1. The Chemical Context of Life
Chapter 2
The Chemical Context of Life

2. The Energy Levels of Electrons
Energy is the capacity to cause change
Potential energy is the energy that matter has because of its location or structure
The electrons of an atom differ in their amounts of potential energy
An electron’s state of potential energy is called its energy level, or electron shell
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3. A ball bouncing down a flight of stairs provides an analogy
Figure 2.8
(a)
A ball bouncing down a flight
of stairs provides an analogy
for energy levels of electrons.
Third shell (highest energy
level in this model)
Energy
absorbed
Second shell (higher
energy level)
Figure 2.8 Energy levels of an atom’s electrons. Electrons exist only at fixed levels of potential energy called electron shells.
First shell (lowest energy
level)
Energy
lost
Atomic
nucleus
(b)

4. Electron Distribution and Chemical Properties
The chemical behavior of an atom is determined by the distribution of electrons in electron shells
The periodic table of the elements shows the electron distribution for each element
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5. Hydrogen 1H 2 He 4.00 Atomic number Helium 2He Mass number
Figure 2.9
Hydrogen 1H 2 He
4.00
Atomic number
Helium
2He
Mass number
Element symbol
First
shell
Electron
distribution
diagram
Lithium
3Li
Beryllium
4Be
Boron 5B
Carbon 6C
Nitrogen 7N
Oxygen 8O
Fluorine 9F
Neon
10Ne
Second
shell
Sodium
11Na
Magnesium
12Mg
Aluminum
13Al
Silicon
14Si
Phosphorus
15P
Sulfur
16S
Chlorine
17Cl
Argon
18Ar
Figure 2.9 Electron distribution diagrams for the first 18 elements in the periodic table. In a standard periodic table (see Appendix B), information for each element is presented as shown for helium in the inset. In the diagrams in this table, electrons are represented as yellow dots and electron shells as concentric circles. These diagrams are a convenient way to picture the distribution of an atom’s electrons among its electron shells, but these simplified models do not accurately represent the shape of the atom or the location of its electrons. The elements are arranged in rows, each representing the filling of an electron shell. As electrons are added, they occupy the lowest available shell.
Third
shell

6. Valence electrons are those in the outermost shell, or valence shell
The chemical behavior of an atom is mostly determined by the valence electrons
Elements with a full valence shell are chemically inert
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7. First shell Second shell
Figure 2.10a
First shell
Neon, with two filled
Shells (10 electrons)
Second shell
(a) Electron distribution diagram
Figure 2.10 Electron orbitals.

8. Concept 2.3: The formation and function of molecules depend on chemical bonding between atoms
Atoms with incomplete valence shells can share or transfer valence electrons with certain other atoms
These interactions usually result in atoms staying close together, held by attractions called chemical bonds
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9. Covalent Bonds
A covalent bond is the sharing of a pair of valence electrons by two atoms
In a covalent bond, the shared electrons count as part of each atom’s valence shell
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10. A molecule consists of two or more atoms held together by covalent bonds
A single covalent bond, or single bond, is the sharing of one pair of valence electrons
A double covalent bond, or double bond, is the sharing of two pairs of valence electrons
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11. This can be abbreviated further with a molecular formula
The notation used to represent atoms and bonding is called a structural formula
For example, H—H
This can be abbreviated further with a molecular formula
For example, H2
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12. Animation: Covalent Bonds
Right-click slide / select “Play”
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13. Figure 2.12
Name and
Molecular
Formula
Electron
Distribution
Diagram
Lewis Dot
Structure and
Structural
Formula
Space-
Filling
Model
(a) Hydrogen (H2)
(b) Oxygen (O2)
(c) Water (H2O)
Figure 2.12 Covalent bonding in four molecules. The number of electrons required to complete an atom’s valence shell generally determines how many covalent bonds that atom will form. This figure shows several ways of indicating covalent bonds.
(d) Methane (CH4)

14. A compound is a combination of two or more different elements
Covalent bonds can form between atoms of the same element or atoms of different elements
A compound is a combination of two or more different elements
Bonding capacity is called the atom’s valence
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15. Atoms in a molecule attract electrons to varying degrees
Electronegativity is an atom’s attraction for the electrons in a covalent bond
The more electronegative an atom, the more strongly it pulls shared electrons toward itself
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16. In a nonpolar covalent bond, the atoms share the electron equally
In a polar covalent bond, one atom is more electronegative, and the atoms do not share the electron equally
Unequal sharing of electrons causes a partial positive or negative charge for each atom or molecule
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17. Figure 2.13 – O H H + +
Figure 2.13 Polar covalent bonds in a water molecule.
H2O

18. Ionic Bonds
Atoms sometimes strip electrons from their bonding partners
An example is the transfer of an electron from sodium to chlorine
After the transfer of an electron, both atoms have charges
A charged atom (or molecule) is called an ion
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19. Na Sodium atom Cl Chlorine atom
Figure Na
Sodium atom Cl
Chlorine atom
Figure 2.14 Electron transfer and ionic bonding. The attraction between oppositely charged atoms, or ions, is an ionic bond. An ionic bond can form between any two oppositely charged ions, even if they have not been formed by transfer of an electron from one to the other.

20. Sodium chloride (NaCl)
Figure + – Na
Sodium atom Cl
Chlorine atom
Na+
Sodium ion
(a cation)
Cl–
Chloride ion
(an anion)
Figure 2.14 Electron transfer and ionic bonding. The attraction between oppositely charged atoms, or ions, is an ionic bond. An ionic bond can form between any two oppositely charged ions, even if they have not been formed by transfer of an electron from one to the other.
Sodium chloride (NaCl)

21. A cation is a positively charged ion
An anion is a negatively charged ion
An ionic bond is an attraction between an anion and a cation
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22. Animation: Ionic Bonds
Right-click slide / select “Play”
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23. Compounds formed by ionic bonds are called ionic compounds, or salts
Salts, such as sodium chloride (table salt), are often found in nature as crystals
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24. Figure 2.15
Na+
Cl–
Figure 2.15 A sodium chloride (NaCl) crystal. The sodium ions (Na+) and chloride ions (Cl–) are held together by ionic bonds. The formula NaCl tells us that the ratio of Na+ to Cl– is 1:1.

25. Figure 2.15a
Figure 2.15 A sodium chloride (NaCl) crystal. The sodium ions (Na+) and chloride ions (Cl–) are held together by ionic bonds. The formula NaCl tells us that the ratio of Na+ to Cl– is 1:1.

26. Figure 2.15b
Na+
Cl–
Figure 2.15 A sodium chloride (NaCl) crystal. The sodium ions (Na+) and chloride ions (Cl–) are held together by ionic bonds. The formula NaCl tells us that the ratio of Na+ to Cl– is 1:1.

27. Hydrogen Bonds
A hydrogen bond forms when a hydrogen atom covalently bonded to one electronegative atom is also attracted to another electronegative atom
In living cells, the electronegative partners are usually oxygen or nitrogen atoms
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28. – + Water (H2O) + Hydrogen bond – Ammonia (NH3) + + +
Figure 2.16 – +
Water (H2O) +
Hydrogen bond –
Ammonia (NH3)
Figure 2.16 A hydrogen bond. + + +