MasteringBiology Chapter 2 Homework

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


What 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

Covalent bonds hold atoms together because they …

fill shells without giving atoms much charge and 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.

Can rotations occurs 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 because the double bond limits all the atoms.

Partial charges occur when …

a covalent bond links atoms of two kinds and 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?


The mass number of an atom is 15, and its atomic number is 7. The atom probably has…

8 neutrons in the nucleus.

What determines 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.

What shell has a highest energy?

The valence 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. The atomic nucleus because unstable if the ration of protons to neutrons is too far from unity

What model most accurately represent the current view of the structure of an atom? Probability or planetary model?

Probability model.

Knowing the atomic mass of an element allows inferences about which of the following?

the number of protons plus neutrons in the element

Which of the following is a trace element required by most living organisms?


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.

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.

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?


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?


An ionic bond involves _____.

an attraction between ions of opposite charge

Hydrogen bonding is most often seen _____.

when hydrogen is covalently bonded to an electronegative atom

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 name is given to the bond between water molecules?


What is the mass number of an ion with 109 electrons, 159 neutrons, and a +1 charge?


The atoms of different phosphorus isotopes _____.

have different numbers of neutrons

Which statement about weak bonds is correct?

Weak bonds are transient and easily reversible.

What bond can form between atoms of equal electronegativity?

Van der Waals interactions can form between atoms of equal electronegativity.

Which statement is true of all atoms that are anions?

The atom has more electrons than protons.

Potential energy is always …

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. In a collision, kinetic energy becomes the potential energy of distortion, which can weaken bonds enough to permit a reaction.

Which statement is true of the transition state?

It can break apart to yield either reactants or products and it is extremely unstable and lasts only an instant.

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

6 electrons.

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

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

The reactivity of an atom arises from _________.

the existence of unpaired electrons in the valence shell

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.

Changing the number of _____ would change an atom into an atom of a different element.

protons in an atom

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


What type of bond shares two pairs of electrons?

Double covalent

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

O, N, C, H

Dr. Haxton says the O-O bond is polar and the C-C bond is nonpolar. A good student would say …

Wrong again, Ralph. Both bonds are non polar. When two atoms of the same kind form a covalent bond, they share electrons equally because their electronegativity is the same.

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

O-H because they are on opposite ends of the electronegativity scale

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

H-N because they differ most in electronegativity

An ionic bond is formed when _____.

one atom transfers an electron to another atom

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

Compared to covalent bonds, ionic bonds …

rely much less on electron-sharing.

Which of the following subatomic particles always has a positive charge?


Catalysts speed reactions mainly by …

lowering EA. Catalysts always lower EA, though they may have other effects as well. With a lower EA, a larger fraction of collisions can produce the transition state.

What type of bond joins the carbon atom to each of the hydrogen atoms?

single (nonpolar) covalent

Which of the following are compounds?

H2O and CH4, but not O2

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


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?

Living cells avoid chemical equilibrium by taking in energy and materials. The only way to avoid equilibrium is to exchange materials and energy with the environment.

In the equation ΔG = ΔH – TΔS, the change in the amount of disorder in the system is symbolized by …

ΔS S is entropy

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. ΔG = ΔH – TΔS = -9 kJ – (+1 kJ) = -10 kJ. Since the free energy decreases, the reaction will run spontaneously.

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 …

the forward reaction will be spontaneous under standard conditions because ΔG is less than zero.

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 The nonstandard concentrations have lowered ΔG by 2 kJ.mol.

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