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Cecie Starr
Christine Evers
Lisa Starr
www.cengage.com/biology/starr
Chapter 2
Life’s Chemical Basis
(Sections 2.1 - 2.3)
Albia Dugger • Miami Dade College
2.2 Start With Atoms
• At life’s first level of organization, atoms interact with other
atoms to form molecules
• The properties of molecules depend on, but differ from, those
of their atomic components
• The behavior of elements, which make up all living things,
depends on the structure of individual atoms
Key Terms
• atom
• Particle that is a fundamental building block of all matter
• element
• A pure substance that consists only of atoms with the
same number of protons
Structure of an Atom
• Atoms consist of
electrons (negative
charge) moving about a
nucleus of protons
(positive charge) and
neutrons (uncharged)
Key Terms
• nucleus
• Core of an atom; occupied by protons and neutrons
• proton
• Positively charged subatomic particle that occurs in the
nucleus of all atoms
• neutron
• Uncharged subatomic particle in the atomic nucleus
Key Terms
• charge
• Electrical property
• Opposite charges attract, and like charges repel
• electron
• Negatively charged subatomic particle that occupies
orbitals around an atomic nucleus
Elements
• The number of protons in the nucleus defines the element
• The periodic table lists elements in order of atomic number
• Elements in columns behave in similar ways
Key Terms
• atomic number
• Number of protons in the atomic nucleus; determines the
element
• periodic table
• Tabular arrangement of known elements by atomic
number
The Periodic Table
ANIMATION: Electron distribution
Isotopes
• Isotopes are atoms of an element that differ in mass number
• isotopes
• Forms of an element that differ in the number of neutrons
their atoms carry
• mass number
• Total number of protons and neutrons in the nucleus of an
element’s atoms
Radioisotopes
• Atoms of radioisotopes spontaneously emit subatomic
particles or energy by the process of radioactive decay,
which can transform one element onto another
• Researchers use radioisotopes to make tracers that track
biological processes inside living organisms
Key Terms
• radioisotope
• Isotope with an unstable nucleus
• radioactive decay
• Process by which atoms of a radioisotope emit energy
and/or subatomic particles when their nucleus
spontaneously disintegrates
• tracer
• Molecule labeled with a detectable substance
Key Concepts
• Atoms and Elements
• Atoms, the building blocks of all matter, differ in their
numbers of protons, neutrons, and electrons
2.3 Why Electrons Matter
• Atoms acquire, share, and donate electrons
• Whether an atom will interact with other atoms depends on
how many electrons it has
• A typical atom has the same number of electrons as protons
Shell Models
• Shell models help us visualize how electrons populate atoms
from the innermost shell outward
• Concentric circles represent successive energy levels
• 1st shell: up to 2 electrons
• 2nd shell: up to 8 electrons
• 3rd shell: up to 8 electrons
Shell Models
A The first shell corresponds to the first
energy level, and it can hold up to 2
electrons. Hydrogen has one proton, so
it has 1 electron and 1 vacancy. A
helium atom has 2 protons, 2 electrons,
and no vacancies. The number of
protons in each model is shown.
B The second shell corresponds to
the second energy level, and it can
hold up to 8 electrons. Carbon has 6
protons, so its first shell is full. Its
second shell has 4 electrons, and
four vacancies. Oxygen has 8
protons and two vacancies. Neon has
10 protons and no vacancies.
first shell
second shell
C The third shell, which corresponds
to the third energy level, can hold up to
8 electrons. A sodium atom has 11
protons, so its first two shells are full;
third shell
the third shell has one electron. Thus,
sodium has seven vacancies. Chlorine
has 17 pro tons and one vacancy.
Argon has 18 protons and no
vacancies.
1 proton
1
1 electron
hydrogen (H)
2
helium (He)
6
8
carbon (C)
oxygen (O)
11
17
sodium (Na)
chlorine (Cl)
10
neon (Ne)
18
argon (Ar)
Stepped Art
Fig. 2.5, p. 26
ANIMATION: Shell models of common
elements
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Atoms and Ions
• An atom is most stable when it has no vacancies
• Atoms fill vacancies by gaining or losing electrons, or by
sharing electrons with other atoms
• An atom with a different number of electrons and protons
carries a charge, and is called an ion
Swapping Electrons
• Electronegativity is a measure of how strongly an atom
attracts electrons from other atoms
• An atom that gains an electron is a negative ion
• An atom that loses an electron is a positive ion
Ion Formation
electron gain
electron loss
Ion Formation
Chlorine
atom
17p+
17e–
_________
charge: 0
Sodium
atom
17
11
Chlorine
ion
17p+
18e–
___________
charge: –1
11p+
11e–
__________
charge: 0
Sodium
ion
17
A A chlorine atom (Cl) becomes a
negatively charged chloride ion
(Cl–) when it gains an electron
and fills the vacancy in its third,
outermost shell.
11
11p+
10e–
_________
charge: +1
B A sodium atom (Na) becomes a
positively charged sodium ion (Na+)
when it (Cl–) when it gains an electron
loses the electron in its third shell. The
atom’s full second shell is now its
outermost, so it has no vacancies.
Fig. 2.6, p. 27
electron gain
Ion Formation
Chlorine
atom
17p+
17e–
_________
charge: 0
17
Chlorine
ion
17p+
18e–
___________
charge: –1
17
A A chlorine atom (Cl) becomes a
negatively charged chloride ion
(Cl–) when it gains an electron
and fills the vacancy in its third,
outermost shell.
Fig. 2.6a, p. 27
electron loss
Ion Formation
Sodium
atom
11
11p+
11e–
__________
charge: 0
Sodium
ion
11
11p+
10e–
_________
charge: +1
B A sodium atom (Na) becomes a
positively charged sodium ion
(Na+) when it (Cl–) when it gains
an electron loses the electron in
its third shell. The atom’s full
second shell is now its
outermost, so it has no
Fig. 2.6b, p. 27
electron gain
electron loss
Ion Formation
Chlorine
atom
17p+
17e–
_________
charge: 0
Sodium
atom
17
11
Chlorine
ion
17p+
18e–
___________
charge: –1
11p+
11e–
__________
charge: 0
Sodium
ion
17
A A chlorine atom (Cl) becomes a
negatively charged chloride ion
(Cl–) when it gains an electron
and fills the vacancy in its third,
outermost shell.
11
11p+
10e–
_________
charge: +1
B A sodium atom (Na) becomes a
positively charged sodium ion (Na+)
when it (Cl–) when it gains an electron
loses the electron in its third shell. The
atom’s full second shell is now its
outermost, so it has no vacancies. Stepped Art
Fig. 2.6, p. 27
ANIMATION: Ionic bonding
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Key Terms
• shell model
• Model of electron distribution in an atom
• ion
• Charged atom
• electronegativity
• Measure of the ability of an atom to pull electrons away
from other atoms
Sharing Electrons
• A chemical bond is an attractive force that unites two atoms
as a molecule
• A compound is a molecule that consists of two or more
elements
• A mixture is an intermingling of substances
Key Terms
• chemical bond
• An attractive force that arises between two atoms when
their electrons interact
• molecule
• Group of two or more atoms joined by chemical bonds
Key Terms
• compound
• Type of molecule that has atoms of more than one
element
• mixture
• An intermingling of two or more types of molecules
• A liquid mixture is a solution
Key Concepts
• Why Electrons Matter
• How an atom interacts with other atoms depends on the
number and arrangement of its electrons
ANIMATION: The shell model of electron
distribution
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ANIMATION: How atoms bond
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