Chapter 2: The Chemical Context of Life

In which of the following will the isotopes of an element always differ? A. atomic number B. number of protons C. number of electrons D. atomic mass

Why? - Atomic mass refers to the number of protons and neutrons in an atom. Atomic forms of an element with the same number of protons (atomic number) but different numbers of neutrons are isotopes of that element.

How would you respond to this reasoning: oxygen is not a greenhouse gas; therefore, gases containing oxygen—such as ozone, nitrous oxide, and carbon dioxide—are not greenhouse gases either. A. False. Compounds can have emergent properties that are very different from those of the elements that form them. B. True. Because oxygen is not a greenhouse gas, it is unlikely to combine with anything at a higher level of organization to form a greenhouse gas. C. True. Compounds display emergent properties that are very similar to those of the elements that form them. D. True. It is impossible for a gas containing oxygen to be a greenhouse gas.

A. False. Compounds can have emergent properties that are very different from those of the elements that form them. Why? - Emergent properties of compounds are more than simply the sum of their parts.

Copper has an atomic number of 29 and a mass number of 64. What would result if an uncharged copper atom lost two electrons? A. The atomic number would remain 29, the mass number would remain 64, and the atom would be a cation with a +2 charge. B. The atomic number would be reduced to 27, the mass number would remain 64, and the atom would be an anion with a +2 charge. C. The atom would have a double negative charge and be an ion. D. The atomic number of the atom would remain 29, the mass number would be reduced to 62, and the atom would be an anion.

A. The atomic number would remain 29, the mass number would remain 64, and the atom would be a cation with a +2 charge. Why? - The loss or gain of electrons does not influence the atomic number (number of protons) or the mass number (number of protons plus neutrons) of an atom.

Ionic bonds form as a result of __________. A. attraction between ions that have opposite charges B. sharing of electron pairs between atoms C. attraction between a hydrogen and other atoms that share electrons unequally D. unequal sharing of electrons between atoms

A. attraction between ions that have opposite charges Why? - Oppositely charged ions attract each other electrically, forming an ionic bond.

What are the four most abundant elements found in living systems? A. hydrogen, oxygen, nitrogen, and carbon B. nitrogen, carbon, sulfur, and oxygen C. hydrogen, oxygen, nitrogen, and carbon dioxide D. hydrogen, oxygen, calcium, and nitrogen

A. hydrogen, oxygen, nitrogen, and carbon Why? - These four elements make up about 96% of living matter.

Which molecule has the shape of a completed tetrahedron? A. methane (CH4) B. water (H2O) C. hydrogen gas (H2) D. oxygen gas (O2)

A. methane (CH4) Why? - The nucleus of the carbon atom is at the center of the tetrahedron, with its four covalent bonds radiating to the hydrogen nuclei at the corners of the tetrahedron.

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? A. -4 B. -2 C. -1 D. +1

B. -2 Why? - . By gaining two electrons from calcium, the sulfate ion now has a charge of -2 and calcium has a charge of +2.

A neutral atom of chlorine has an atomic number of 17. It has __________ electrons in its third shell. A. 2 B. 7 C. 8 D. 17

B. 7 Why? - The first shell holds two electrons and is full; the second shell holds eight electrons and is also full, which leaves seven electrons to occupy the third shell.

An element has eight protons, nine neutrons, and eight electrons. Its atomic number and atomic mass, respectively, are __________. A. 8 and 16 B. 8 and 17 C. 9 and 16 D. 9 and 17

B. 8 and 17 Why? - The number of protons, eight, gives the atomic number. The atomic mass is the sum of the number of protons and neutrons; thus, 8 + 9 = 17.

Which of the following is a trace element? A. Hydrogen B. Copper C. Oxygen D. Nitrogen

B. Copper Why? - Copper (Cu) is a trace element. Copper accounts for less than 0.01% of living matter.

Which of the following has negligible mass? A. Neutron B. Electron C. Atom D. Proton

B. Electron Why? - Because the mass of an electron is only about 1/2,000 that of a proton or neutron, electrons are not considered when computing atomic mass.

When one or more pairs of valence electrons are shared by two neutral atoms, what type of bond is formed? A. a hydrogen bond B. a covalent bond C. an ionic bond D. an electronegative bond

B. a covalent bond Why? - Covalent bonds are formed between atoms that share one or more pairs of valence electrons.

Some groups of chemical elements react similarly to one another. For example, the chemistries of sodium and of lithium are similar, as are the chemistries of chlorine and of iodine. These similarities in chemistry result when different elements have similar __________. A. number of electron shells present in the atoms B. number of electrons in its outermost shell C. mean energy level of its electrons D. degree to which it has more or fewer electrons than protons

B. number of electrons in its outermost shell Why? - Only electrons are directly involved in the interactions between atoms, and the number of electrons in the outermost shell determines the chemical behavior or reactivity of an atom.

How many electrons would be present in the valence shell of a sulfur atom (atomic number 16, mass number 32)? A. total numbers of electrons B. numbers of outer-shell electrons C. energy per electron D. atomic size

B. numbers of outer-shell electrons Why? - Atoms with the same number of electrons in their valence shells exhibit similar chemical behavior or reactivity.

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? A. the atomic numbers of the atoms involved B. the molecule's shape C. the type of bonding in the molecule D. the number of valence electrons

B. the molecule's shape Why? - The molecule would have to have the correct shape to bind to the receptor protein and block it.

When do hydrogen bonds occur? A. when a molecule with a low molecular weight is bonded to a molecule with a high molecular weight B. when partial opposite charges on molecules come close enough to attract each other C. when a molecule with partial charges contacts a molecule without partial charges D. when two atoms achieve stable electron configurations by sharing electrons with each other

B. when partial opposite charges on molecules come close enough to attract each other Why? - A hydrogen bond occurs when a hydrogen atom covalently bonded to an electronegative atom is attracted to another electronegative atom.

There are __________ naturally occurring elements. A. 4 B. 25 C. 92 D. 108

C. 92 Why? - There are 92 naturally occurring elements, and about 25 of these are essential to human life.

What is the role of van der Waals interactions in biological molecules? A. Van der Waals interactions are too weak to be significant. B. Van der Waals interactions are important only in water. C. Although they are weak bonds, van der Waals interactions help to reinforce the three-dimensional shapes of large molecules. D. Van der Waals interactions are strong bonds that play a major structural role in holding proteins together.

C. Although they are weak bonds, van der Waals interactions help to reinforce the three-dimensional shapes of large molecules. Why? - Weak bonds are important in the three-dimensional structure of most large biological molecules, and they play a role in molecular interactions.

Which of the following subatomic particles has appreciable mass but no charge? A. Electron B. Proton C. Neutron D. Both proton and neutron.

C. Neutron Why? - A neutron weighs about 1.7 × 10^-24 grams (has a mass close to 1 dalton), and it is electrically neutral.

Why are radioactive isotopes useful in scientific research? A. because they are not readily incorporated into biological reactions B. because they are difficult to detect in small amounts, but living cells cannot distinguish them from the corresponding stable isotopes C. because they can be used as tracers to follow particular atoms and molecules through metabolic pathways D. because they decay spontaneously and give off energy and subatomic particles

C. because they can be used as tracers to follow particular atoms and molecules through metabolic pathways Why? - Because they are detectable in very small amounts, a few labeled molecules can be traced through an organism's body to see how they are used.

The number of protons in an uncharged atom __________. A. equals the number of neutrons B. varies from isotope to isotope C. equals the number of electrons D. C. equals the number of electrons

C. equals the number of electrons Why? - An uncharged atom has an equal number of protons and electrons.

When the proton number and electron number are unequal, the atom or molecule __________. A. forms a covalent bond with another atom B. becomes part of a molecule C. is an ion D. gains or loses a proton

C. is an ion Why? - When the proton number and electron number are unequal, the atom or molecule has a net positive charge (cation) or a net negative charge (anion).

Phosphorus-32 (radioactive) has __________ than phosphorus-35 (normal). (Concept 2.2) A. three more neutrons B. three more protons C. three fewer neutrons D. three fewer protons

C. three fewer neutrons Why? - 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 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 __________. A. 6, 8, and 6 B. 6, 6, and 8 C. 8, 6, and 8 D. 8, 6, and 6

D. 8, 6, and 6 Why? - The atomic number, 6, is equal to the number of protons. Therefore, the number of electrons in an electrically neutral atom would also be 6. The number of neutrons must be 8 because the atomic mass is 14 (14 - 6 = 8).

What is generally true about essential elements and living organisms? A. The elements carbon, hydrogen, nitrogen, and oxygen make up 20-25% of living mass. B. Given their low concentrations in nature, the toxicity of some elements is generally not a factor in the evolution of biological communities. C. All organisms require 25 of the 92 naturally occurring elements to survive. D. Although all forms of life require iron, other elements are required only by certain species.

D. Although all forms of life require iron, other elements are required only by certain species. Why? - Humans and other vertebrates require small amounts of iodine in their diets to maintain thyroid function. This is a requirement not observed among invertebrates.

If you were told that concentrations of trace elements above what is required by organisms generally have no effect on those organisms, what would you say? A. True. Most organisms have evolved adaptations that allow them to tolerate very high concentrations of trace elements in the environment. B. False. Increasing the concentration of trace elements above what is required generally has a beneficial effect on organisms. C. True. Only trace element limitation can lead to disease among organisms. D. False. Elevated concentrations of some trace elements such as cobalt and chromium can be toxic.

D. False. Elevated concentrations of some trace elements such as cobalt and chromium can be toxic. Why? - Although considered essential to life at minute amounts, high concentrations of trace elements (such as heavy metals) in the environment can lead to disease and mortality among organisms.

Which statement describes a reversible reaction that has reached chemical equilibrium? A. The rate of the reverse reaction exceeds the rate of the forward reaction. B. The forward and the reverse reactions have stopped. C. The concentration of the reactants is equal to the concentration of the products. D. The rate of the reverse reaction equals the rate of the forward reaction.

D. The rate of the reverse reaction equals the rate of the forward reaction. Why? - This is the definition of chemical equilibrium.

Which statement is true about chemical reactions? A. They may have different numbers of a given atom on each side of the equation arrow. B. They represent the way matter is created and destroyed. C. They reach chemical equilibrium when the amounts of products and reactants are equal. D. They involve the making and breaking of chemical bonds.

D. They involve the making and breaking of chemical bonds. Why? - This is how chemical changes occur.

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? A. an atom with 21 protons, 20 neutrons, and 21 electrons B. an atom with 20 protons, 20 neutrons, and 18 electrons C. an atom with 21 protons, 21 neutrons, and 21 electrons D. an atom with 20 protons, 21 neutrons, and 20 electrons

D. an atom with 20 protons, 21 neutrons, and 20 electrons Why? - Isotopes of an element have the same number of protons but different numbers of neutrons.

An atom that normally has __________ in its outer shell would NOT tend to form chemical bonds with other atoms. A. one electron B. three electrons C. four electrons D. eight electrons

D. eight electrons Why? - Eight electrons will leave no unpaired electrons in the valence shell. In this configuration, atoms are chemically unreactive or inert.

A hydrogen bond __________. A. is one in which two hydrogen atoms bond to each other B. is one in which hydrogen is strongly attracted to a positively charged atom C. is a function of hydrophobic interactions D. is a weak chemical bond

D. is a weak chemical bond Why? - Hydrogen bonds are weak chemical bonds that are very important in the chemistry of life.

covalent bond is likely to be polar if __________. A. the two atoms sharing electrons are equally electronegative B. it is between two atoms that are both very strong electron acceptors C. the two atoms sharing electrons are of the same element D. one of the atoms sharing electrons is much more electronegative than the other

D. one of the atoms sharing electrons is much more electronegative than the other Why? - If one atom is more electronegative than the other, electrons of the bond will not be shared equally, resulting in a polar covalent bond.

An uncharged atom of nitrogen (atomic number = 7) has __________. A. seven protons and seven neutrons B. seven neutrons and seven electrons C. seven neutrons and six protons D. seven protons and seven electrons

D. seven protons and seven electrons Why? - The atomic number is the number of protons, and this equals the number of electrons in an electrically neutral (uncharged) atom.

For most atoms, when is an unreactive configuration of electrons attained? A. one electron B. two electrons C. four electrons D. six electrons

D. six electrons Why? - An atom of sulfur would have 16 electrons, with 2 electrons in the innermost electron shell, 8 electrons in the next electron shell, and 6 electrons in the outermost or valence shell.

For most atoms, when is an unreactive configuration of electrons attained? A. when the atom has moved all its electrons to its outermost shell B. when the atom has as many protons as neutrons C. when the atom achieves a zero net charge D. when the atom has eight electrons in its outermost shell

D. when the atom has eight electrons in its outermost shell Why? - An atom with a complete valence shell is unreactive.