1. Atoms, Molecules, and Life2
Atoms, Molecules, and Life 1

2. Chapter 2 At a Glance 2.1 What Are Atoms?
2.2 How Do Atoms Interact to Form Molecules?
2.3 Why Is Water So Important to Life?

3. 2.1 What Are Atoms?
Atoms are the basic structural units of elements
Elements are substances that cannot be broken down into simpler substances
Elements cannot be transformed into other substances under ordinary conditions
Elements form all matter alone and when combined with other elements
An atom is the smallest unit of an element, and retains all the chemical properties of that element
All atoms belong to one of the 98 naturally occurring elements total as of 2017.

4. Table 2-1 4

5. 2.1 What Are Atoms? Atoms are composed of subatomic particles
Atoms are composed of still-smaller particles
Atoms are composed of subatomic particles
In the central atomic nucleus, there are positively charged protons and uncharged neutrons
In orbit around the nucleus are negatively charged particles called electrons
Atoms are electrically neutral because their number of positive protons and negative electrons is equal

6. Table 2-2 6

7. 2.1 What Are Atoms?
Atoms are composed of still-smaller particles (continued)
Subatomic particles are measured in atomic mass because they are very small
The mass number of an atom is the total mass of the protons and neutrons contained in the nucleus
The atomic nucleus is formed by the cluster of protons and neutrons in the center of the atom
Electrons rapidly orbit around the nucleus in a continuous motion

8. 2.1 What Are Atoms? Elements are defined by their atomic numbers
The number of protons in the nucleus of an atom is known as the atom’s atomic number
For example, hydrogen atoms have one proton, so the atomic number for hydrogen is one
The atomic number is the defining value of each element
For example, carbon has six protons and oxygen has eight

10. 2.1 What Are Atoms?
Isotopes are atoms of the same element with different numbers of neutrons
Variant atomic forms of an element are called isotopes
Some isotopes are radioactive (meaning that they spontaneously break apart, forming different atoms and releasing energy) and are used in research
At room temperature, elements may occur as solids, liquids, or gases

11. Hydrogen (H) Helium (He) e electron e shell p p p n n atomic
Figure 2-1 Atomic models e e
electron
shell p p p n n
atomic
nucleus e
Hydrogen (H)
Helium (He) 11

12. 2.1 What Are Atoms?
Nuclei and electrons play complementary roles in atoms
Nuclei provide stability by resisting external forces such as energy, heat, and electricity
For example, the stable nuclei of a carbon atom (12C) keeps its carbonic identity regardless of the different structures it may be found in
Electrons are dynamic
They can capture and release energy that forms bonds used to combine atoms

13. 2.1 What Are Atoms?
Electrons occupy complex regions around the nucleus
Electrons are distributed around the nucleus of an atom in electron shells

14. Figure 2-2 Electron shells in atoms
6p
8p
15p
20p 6n 8n
16n
20n
Carbon (C)
Oxygen (O)
Phosphorus (P)
Calcium (Ca) 14

15. 2.1 What Are Atoms? Electrons can capture and release energy
When energy excites an atom, it causes an electron to jump from a lower- to a higher-energy shell
Later, the electron falls back into its original shell, releasing the energy
Life depends on electrons capturing and releasing energy
For example, the use of the light bulb

16. Figure 2-3 Energy capture and release
The energy boosts
the electron to a
higher-energy shell
The electron drops back
into lower-energy shell,
releasing energy as light
An electron
absorbs energy
heat energy
light 16

17. 2.1 What Are Atoms?
As atomic number increases, electrons fill shells increasingly distant from the nucleus
Each electron shell holds a specific number of electrons
The first shell, or energy level, holds two electrons
The second shell holds up to eight
Electrons fill the shell closest to the nucleus first in an effort to maintain stability

18. 2.2 How Do Atoms Interact to Form Molecules?
Matter consists of atoms
Molecules consist of two or more atoms, from the same or different elements, that are held together by interactions among their outermost electron shells
For example, hydrogen and oxygen are linked to form water (H2O is two atoms of hydrogen and one atom of oxygen)
Table salt is another example (NaCl is one atom of sodium and one atom of chloride)

19. 2.2 How Do Atoms Interact to Form Molecules?
Atoms form molecules to fill vacancies in their outer electron shells
Atoms will not react with other atoms if the outermost shell is completely empty or full (such atoms are considered inert)
Example: Neon, with eight electrons in the outermost shell, is full and, therefore, inert

20. 2.2 How Do Atoms Interact to Form Molecules?
Atoms form molecules to fill vacancies in their outer electron shells (continued)
Atoms will react with other atoms if the outermost shell is partially full (such atoms are considered reactive)
Example: Oxygen, with six electrons in its outermost shell, can hold two more electrons, and so is susceptible to reacting

21. 2.2 How Do Atoms Interact to Form Molecules?
Chemical bonds hold atoms together in molecules
There are three types of chemical bonds, which are the attractive forces holding atoms together in molecules: ionic bond, covalent bond, and hydrogen bond
Reactive atoms gain stability through electron interactions (chemical bonds)
Hydrogen and oxygen atoms gain stability by interacting with each other
Single electrons from each of two hydrogen molecules fill the outer shell of an oxygen atom

22. Table 2-3 22

23. 2.2 How Do Atoms Interact to Form Molecules?
Ionic bonds form among ions
Atoms that have lost or gained electrons, thereby altering the balance between protons and electrons, are charged, and are called ions
Atoms that have lost electrons become positively charged ions (e.g., sodium: Na)
Atoms that have gained electrons become negatively charged ions (e.g., chlorine: Cl)
Oppositely charged ions that are attracted to each other are bound into a molecule by ionic bonds

24. Figure 2-4 The formation of ions and ionic bonds
Sodium atom (neutral)
Chlorine atom (neutral)
11p
17p
11n
18n
Electron transferred
Neutral atoms
Sodium ion ()
Chloride ion ()
11p
17p
11n
18n
Attraction between
opposite charges
Ions
Cl
Na
Cl
Na
Cl
Na
Cl
Na
Cl
An ionic compound: NaCl 24

25. 2.2 How Do Atoms Interact to Form Molecules?
Covalent bonds form by sharing electrons
An atom with a partially full outermost electron shell can become stable by sharing electrons with another atom, forming a covalent bond
Two electrons (one from each atom), when shared, form a covalent bond

26. 2.2 How Do Atoms Interact to Form Molecules?
Covalent bonds form by sharing electrons (continued)
Covalent bonds are found in H2 (single bond), O2 (double bond), N2 (triple bond), and H2O
Covalent bonds are stronger than ionic bonds but vary in their stability

27. 2.2 How Do Atoms Interact to Form Molecules?
Covalent bonds form by sharing electrons (continued)
Because biological molecules must function in a watery environment where ionic bonds rapidly dissociate (break down), the atoms in most biological molecules, such as those found in proteins, sugars, and fats, are joined by covalent bonds

28. Table 2-4 28

29. 2.2 How Do Atoms Interact to Form Molecules?
Covalent bonds may produce nonpolar or polar molecules
Atoms within a molecule may have different nuclear charges
Those atoms with a greater positive nuclear charge pull more strongly on electrons in a covalent bond

30. 2.2 How Do Atoms Interact to Form Molecules?
Covalent bonds may produce nonpolar or polar molecules (continued)
In molecules such as H2, both atoms exert the same pulling force on bond electrons and the bond is called a nonpolar covalent bond
In molecules where atoms of different elements are involved (H2O), the electrons are not always equally shared and these covalent bonds are called polar covalent bonds

31. 2.2 How Do Atoms Interact to Form Molecules?
Covalent bonds may produce nonpolar or polar molecules (continued)
A molecule with polar bonds may be polar overall
H2O is a polar molecule
The (slightly) positively charged pole is around each hydrogen
The (slightly) negatively charged pole is around the oxygen

32. Figure 2-5 Covalent bonds involve shared electrons
(oxygen: slightly negative)
()
Larger
positive
charge
Same charge
on both nuclei
Electrons
spend more
time near the
larger nucleus
8p 8n
Electrons spend
equal time near
each nucleus
Smaller
positive
charge
(hydrogens:
slightly positive)
(hydrogens: uncharged)
()
()
Nonpolar covalent bonding in
hydrogen gas (H2)
Polar covalent bonding in water (H2O) 32

33. 2.2 How Do Atoms Interact to Form Molecules?
Hydrogen bonds are attractive forces between polar molecules
Polar molecules—such as those in water—have partially charged atoms at their ends
Hydrogen bonds form when partial opposite charges in different molecules attract each other
The partially positive hydrogen atoms of one water molecule are attracted to the partially negative oxygen on another

34. 2.2 How Do Atoms Interact to Form Molecules?
Hydrogen bonds are attractive forces between polar molecules (continued)
Polar biological molecules can form hydrogen bonds with water, each other, or even within the same molecule
Hydrogen bonds are comparatively weak but, collectively, can be quite strong

35. Figure 2-6 Hydrogen bonds in water
()
() H
() H
() O
() H
()
hydrogen
bonds 35

36. 2.3 Why Is Water So Important to Life?
Water molecules attract one another
Cohesion is the tendency of the molecules of a substance to stick together
Hydrogen bonding between water molecules produces high cohesion
Water cohesion explains how water molecules can form a chain in delivering moisture to the top of a tree

37. 2.3 Why Is Water So Important to Life?
Water molecules attract one another (continued)
Cohesion of water molecules along a surface produces surface tension
Spiders and water striders rely on surface tension to move across the surface of ponds

38. Figure 2-7 Cohesion among water molecules
Cohesion helps water to
reach treetops
Cohesion causes surface tension 38

39. 2.3 Why Is Water So Important to Life?
Water interacts with many other molecules
Water is an excellent solvent
A wide range of substances dissolve (are completely surrounded and dispersed) in water to form solutions

40. Figure 2-8 Water as a solvent
Cl
Na H
Na
Cl O H
Cl
Na 40

41. 2.3 Why Is Water So Important to Life?
Water interacts with many other molecules (continued)
Water-soluble molecules are hydrophilic
Water molecules are attracted to and can surround (and thereby dissolve) ions or polar molecules, such as sugars and amino acids

42. 2.3 Why Is Water So Important to Life?
Water interacts with many other molecules (continued)
Water-insoluble molecules that repel and drive together uncharged and nonpolar molecules such as fats and oils are hydrophobic
The “clumping” of nonpolar molecules is called hydrophobic interaction

43. Figure 2-9 Oil and water don’t mix43

44. 2.3 Why Is Water So Important to Life?
Water moderates the effects of temperature changes
Very low or very high temperatures may damage enzymes or slow down important chemical reactions
A lot of energy is required to heat water
The evaporation of water also requires a lot of energy

45. 2.3 Why Is Water So Important to Life?
Water moderates the effects of temperature changes (continued)
It takes a lot of energy to heat water
The energy required to heat 1 gram of a substance by 1°C is called its specific heat
Temperature reflects the speed of molecular motion
It requires 1 calorie of energy to raise the temperature of 1g of water 1°C (the specific heat of water), which is a very slow process

46. 2.3 Why Is Water So Important to Life?
Water moderates the effects of temperature changes (continued)
It takes a lot of energy to evaporate water
The heat of vaporization is the amount of heat needed to cause a substance such as water to evaporate (to change from a liquid to a vapor)
Evaporating water uses up heat from its surroundings, cooling the nearby environment (as occurs during sweating)
Because the human body is mostly water, a sunbather can absorb a lot of heat energy without sending her/his body temperature soaring

47. 2.3 Why Is Water So Important to Life?
Water forms an unusual solid: ice
Most substances become denser when they solidify from a liquid
Ice is unusual because it is less dense than liquid water
Water molecules spread apart slightly during the freezing process

48. Figure 2-11 Liquid water and ice48

49. 2.3 Why Is Water So Important to Life?
Water forms an unusual solid: ice (continued)
Ice floats in liquid water
Ponds and lakes freeze from the top down and never freeze completely to the bottom
Many plants and fish are therefore saved from freezing

50. Figure 2-12 Ice floats 50

51. 2.3 Why Is Water So Important to Life?
Water-based solutions can be acidic, basic, or neutral
A small fraction of water molecules break apart into ions
H2O  OH  H
Solutions in which H  OH are acidic
For example, hydrochloric acid ionizes in water
HCl  H  Cl
Lemon juice and vinegar are naturally occurring acids

52. 2.3 Why Is Water So Important to Life?
Water-based solutions can be acidic, basic, or neutral (continued)
Solutions in which OH  H are basic
For example, sodium hydroxide ionizes in water
NaOH  Na  OH
Baking soda, chlorine bleach, and ammonia are basic

53. Figure 2-13 Some water is always ionized
()
() O O H H H H
water
hydroxide ion
hydrogen ion
(H2O)
(OH)
(H) 53

54. 2.3 Why Is Water So Important to Life?
Water-based solutions can be acidic, basic, or neutral (continued)
The degree of acidity of a solution is measured using the pH scale
pH 0–6 is acidic (H  OH)
pH 7 is neutral (H  OH)
pH 8–14 is basic (OH  H)

55. H concentration in moles/liter
Figure 2-14 The pH scale
household ammonia (11.9)
1 molar hydrochloric
acid (HCI)
“acid rain” (2.5–5.5)
chlorine bleach (12.6)
seawater (7.8–8.3)
drain cleaner (14.0)
hydroxide (NaOH)
1 molar sodium
stomach acid (2)
lemon juice (2.3)
vinegar, cola (3.0)
black coffee (5.0)
blood, sweat (7.4)
baking soda (8.4)
washing soda (12)
oven cleaner (13.0)
normal rain (5.6)
pure water (7.0)
tomatoes (4.5)
orange (3.5)
antacid (10)
beer (4.1)
urine (5.7)
milk (6.4) 1 2 3 4 5 6 7 8 9 10 11 12 13 14
pH value
(H  OH)
(H  OH)
neutral
increasingly acidic
increasingly basic
(H  OH)
100
101
102
103
104
105
106
107
108
109
1010
1011
1012
1013
1014
H concentration in moles/liter 55

56. 2.3 Why Is Water So Important to Life?
Water-based solutions can be acidic, basic, or neutral (continued)
A buffer is a type of molecule that helps a solution maintain constant pH
Buffers accept or release H in response to a pH change
The bicarbonate buffer found in our bloodstream prevents pH changes

57. 2.3 Why Is Water So Important to Life?
Water-based solutions can be acidic, basic, or neutral (continued)
If the blood becomes too acidic, bicarbonate accepts (and absorbs) H to make carbonic acid
HCO3  H  H2CO3
bicarbonate hydrogen ion carbonic acid

58. 2.3 Why Is Water So Important to Life?
Water-based solutions can be acidic, basic, or neutral (continued)
If the blood becomes too basic, carbonic acid liberates hydrogen ions to combine with OH to form water
H2CO3  OH  HCO3  H2O
carbonic acid hydroxide ion bicarbonate water