the molecular formula of the compound.
What coefficients must be placed in the following blanks so that all atoms are accounted for in the products?

C6H12O6 → _____ C2H6O + _____ CO2

2; 2
Which of the following bonds can form between atoms of equal electronegativity?
Van der Waals interactions can form between atoms of equal electronegativity.
What does the term electron orbital describe?
An electron orbital describes a three-dimensional space where an electron can be found 90% of the time.
Which statement about relative potential energy of electrons is correct?
An electron in the 3 p orbital of the third electron shell has more potential energy than an electron in the 2 p orbital of the second electron shell.
Which statement about weak bonds is correct?
Weak bonds are transient and easily reversible.
What happens when two atoms form a chemical bond?
A chemical bond forms when two atoms transfer or share outer electrons to complete their outer shells.
Undersea deposits that contain metals such as copper, gold, and silver are collectively known as what?
massive sulfides
The discovery of which of the following led to a drastic change in deep sea mining?
black smokers
You are an explorer in search of deep sea metal deposits. Which of the following would likely be the best place to look?
near volcanic fissures
Which of the following is true?
The ores found in seabeds often have substantially higher purities than those found on land.
Your mining company wishes to export its metals to the country that consumes the most copper and gold. Where are you looking to send your products?
Can you match these prefixes, suffixes, and word roots with their definitions?
with or together with: co-
not: non-
equal: ios-
many: poly-
four: quad- (or quat-)
against: anti-
two: di-
first: prim-
single: mono-
three: tri-
Can you match these prefixes, suffixes, and word roots with their definitions?
first: proto-
water: hydro-
of neither gender or type: neutr-
fat: lip-
to fear: -phob
loosening, split apart: -lysis
to: ad-
to love: -phil
from, out of, remove: de-
A substance that cannot be broken down into other substances by ordinary chemical procedures is a(n) _____.
Which of the following is a trace element required by most living organisms?
Given what you know about the human body, what do you think accounts for the high percentage of oxygen (65.0%)?
The human body is made up mainly of water, and there is one oxygen atom in every water molecule.
About twenty-five of the ninety-two natural elements are known to be essential to life. Which four of these twenty-five elements make up approximately 96 percent of living matter?
carbon, hydrogen, nitrogen, oxygen
Trace elements are those required by an organism in only minute quantities. Which of the following is a trace element that is required by humans and other vertebrates, but not by other organisms such as bacteria or plants?
Which of the following statements is FALSE?
All life requires the same essential elements.
Which of the following are compounds?
H2O and CH4, but not O2
What is the mass number of an ion with 109 electrons, 159 neutrons, and a +1 charge?
Match each property to the appropriate subatomic particle.
Proton: symbol P, +1 charge
Neutron: symbol N, 0 charge
Electron: symbol e, -1 charge
Sort the subatomic particles according to their masses.
mass of ~1 amu: neutron & proton
masss of ~1/2000: electron
What is the atomic number of an atom that has 6 protons, 6 neutrons, and 6 electrons?
Which of these refers to atoms with the same atomic number but different atomic masses?
These atoms are isotopes.
Fluorine’s atomic number is 9 and its atomic mass is 19. How many neutrons does fluorine have?
An uncharged atom of boron has an atomic number of 5 and an atomic mass of 11. How many protons does boron have?
The innermost electron shell of an atom can hold up to _____ electrons.
Which of these relationships is true of an uncharged atom?
The number of protons is equal to the number of electrons.
What determines the types of chemical reactions that an atom participates in?
the number of electrons in the outermost electron shell
Which of the following subatomic particles always has a positive charge?
Changing the number of _____ would change an atom into an atom of a different element.
protons in an atom
The atoms of different phosphorus isotopes _____.
have different numbers of neutrons
The type of bonding and the numbers of covalent bonds an atom can form with other atoms is determined by _____.
the number of unpaired electrons in the valence shell
The mass number of an atom is 15, and its atomic number is 7. The atom probably has…
8 neutrons in the nucleus.
Which statement is true of atoms?
Electrons determine the atom’s size.
Dr. Jones says an atom has 3 electrons in the first shell and four electrons in the second shell. Someone should tell Dr. Jones that …
the first shell shouldn’t have 3 electrons.
Which statement is true of the energy levels of electrons in shells?
The valence shell has higher energy than other occupied shells.
Which statement is true of the atom shown in the diagram?
The atom has more than one valence electron.

The atom is in the excited state.

An electron will move from the outer to the inner shell.

Two atoms always represent the same element if they have …
the same number of protons.
An atom has 6 electrons, 6 protons, and 6 neutrons. You can tell that this atom belongs to the element _____ because _____________________.
C; it has 6 protons.
An atom has 8 protons, 8 neutrons, and 8 electrons. Another isotope of the same element might have …
10 neutrons.
Radioactive decay is likely to occur when …
an atom has too many neutrons.
Which model most accurately represents the current view of the structure of the atom?
Probability model
Which statement is most useful in explaining why chemists assign atoms to chemical elements by counting protons?

The proton’s negative charge holds electrons in the atom.
99% of the atom’s mass consists of protons.
Protons at the atom’s surface determine the atom’s behavior.
The nucleus doesn’t change in stable isotopes.

None of these. Elements are defined by the number of protons.
Part A
Carbon-14 (14C) is a radioactive isotope of carbon that decays to 14N at a constant rate. 14C is present in the atmosphere in small amounts at a constant ratio to both 13C and 12C, two stable isotopes of carbon. When carbon is taken up from the atmosphere by a plant during photosynthesis, 12C, 13C, and 14C isotopes are incorporated into the plant in the same proportions in which they are present in the atmosphere. These proportions remain the same in the tissues of an animal that eats the plant.

While an organism is alive, the 14C in its body decays to 14N but is continually replaced by new carbon from the environment. Once an organism dies, it stops taking in new 14C, but the 14C in its tissues continues to decay, while the 12C in its tissues remains the same because it is not radioactive and does not decay. Thus, scientists can calculate how long the pool of original 14C has been decaying in a fossil by measuring the ratio of 14C to 12C and comparing it to the ratio of 14C to 12C present originally in the atmosphere. The fraction of 14C in a fossil compared to the original fraction of 14C can be converted to years because we know that the half-life of 14C is 5,730 years—in other words, half of the 14C in a fossil decays every 5,730 years.

The graph below shows a standard graph of radioactive isotope decay. The red curve indicates the fraction of the radioactive isotope remaining (on the y-axis) over time in units of half-lives (on the x-axis). Recall that a half-life is the amount of time it takes for half of the radioactive isotope to decay.

Time (half-lives): fraction of isotope remaining in fossil

1: 1/2
2: 1/4
3: 1/8
4: 1/16
5: 1/32
6: 1/64
7: 1/128
8: 1/256
9: 1/512
10: 1/1024

The graph below is the same as in Part A, but with the fractions converted to decimal values. For each decimal value, fill in the exponent to complete the scientific notation. Don’t forget to indicate if the exponent should be negative (-). The first one has been done for you.
Time (half-lives): fraction of isotope remaining in fossil

1: 10^-1
2: 10^-1
3: 10^-1
4: 10^-2
5: 10^-2
6: 10^-2
7: 10^-3
8: 10^-3
9: 10^-3
10: 10^-4

The graph you constructed so far applies to decay of any radioisotope. To calibrate it specifically to 14C, you need to calculate how many years must pass to reach the amount of remaining isotope for each point of the half-life curve.
To calibrate the x-axis for 14C decay, you have to convert half-lives to number of years. Recall that 14C has a single half-life of 5,730 years. How many years make up 5 half-lives for 14C?
28,650 years
The researchers found that the Neanderthal fossil had approximately 7.80 × 10-3 as much 14C as was present in the atmosphere around the living Neanderthal and the plants it ate.
Approximately how many half-lives have passed since the Neanderthal died?
7 half-lives
Using the 14C calibration on the x-axis, what is the approximate age of the Neanderthal fossil?
40,110 years old
Recall that the analyzed fossil was found in the most historically recent (uppermost) layer containing Neanderthal bones of the excavated archeological site.
Approximately when did Neanderthals become extinct at this location, according to the study?
40,110 years ago or less
The researchers cite evidence that modern humans (H. sapiens) became established in the same region as the last Neanderthals approximately 39,000-42,000 years ago.
What does this suggest about the possible overlap of Neanderthals and modern humans in the region?
Neanderthals and modern humans may have lived in the same region for up to approximately 2,000 years, or they may not have overlapped at all.
Carbon-14 dating works for fossils up to about 75,000 years old.
Most dinosaurs went extinct 65.5 million years ago. Can 14C be used to date dinosaur bones?
No, the 14C present in the dinosaur when it died would decay too much to be measured after 65.5 million years.
Radioactive uranium-235 has a half-life of 704 million years. If it was incorporated into dinosaur bones, could it be used to date the dinosaur fossils?
Yes, after 65.5 million years only about one tenth of the 235U would have decayed, leaving plenty to measure in the fossils.
Knowing the atomic mass of an element allows inferences about which of the following?
the number of protons plus neutrons in the element
In what way are elements in the same column of the periodic table the same? They have the same number of _____.
electrons in their valence shells when neutral
Molybdenum has an atomic number of 42. Several common isotopes exist, with mass numbers from 92-100. Therefore, which of the following can be true?
Molybdenum atoms can have between 50 and 58 neutrons.
Carbon-12 is the most common isotope of carbon and has a mass number of 12. However, the average atomic mass of carbon found on a periodic table is slightly more than 12 daltons. Why?
Some carbon atoms in nature have more neutrons.
Which of the following best describes the relationship between the atoms described below?

Atom 1:

Atom 2:

They are isotopes.
The atomic number of nitrogen is 7. Nitrogen-15 has a greater mass number than nitrogen-14 because the atomic nucleus of nitrogen-15 contains _____.
8 neutrons
From its atomic number of 15, it is possible to predict that the phosphorus atom has _____.
15 protons and 15 electrons
Fluorine has an atomic number of 9. Which of the following would you do to a neutral fluorine atom to complete its valence shell?
add 1 electron
Magnesium has an atomic number of 12. What is the most stable charge for a magnesium ion?
a +2 charge
Refer to the figure above (first three rows of the periodic table). What element has properties most similar to carbon?
How many neutrons are present in the nucleus of a phosphorus-32 (32P) atom (see the figure above)?
How many electrons will a single atom of sulfur with no charge and no bonds have in its valence shell (see the figure above)?
Based on electron configuration, which of the elements in the figure above would exhibit a chemical behavior most like that of oxygen?
Which one of the atoms shown would be most likely to form a cation with a charge of +1?
1st shell: two e-
2nd shell: one e-
Which one of the atoms shown would be most likely to form an anion with a charge of -1?
1st shell: two e-
2nd shell: seven e-
Oxygen has an atomic number of 8 and most commonly, a mass number of 16. Thus, what is the atomic mass of an oxygen atom?
approximately 16 daltons
If you change the number of neutrons in an atom, you create _____.
an isotope
Can the atomic mass of an element vary?
Yes. Adding or losing neutrons will change the atomic mass without forming a different element.
Which of the following is the best description of an atom’s physical structure?
Atoms are little bubbles of space with mass concentrated at the center of the bubble.
What type of bond is joining the two hydrogen atoms?
A(n) _____ refers to two or more atoms held together by covalent bonds.
This atom can form up to _____ single covalent bond(s).
A(n) _____ bond joins these two oxygen atoms.
double covalent
Which of these figures correctly illustrates the nature of the bonding of H2O?
O has 4 electrons bonded to 2 H atoms. Each atom bond is 2 electrons. O atom has negative charge, H atoms have a positive charge.
What type of bond joins the carbon atom to each of the hydrogen atoms?
single (nonpolar) covalent
Atoms with the same number of protons but with different electrical charges _____.
are different ions
In salt, what is the nature of the bond between sodium and chlorine?
What is the result of the animated process?
a positively charged sodium ion and a negatively charged chlorine ion
An ionic bond involves _____.
an attraction between ions of opposite charge
The brackets are indicating a(n) _____ bond.
What name is given to the bond between water molecules?
A carbon atom and a hydrogen atom form what type of bond in a molecule?
nonpolar covalent bond
An ionic bond is formed when _____.
one atom transfers an electron to another atom
Hydrogen bonding is most often seen _____.
when hydrogen is covalently bonded to an electronegative atom
A cation with two units of charge has 10 neutrons and 8 protons. The ion also has …
6 electrons.
Which statement is true of the molecule shown here?
It’s both a cation and an anion.
The atoms shown here will become ions if they meet because …
atoms tend to avoid having a partly filled valence shell.
True or false: Ions may form by transferring an atomic nucleus (with or without electrons) to another molecule.
When chemists and biologists want to show how atoms are bonded in a molecule, they usually use a …
structural formula.
A phrase that applies to covalent bonding and not other kinds of bonds is …
Covalent bonds hold atoms together because they …
(a) fill shells without giving atoms much charge.
(b) bring electrons closer to protons.
In molecules, C, H, O, and N atoms usually make __, __, __, and __ bonds respectively.
4, 1, 2, 3
An atom’s atomic number is 7. Its valence is most likely …
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.
In a double covalent bond, a carbon atom shares …
electrons in two orbitals.
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.

All of the above.
Without making or breaking bonds, the pictured molecule can change its shape because …
rotation can occur around single bonds.
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. (b) atoms share electrons unequally.
Partial charges occur when …
atoms share electrons unequally.
To fill the valence shell, an electrically neutral, unbonded atom with atomic number 8 must add …
2 electrons.
Which answer helps to explain why carbon atoms tend to make 4 covalent bonds?
The valence shell needs 8 electrons.
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?
Which answer correctly ranks the atoms in terms of decreasing electronegativity (the highest electronegativity first)?
O, N, C, H
Which atom in the pictured molecule will have the strongest partial positive charge?
The C that’s in C=O.
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.
Use your knowledge of electronegativity to pick out the most polar bond in the list below.
If you want a molecule that is highly polar, look for one that contains:
Compared to covalent bonds, ionic bonds …
rely much less on electron-sharing.
A salamander relies on hydrogen bonding to stick to various surfaces. Therefore, a salamander would have the greatest difficulty clinging to a _____.
surface of hydrocarbons
A covalent chemical bond is one in which _____.
outer-shell electrons of two atoms are shared so as to satisfactorily fill their respective orbitals
What is the maximum number of covalent bonds that an oxygen atom with atomic number 8 can make with hydrogen?
Nitrogen (N) is more electronegative than hydrogen (H). Which of the following is a correct statement about the atoms in ammonia (NH3)?
Each hydrogen atom has a partial positive charge; the nitrogen atom has a partial negative charge.
Bonds between two atoms that are equally electronegative are _____.
nonpolar covalent bonds
What results from an unequal sharing of electrons between atoms?
a polar covalent bond
A covalent bond is likely to be polar when _____.
one of the atoms sharing electrons is more electronegative than the other atom
What is the difference between covalent bonds and ionic bonds?
Covalent bonds involve the sharing of electrons between atoms; ionic bonds involve the electrical attraction between charged atoms.
The atomic number of chlorine is 17. The atomic number of magnesium is 12. What is the formula for magnesium chloride?
How many electron pairs are shared between carbon atoms in a molecule that has the formula C2H4?
Which bond or interaction would be difficult to disrupt when compounds are put into water?
covalent bonds between carbon atoms
Water molecules are attracted to one another by _____.
hydrogen bonds
Van der Waals interactions may result when _____.
electrons are not symmetrically distributed in a molecule
What is the maximum number of hydrogen atoms that can be covalently bonded in a molecule containing two carbon atoms?
What results from the chemical reaction illustrated above? The reactants have no charge.
a cation with a net charge of +1 and an anion with a net charge of -1
What is the atomic number of the cation formed in the reaction illustrated above?
What causes the shape of the molecule shown above?
the shape of the sp3 hybrid orbitals of the electrons shared between the carbon and hydrogen atoms
How many electrons are involved in a single covalent bond?
How many electrons are involved in a double covalent bond?
If an atom has a charge of +1, which of the following must be true?
It has one more proton than it does electrons.
Elements found on the left side of the periodic table contain outer shells that are _____; these elements tend to form _____ in solution.
almost empty; cations
An atom has four electrons in its valence shell. What types of covalent bonds is it capable of forming?
single, double, or triple
When are atoms most stable?
when all of the electron orbitals in the valence shell are filled
When the atoms involved in a covalent bond have the same electronegativity, what type of bond results?
a nonpolar covalent bond
Nitrogen (N) normally forms three covalent bonds with a valence of 5. However, ammonium has four covalent bonds, each to a different hydrogen (H) atom (H has a valence of 1). What do you predict to be the charge on ammonium?
You need to write down information about a molecule, but need to indicate only the type and number of atoms it contains. Which representation would work best?
molecular formula
You need to represent a molecule to best illustrate the relative sizes of the atoms involved and their interrelationships. Which representation would work best?
space-filling model
Chemical equilibrium is reached when _____.
the forward and reverse reactions occur at the same rate so that the concentrations of reactants and products remain the same
Which of the following is a chemical reaction?

Melting ice.
Making a hydrogen bond between a water molecule and a sugar molecule.
Changing a carbon atom to a nitrogen atom by radioactive decay.
Dissolving a lump of sugar in a cup of coffee.

None of the above.
What kind of chemical reaction is shown in this animation below?


None of the above.
Potential energy is always … Which fact is most important in explaining how molecules convert thermal energy to activation energy (EA)?
associated with position or arrangement.
Which of the following is true of activation energy?
Molecules usually get it from thermal energy.
Which fact is most important in explaining how molecules convert thermal energy to activation energy (EA)?
Distortion increases a molecule’s potential energy.
Which statement is true of the transition state?
(a) It can break apart to yield either reactants or products.
(b) It is extremely unstable and lasts only an instant.
Which statement about the following reaction is true? 4 Fe + 3 O2 → 2 Fe2O3
Four atoms of Fe react with 3 molecules of O2 to form 2 molecules of Fe2O3
Choose the appropriate balanced equation for the following chemical reaction. C6H6 + H2 → C6H12
C6H6 + 3 H2 → C6H12
Choose the correct balanced equation for the following reaction. C8H18 + O2 → CO2 + H2O
2 C8H18 + 25 O2 → 16 CO2 + 18 H2O
What is the molecular mass of caffeine (C8H10N4O2)?
How many grams of water (H2O) are needed to react with 5.0 grams of ferric trichloride (FeCl3)?
FeCl3 + 3 H2O → Fe(OH)3 + 3 HCl
1.7 g H2O
How many grams of glucose would you need to make 1 liter of an aqueous 0.5M glucose solution? (Atomic masses: C = 12.01, H = 1.008, O = 16.00.)
90.08 grams of glucose
Dr. Haxton says he has found a reaction that is truly irreversible. What would a good student say?
Sorry, Doctor. All reactions are reversible.
Which statement is true of the relation between life and chemical equilibrium?
(b) Living cells avoid chemical equilibrium by taking in energy and materials.
The reaction shown below is at equilibrium. What could you do to make this reaction produce more methyl benzoate?
(a) Add more benzoic acid.
(b) Remove some methanol (HOCH3).
In the equation ΔG = ΔH – TΔS, the change in the amount of disorder in the system is symbolized by …
A chemist calculates that a certain reaction would result in ΔH = -9 kJ and TΔS = +1 kJ. What can you conclude?
The reaction will occur spontaneously, because it decreases G.
For the reaction ATP + H2O → ADP + Pi, the change in standard free energy (ΔG∘) is -30.5 kJ/mol. From this information we can conclude that …
(a) the forward reaction will be spontaneous under standard conditions.
Under standard conditions the reversible reaction A + B → C + D has a ΔG∘ = -25 kJ/mol. What is ΔG when: T = 475 K, [A] = 0.5M, [B] = 0.2 M, [C] = 0.2 M, and [D] = 0.3 M?
-27 kJ/mol
Which statement is true of the relationship between life and chemical equilibrium?
(d) Living cells avoid chemical equilibrium by taking in energy and materials.
Catalysts speed reactions mainly by …
lowering EA.
Which of the following is true for this reaction?

3H2 + N2 <--> 2NH3

Ammonia is being formed and decomposed simultaneously.
Which of the following correctly describes all chemical equilibrium?
Forward and reverse reactions continue with no net effect on the concentrations of the reactants and products.
Which of the following correctly describes a reaction that has reached chemical equilibrium?
The rate of the forward reaction is equal to the rate of the reverse reaction.
Ionic bonds form as a result of __________.
attraction between ions that have opposite charges
Which of the following molecules has the shape of a completed tetrahedron?
methane (CH4)
Which of the following four statements, if any, is true regarding essential elements and living organisms?
Although all forms of life require iron, other elements are required only by certain species.
In which of the following will isotopes of an element always differ?
atomic mass
Which of the following statements is true about chemical reactions?
They involve the making and breaking of chemical bonds.
When the proton number and electron number are unequal, the atom or molecule __________.
is an ion
The chemical characteristics or reactivity of an element depend mostly on the __________.
number of electrons in its outermost shell
An atom that normally has __________ in its outer shell would NOT tend to form chemical bonds with other atoms.
eight electrons
A hydrogen bond __________.
is a weak chemical bond
Which of the following has negligible mass?
Pharmaceutical researchers are often interested in blocking particular receptor proteins on cell surfaces. What chemical property of a molecule would be most important for this type of application?
the molecule’s shape
Which of the following is a trace element?
A particular carbon isotope has an atomic number of 6 and an atomic mass of 14. The respective number of neutrons, protons, and electrons that this carbon isotope has is __________.
8, 6, and 6
Copper has an atomic number of 29 and a mass number of 64. What would result if an uncharged copper atom lost two electrons?
The atomic number would remain 29, the mass number would remain 64, and the atom would be a cation with a +2 charge.
Which of the following statements describes a reversible reaction that has reached chemical equilibrium?
The rate of the reverse reaction equals the rate of the forward reaction.
Some groups of elements react chemically in similar ways. For example, the chemistry of sodium and the chemistry of lithium are similar. The chemistry of chlorine and the chemistry of iodine are also similar. These similarities in chemistry result when different elements have similar __________.
numbers of outer-shell electrons
When one or more pairs of valence electrons are shared by two neutral atoms, what type of bond is formed?
a covalent bond
When do hydrogen bonds occur?
when partial opposite charges on molecules come close enough to attract each other
Which of the following subatomic particles has appreciable mass and lacks a charge?
What are the four most abundant elements found in living systems?
hydrogen, oxygen, nitrogen, and carbon
There are __________ naturally occurring elements.
Phosphorus-32 (radioactive) has __________ than phosphorus-35 (normal).
three fewer neutrons
The compound CaSO4 ionizes into a calcium ion and a sulfate ion (SO4). Calcium has two electrons in its outer shell. Upon ionization, what would you expect the charge on the sulfate ion to be?
Refer to this summary equation of photosynthesis to complete the sentences about chemical bonds and reactions.

6 CO2 + 6 H2O → C6H12O6 + 6 O2

The equation shows a [chemical reaction]– the breaking and forming of chemical bonds that leads to a change in the composition of matter.

In the equation, Co2 is a [reactant].

In the equation, C6H12O6 is a [product].

In 02, the type of bond that holds the two oxygen atoms together is a [nonpolar covalent bond].

In H2O, the type of bond that holds one of the hydrogen atoms to the oxygen atom is a [polar covalent bond].

The number of oxygen atoms on the left side of the equation is [equal to] the number of oxygen atoms on the right side.

A covalent bond is likely to be polar if __________.
one of the atoms sharing electrons is much more electronegative than the other
What is the role of van der Waals interactions in biological molecules?
Although they are weak bonds, van der Waals interactions help to reinforce the three-dimensional shapes of large molecules.
The number of protons in an uncharged atom __________.
equals the number of electrons
Consider this statement: Concentrations of trace elements in an environment above what is required by organisms generally have no effect on those organisms. Which of the following is a valid response to this statement?
False. Elevated concentrations of some trace elements such as cobalt and chromium can be toxic.
The most common form of calcium has 20 protons, 20 neutrons, and 20 electrons. Which of the following elements would be an isotope of calcium?
an atom with 20 protons, 21 neutrons, and 20 electrons
An uncharged atom of nitrogen (atomic number = 7) has __________.
seven protons and seven electrons