Chapter 2: The Chemical Context of Life.

Practice questions from Pearson or books.

Which of the following lists some of the levels of biological organization from less complex to more complex?

Answer: atoms, molecules, cells, organs.

Explanation: Molecules are made up of atoms, which are the least complex components on the list. A cell has many different molecules in it, and an organ is an organized structure made up of many cells. As complexity increases, novel properties emerge at each level that are absent from the preceding level.

Which of the following is a true statement about how living organisms use energy and chemicals to carry out life’s activities?

Answer: Energy flows through an ecosystem, but chemicals cycle within it.

Explanation: Energy flows through an ecosystem in one direction, usually entering as light and exiting as heat. In contrast, chemicals cycle within an ecosystem, where they are used and then recycled.

Which of the following statements about Darwin’s theory of natural selection is true?

Answer: Individuals with inherited traits that are better suited to the local environment are more likely to survive and reproduce than less well-suited individuals.

Explanation: Darwin’s observations of the natural world led him to reason that over many generations, a higher and higher proportion of individuals in a population will have advantageous traits that favor survival and reproduction. Populations, not individuals, evolve such that after multiple generations, individuals are better adapted to their environment. Evolution occurs as the unequal reproductive success of individuals ultimately leads to adaptation to their environment, as long as the environment remains the same.

The information in a gene that directs the production of a cellular product is gene expression. The correct sequence for the transfer of information from a gene to a function protein is __________.

Anwser: transcription, translation, and protein folding.



Explnation: Many genes provide the blueprints for making proteins, which are the major players in building and maintaining a cell and carrying out its activities. For instance, a given bacterial gene may specify a particular protein (an enzyme) required to break down a certain sugar molecule, while a human gene may denote a different protein (an antibody) that helps fight off infection. Genes control protein production indirectly, using a related molecule called RNA as an intermediary. The sequence of nucleotides along a gene is transcribed into RNA, which is then translated into a linked series of protein building blocks called amino acids. These two stages result in a specific protein with a unique shape and function. The entire process, by which the information in a gene directs the manufacture of a cellular product, is called gene expression.

Which of the following four statements, if any, is true regarding essential elements and living organisms?

Anwser: Although all forms of life require iron, other elements are required only by certain species.

Explnation: All forms of life require iron, whereas other elements are needed by certain species only; for example, humans and vertebrates need iodine, but invertebrates do not.

Which of the following has negligible mass?

Anwser: Electron

Explnantion: An electron has negligible mass. “Negligible” mass means a mass so low that it is not taken into account in ordinary calculations of atomic mass. The subatomic particle that fits this description is the electron. Its mass is only about 1/1,200 the mass of a proton or neutron.

Nonpolar covalent bonds are always formed from atoms that are __________.

Answer: equal in their electronegativity.


Explnation: Atoms in a molecule attract shared bonding electrons to varying degrees, depending on the element. The attraction of a particular atom for the electrons of a covalent bond is called its electronegativity. The more electronegative an atom is, the more strongly it pulls shared electrons toward itself. In a covalent bond between two atoms of the same element, the electrons are shared equally because the two atoms have the same electronegativity—the tug-of-war is at a standoff. Such a bond is called a nonpolar covalent bond.

The difference between a stable isotope and an unstable isotope of an element is that __________.

Anwser: The unstable isotope has a nucleus that decays spontaneously, is radioactive, and can be transformed into an atom of another element.

Explanation: All atoms of a given element have the same number of protons, but some atoms have more neutrons than other atoms of the same element and therefore have greater mass. These different atomic forms of the same element are called isotopes of the element.

Ionic bonds form as a result of __________.

Answer:attraction between ions that have opposite charges.

Explanation: Ionic bonds are attractions that form between ions of opposite charges. In an ionic bond, electrons are transferred from one ion (the cation) to the other (the anion), not shared, as is the case for covalent bonds.

The behavior of an atom depends on the __________.

Answer: valence electrons in the outermost electron shell.

Explanation: The behavior of an atom depends on the valence electrons in the outermost electron shell.

Isotopes of an element will always differ in __________.

Correct anwser: atomic mass.

Explantation: Isotopes of an element will always differ in atomic mass.

An element’s atomic number is the number of protons, whereas its atomic mass is the sum of the number of protons and neutrons, in the nucleus.

 The isotopes of an element have the same number of protons and therefore the same atomic number; however, they have different numbers of neutrons and therefore differ in atomic mass.

 

Elements with the same number of protons but different numbers of electrons carry a charge; they are called ions of the element.

Radioactive tracers _________.

Correct anwser: are important tools in biological research and medicine.

Explnation: Radioactive isotopes are often used as diagnostic tools in medicine. Cells can use radioactive atoms just as they would use nonradioactive isotopes of the same element.

The chemical characteristics or reactivity of an element depend mostly on the __________.

Explnation: The chemical characteristics and reactivity of an element depend mainly on the number of electrons in its atoms’ outermost valence shell. These are the electrons that are most free to interact chemically with the electrons of other atoms.

Weak chemical bonds such as hydrogen bonds __________.

Answer: form when a hydrogen atom that is covalently bonded to an electronegative atom like oxygen or nitrogen is attracted to another electronegative atom.

Explanation: When a hydrogen atom is covalently bonded to an electronegative atom, the hydrogen atom has a partial positive charge that allows it to be attracted to a different electronegative atom nearby. This attraction between a hydrogen and an electronegative atom is called a hydrogen bond. In living cells, the electronegative partners are usually oxygen or nitrogen atoms.

A covalent bond __________.

Anwser: A covalent bond is the sharing of a pair of valence electrons by two atoms.

Explnation: For example, let’s consider what happens when two hydrogen atoms approach each other. Recall that hydrogen has 1 valence electron in the first shell, but the shell’s capacity is 2 electrons. When the two hydrogen atoms come close enough for their 1s orbitals to overlap, they can share their electrons. Each hydrogen atom is now associated with 2 electrons in what amounts to a completed valence shell. Two or more atoms held together by covalent bonds constitute a molecule, in this case a hydrogen molecule. The strongest kinds of chemical bonds are covalent bonds and ionic bonds (when in dry ionic compounds). 

When one or more pairs of valence electrons are shared by two neutral atoms, what type of bond is formed?

Anwser: A covalent bond.

Explnation: When electrons are shared, a covalent bond is formed.

What are the four most abundant elements found in living systems?

Anwser: Hydrogen, oxygen, nitrogen, and carbon

Explnation: The four most abundant elements in living systems are hydrogen, oxygen, nitrogen, and carbon, which together make up about 96 percent of the mass of living organisms.

 

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

Answer: It is an ion.

Explanation: When the proton number and electron number are unequal, the atom or molecule is an ion.

The charge can be either positive or negative, depending on if there are more protons or more electrons, and this atom is called an ion.

Which of the following subatomic particles has appreciable mass and lacks a charge?

Anwser: Neutron.

Explanation: You can get a hint about the charge from the name: a “neutron” is "neutral," meaning that it lacks a charge and is the correct answer.

The fact that oxygen is an atom that is strongly electronegative means that __________.

Answer: it more strongly pulls shared electrons toward itself when forming a covalent bond with a less electronegative atom.


Explanation: The fact that oxygen is an atom that is strongly electronegative means that it more strongly pulls shared electrons toward itself when forming a covalent bond with a less electronegative atom.

In a process called radiometric dating, scientists measure the __________ of an isotope to determine the age of the Earth.

Anwser: Half-Life

Explanation: Researchers use radioactive isotopes to date relics of past life. A radioactive isotope is one in which the nucleus decays spontaneously, giving off particles and energy. When the radioactive decay leads to a change in the number of protons, it transforms the atom to an atom of a different element.

Orbitals are best described as __________.

Anwser: a three-dimensional space where an electron spends most of its time with no more than 1 other electron, forming a pair.

The reactivity of an atom arises from __________.

Answer: the existence of unpaired electrons in the valence shell.



Explanation: It is neither a function of the average distance of the outermost shell from the nucleus, nor of the potential energies of the valence shell or all the electron shells. The atom’s reactivity is not related to the potential energy difference between the s and

Weak chemical bonds such as hydrogen bonds __________.

Anwser: form when a hydrogen atom that is covalently bonded to an electronegative atom like oxygen or nitrogen is attracted to another electronegative atom.

Explanation: When a hydrogen atom is covalently bonded to an electronegative atom, the hydrogen atom has a partial positive charge that allows it to be attracted to a different electronegative atom nearby. This attraction between a hydrogen and an electronegative atom is called a hydrogen bond.In living cells, the electronegative partners are usually oxygen or nitrogen atoms.

In ionic interactions, __________.

Anwser: the transfer of valence electrons results in a completed valence shell for each ion.

Explanation:

When two atoms are very unequal in their attraction for valence electrons, the more electronegative atom strips an electron completely away from its partner. The two resulting oppositely charged atoms (or molecules) are called ions. A positively charged ion is called a cation, while a negatively charged ion is called an anion. Because of their opposite charges, cations and anions attract each other; this attraction is called an ionic bond. Note that the transfer of an electron is not, by itself, the formation of a bond; rather, it allows a bond to form because it results in two ions of opposite charge. Any two ions of opposite charge can form an ionic bond. The ions do not need to have acquired their charge by an electron transfer with each other.

What is generally true about essential elements and living organisms?

Answer: Although all forms of life require iron, other elements are required only by certain species.


Explanation: 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?

Anwser: False. Elevated concentrations of some trace elements such as cobalt and chromium can be toxic.

Explanation: 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.

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.

Answer: False. Compounds can have emergent properties that are very different from those of the elements that form them.

Explnation: Correct. Emergent properties of compounds are more than simply the sum of their parts.

There are __________ naturally occurring elements.

Answer: 92


Explanation: There are 92 naturally occurring elements, and about 25 of these are essential to human life.

Which of the following is a trace element?

Answer: Copper.


Explanation: Correct. Copper (Cu) is a trace element. Copper accounts for less than 0.01% of living matter.

The number of protons in an uncharged atom __________.

Answer: equals the number of electrons.

Explanation: An uncharged atom has an equal number of protons and electrons.

An element has eight protons, nine neutrons, and eight electrons. Its atomic number and atomic mass, respectively, are __________.

Answer: 8 and 17

Explanation: 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.

An uncharged atom of nitrogen (atomic number = 7) has __________.

Answer: seven protons and seven electrons.

Explanation: The atomic number is the number of protons, and this equals the number of electrons in an electrically neutral (uncharged) atom.

In which of the following will isotopes of an element always differ?

Answer: Atomic mass

Explanation: 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.

A particular carbon isotope has an atomic number of 6 and an atomic mass of 14. The respective numbers of neutrons, protons, and electrons that this carbon isotope has are __________.

Answer: 8, 6, and 6

Explanation: 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).

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

Answer: three fewer neutrons.

Explanation: 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.

Question15

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?

Answer: An atom with 20 protons, 21 neutrons, and 20 electrons.

Explanation: Isotopes of an element have the same number of protons but different numbers of neutrons.

Why are radioactive isotopes useful in scientific research?

Anwser: Because they can be used as tracers to follow particular atoms and molecules through metabolic pathways.

Explanation: 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.

A neutral atom of chlorine has an atomic number of 17. It has __________ electrons in its third shell.

Answer: 7.

Explanation: 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.

The chemical characteristics or reactivity of an element depend mostly on the __________.

Answer: number of electrons in its outermost shell.

Explanation: 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.

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 __________.

Answer: numbers of outer-shell electrons.

Explanation: Atoms with the same number of electrons in their valence shells exhibit similar chemical behavior or reactivity.

How many electrons would be present in the valence shell of a sulfur atom (atomic number 16, mass number 32)?

Answer: Six electrons.

Explanation: 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 does the configuration of electrons make the atom unreactive?

Answer: When the atom has eight electrons in its outermost shell.

Explanation: An atom with a complete valence shell is unreactive.

An atom that normally has __________ in its outer shell would not tend to form chemical bonds with other atoms.

Answer: eight electrons.

Explanation: Eight electrons will leave no unpaired electrons in the valence shell. In this configuration, atoms are chemically unreactive or inert.

When one or more pairs of valence electrons are shared by two neutral atoms, what type of bond is formed?

Answer: A covalent bond.

Explanation: Covalent bonds are formed between atoms that share one or more pairs of valence electrons.

A covalent bond is likely to be polar if __________.

Answer: one of the atoms sharing electrons is much more electronegative than the other.

Explanation: If one atom is more electronegative than the other, electrons of the bond will not be shared equally, resulting in a polar covalent bond.

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

Answer: It is an ion.

Explanation: 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).

Copper has an atomic number of 29 and a mass number of 64. What would result if an uncharged copper atom lost two electrons?

Answer: The atomic number would remain 29, the mass number would remain 64, and the atom would be a cation with a +2 charge.


Explanation: 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.

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?

Answer: -2

Explanation: By gaining two electrons from calcium, the sulfate ion now has a charge of –2 and calcium has a charge of +2. alcium has two electrons in its outer shell, so the sulfate ion can pick up only two electrons, not four.

Trace elements __________.

Answer: are required by an organism in minute quantities.

Explanation: It is correct because trace elements are needed by all forms of life.

Ionic bonds form as a result of __________.

Answer: attraction between ions that have opposite charges.

Explanation: Oppositely charged ions attract each other electrically, forming an ionic bond.

A hydrogen bond __________.

Answer: is a weak chemical bond.


Explanation: Hydrogen bonds are weak chemical bonds that are very important in the chemistry of life.

When do hydrogen bonds occur?

Answer: When partial opposite charges on molecules come close enough to attract each other.

Explanation: A hydrogen bond occurs when a hydrogen atom covalently bonded to an electronegative atom is attracted to another electronegative atom.

What is the role of van der Waals interactions in biological molecules?

Answer: Although they are weak bonds, van der Waals interactions help to reinforce the three-dimensional shapes of large molecules.

Explantion: Weak bonds are important in the three-dimensional structure of most large biological molecules, and they play a role in molecular interactions.

Which molecule has the shape of a completed tetrahedron?

Answer: Methane (CH₄)

Explanation: 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.

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?

Answer: The molecule's shape.

Explanation: The molecule would have to have the correct shape to bind to the receptor protein and block it.

Which statement is true about chemical reactions?

Answer: They involve the making and breaking of chemical bonds.

Explanation: This is how chemical changes occur.

Which statement describes a reversible reaction that has reached chemical equilibrium?

Answer: The rate of the reverse reaction equals the rate of the forward reaction.


Explanation: This is the definition of chemical equilibrium.