Chapter 2 Flashcards

Question

radioactive isotope

Answer 1

. 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. For example, when an atom of carbon-14 (14C) decays, it loses a proton, becoming an atom of nitrogen (14N). Radioactive isotopes have many useful applications in biology.

Answer 2

Cells can use radioactive atoms just as they would use nonradioactive isotopes of the same element. The radioactive isotopes are incorporated into biologically active molecules, which are then used as tracers to track atoms during metabolism, the chemical processes of an organism. diagnostic medical tool.

Answer 3

fixed rate, expressed as the half-life of the isotope—the | time it takes for 50% of the parent isotope to decay

Answer 4

temp, pressure or any other environmental vble

Answer 5

. Using a process called radiometric dating, scientists measure the ratio of different isotopes and calculate how many half-lives (in years) have passed since an organism was fossilized or a rock was formed

Answer 6

“measure” a particular range of | years:

Answer 7

, their nuclei do not come close enough to interact. Of the three subatomic particles we have discussed, only electrons are directly involved in chemical reactions.

Answer 8

is defined as the capacity to cause change— | for instance, by doing work

Answer 9

Potential energy is the energy that matter possesses because of its location or structure. For example, water in a reservoir on a hill has potential energy because of its altitude. When the gates of the reservoir’s dam are opened and the water runs downhill, the energy can be used to do work, such as moving the blades of turbines to generate electricity. Because energy has been expended, the water has less energy at the bottom of the hill than it did in the reservoir. Matter has a natural tendency to move toward the lowest possible state of potential energy; in our example, the water runs downhill. To restore the potential energy of a reservoir, work must be done to elevate the water against gravity.

Answer 10

Electrons are found in different electron shells, each with a characteristic average distance and energy level. In diagrams, shells can be represented by concentric circles

Answer 11

An electron can move from one shell to another, but only by absorbing or losing an amount of energy equal to the difference in potential energy between its position in the old shell and that in the new shell.

Answer 12

For example, light energy can excite an electron to a higher energy level. (Indeed, this is the first step taken when plants harness the energy of sunlight for photosynthesis, the process that produces food from carbon dioxide and water. You’ll learn more about photosynthesis in Chapter 10.) When an electron loses energy, it “falls back” to a shell closer to the nucleus, and the lost energy is usually released to the environment as heat.

Answer 13

Electrons, like all matter, tend to exist in the | lowest available state of potential energy.

Answer 14

1. the number of electrons in its outermost shell. 2. valence e 3. valence shell

Answer 15

similar chemical behavior

Answer 16

. Accordingly, the concentric-circle diagrams do not give a real picture of an atom. In reality, we can never know the exact location of an electron. What we can do instead is describe the space in which an electron spends most of its time. The three-dimensional space where an electron is found 90% of the time is called an orbital.

Answer 17

No more than 2 electrons can occupy a single orbital. The first electron shell can therefore accommodate up to 2 electrons in its s orbital

Answer 18

As you will see in the next section, atoms interact in a way that completes their valence shells. When they do so, it is the unpaired electrons that are involved.

Answer 19

Atoms with incomplete valence shells can interact with certain other atoms in such a way that each partner atom completes its valence shell: The atoms either share or transfer valence electrons

Answer 20

These interactions usually result in atoms staying close together, held by attractions called chemical bonds

Answer 21

. The strongest kinds of chemical bonds are covalent bonds and ionic bonds in dry ionic compounds. (Ionic bonds in aqueous, or water-based, solutions are weak interactions, as we will see later.)

Answer 22

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

Answer 23

s. When the two hydrogen atoms come close enough | for their 1s orbitals to overlap, they can share their electrons

Answer 24

two or more atoms held together by covalent bonds

Answer 25

a pair of shared e

Answer 26

Each atom that can share valence electrons has a bonding capacity corresponding to the number of covalent bonds the atom can form. When the bonds form, they give the atom a full complement of electrons in the valence shell. The bonding capacity of oxygen, for example, is 2. This bonding capacity is called the atom’s valence and usually equals the number of unpaired electrons required to complete the atom’s outermost (valence) shell.

Answer 27

The molecules H2 and O2 are pure elements rather than compounds because a compound is a combination of two or more different element

Answer 28

The attraction of a particular atom for the electrons of a covalent bond. The more electronegative an atom is, the more strongly it pulls shared electrons toward itself

Answer 29

unequal charge distribution. d. Such bonds vary in their polarity, depending on the relative electronegativity of the two atoms. For example, the bonds between the oxygen and hydrogen atoms of a water molecule are quite polae

Answer 30

attracting shared electrons much more strongly than hydrogen does.

Answer 31

In a covalent bond between oxygen and hydrogen, the electrons spend more time near the oxygen nucleus than near the hydrogen nucleus. Because electrons have a negative charge and are pulled toward oxygen in a water molecule, the oxygen atom has two regions of partial negative charge (each indicated by the Greek letter δ with a minus sign, δ-, “delta minus”), and each hydrogen atom has a partial positive charge (δ+, “delta plus”)

Answer 32

rather, it allows a | bond to form because it results in two ions of opposite charge.

Answer 33

When these two atoms meet, the lone valence electron of sodium is transferred to the chlorine atom, and both atoms end up with their valence shells complete. (Because sodium no longer has an electron in the third shell, the second shell is now the valence shell.)

Answer 34

The electron transfer between the two atoms moves one unit of negative charge from sodium to chlorine. Sodium, now with 11 protons but only 10 electrons, has a net electrical charge of 1+; the sodium atom has become a cation. Conversely, the chlorine atom, having gained an extra electron, now has 17 protons and 18 electrons, giving it a net electrical charge of 1- ; it has become a chloride ion—an anion.

Answer 35

Salts are often found in nature as crystals of various sizes and shapes. Each salt crystal is an aggregate of vast numbers of cations and anions bonded by their electrical attraction and arranged in a three-dimensional lattice

Answer 36

The formula for an ionic compound, such as NaCl, indicates only the ratio of elements in a crystal of the salt. “NaCl” by itself is not a molecule.

Answer 37

``` n. Magnesium (12Mg) must lose 2 outer electrons if the atom is to have a complete valence shell, so it has a tendency to become a cation with a net charge of 2+ (Mg2+ ). One magnesium cation can therefore form ionic bonds with two chloride anions (Cl- ) ```

Answer 38

r, however, the ionic bonds are much weaker because each ion is partially shielded by its interactions with water molecules

Answer 39

The reversibility of weak interactions can be an advantage: Two molecules can come together, affect one another in some way, and then separate.

Answer 40

. 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

Answer 41

These van der Waals interactions are individually weak and occur only when atoms and molecules are very close together. When many such interactions occur simultaneously, however, they can be powerful

Answer 42

These shapes are determined by the positions of the atoms’ | orbitals

Answer 43

all four hybrid orbitals of the carbon atom are shared with hydrogen atoms (see Figure 2.15b). The carbon nucleus is at the center, with its four covalent bonds radiating to hydrogen nuclei at the corners of the tetrahedron. Larger molecules containing multiple carbon atoms, including many of the molecules that make up living matter, have more complex overall shapes.

Answer 44

n. Endorphins are signaling molecules made by the pituitary gland that bind to the receptors, relieving pain and producing euphoria during times of stress, such as intense exercise.

Answer 45

The making and breaking of chemical bonds, leading to changes in the composition of matter, are called chemical reactions

Answer 46

This reaction breaks the covalent bonds of H2 and O2 and | forms the new bonds of H2O.

Answer 47

The raw materials of photosynthesis are carbon dioxide (CO2) and water (H2O), which land plants absorb from the air and soil, respectively. Within the plant cells, sunlight powers the conversion of these ingredients to a sugar called glucose (C6H12O6) and oxygen molecules (O2), a by-product that can be seen when released by a water plant (Figure 2.17). Although photosynthesis is actually a sequence of many chemical reactions, we still end up with the same number and types of atoms that we had when we started. Matter has simply been rearranged, with an input of energy provided by sunlight.

Answer 48

reversible, with the products of the forward reaction becoming the reactants for the reverse reaction

Answer 49

s. As products accumulate, collisions resulting in the reverse reaction become more frequent. Eventually, the forward and reverse reactions occur at the same rate, and the relative concentrations of products and reactants stop changing. The point at which the reactions offset one another exactly is called chemical equilibrium.

Answer 50

An electron can move from one shell to another only if the energy it gains or loses is exactly equal to the difference in energy between the energy levels of the two shells.

Answer 51

``` When two hydrogen atoms approach each other, the electron of each atom is also attracted to the proton in the other nucleus. ```

Answer 52

Valence is typically the number of electrons needed to fill the outermost shell of an atom