BIOLOGY 2E

Chapter 2 THE CHEMICAL FOUNDATION OF LIFE

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IN THE BEGINNING…

Beginning = Time
Heaven = Space
Earth = Matter

In the Beginning, God Created the Heaven and

the Earth
Genesis 1:1
ATOMS ARE THE BUILDING
BLOCKS OF MOLECULES

• Life is composed of matter

• Matter:
• occupies space
• has mass

• Elements are unique forms of matter

1. Specific chemical properties
2. Specific physical properties
ELEMENTS AND THE LIVING WORLD

• Elements cannot be broken down into smaller substances

• Each element is designated by a chemical symbol (one or
two letters)
• Sulfur = S
• Calcium = Ca

• Four most common elements of living organisms

• Carbon C
• Oxygen = O
• Hydrogen = H
• Nitrogen = N
ELEMENTS AND THE LIVING WORLD

Comparing the elements of living

organisms to the non-living world

Element Life Atmosphere Earth’s Crust

(Humans)

Oxygen (O) 65% 21% 46%

Carbon (C) 18% trace trace

Hydrogen (H) 10% trace 0.1%

Nitrogen (N) 3% 78% trace

 ATOMS ARE THE BUILDING
BLOCKS OF ELEMENTS
Atom – smallest unit of matter that retains all
chemical properties of an element

Atoms contains two regions

1. Nucleus – center of the atom and contains
protons and neutrons
2. Outermost region – holds electrons in orbit
around the nucleus

Protons, neutrons, and electrons are referred to

as sub-atomic particles
KEY PROPERTIES OF SUB-ATOMIC PARTICLES
Charge Mass (amu) Location

Proton +1 1 nucleus

Neutron 0 1 nucleus

Electron -1 0 orbitals
 THE ARRANGEMENT OF SUB-ATOMIC
PARTICLES IN AN ATOM
Example of an atom
from the element
Helium (He)

Simplified depiction of how sub-atomic particles are

arranged within an atom.
 WHAT’S THE DIFFERENCE BETWEEN
ATOMIC NUMBER AND ATOMIC MASS?

Atoms of each element have a standard number of protons

and electrons
• Atomic Number is the number of protons
• Each element has a distinct atomic number

• Atomic Mass is the mass of the atom, roughly equal to

number of protons and neutrons
• The number of neutrons can vary in an element
• Elements with different number of neutrons are isotopes
• Electrons don’t need to be included in calculation of atomic mass
• Atomic mass is expressed in atomic mass units (amu)

• We can calculate the number of neutrons in an element by

subtracting the atomic number from the atomic mass
FIGURE 2.3 ATOMIC MASS VS. ATOMIC NUMBER

Carbon has an atomic number of six, and two stable

isotopes with mass numbers of twelve and thirteen,
respectively. Carbon-12’s atomic mass is approx. 12.11.
 ISOTOPES
Forms of an element with different numbers of
neutrons, and thus different mass numbers
Example:
1H has 0 neutrons

2H has 1 neutron
3H has 2 neutrons
ISOTOPE SIMULATION

https://phet.colorado.edu/sims/html/isotopes-and-atomic-
mass/latest/isotopes-and-atomic-mass_en.html
 ISOTOPES CAN BE USED AS A RESEARCH TOOL

Radioisotopes – isotopes that

emit neutrons, protons, and
electrons

Radiometric dating takes

advantage of this natural
phenomenon

Example: Over time Carbon-14

decays to Nitrogen-14.
Researchers can compare
Carbon-14 in atmosphere to
carbon-14 in fossils remains and
estimate fossil age

The age of carbon-containing remains less than about 50,000 years old, such as this
pygmy mammoth, can be determined using carbon dating.
PERIODIC TABLE

The periodic table shows the atomic mass and atomic number of each element. The
atomic number appears above the symbol for the element and the approximate atomic
mass appears below it.
ELECTRON SHELLS AND THE BOHR MODEL

• Atoms with neutral charge

• Number of protons = number of electrons
• Number of electrons = atomic number

Bohr model – early model with protons in nucleus and

electrons in circular orbits at specific distance from
nucleus

Orbits: electron shells or energy levels

• An electron normally exists in the lowest available
energy shell (closest to the nucleus)
• Electrons fill orbitals closest to nucleus first, then those
further away in order (i.e. 1n fills first, then 2n, then 3n,
etc
FIGURE 2.7 – ELECTRONS FILLING THEIR SHELLS

Outer shell: Valence shell

• The most stable configuration occurs when the valence shell is filled
• The elements in group 18 of this diagram have full valence shells
FIGURE 2.7 – ELECTRONS FILLING THEIR SHELLS

When the first two outer shells are filled each will have eight electrons (octet rule)
• Group 1 elements (H, Li, NA) could achieve stability by losing an outer electron
• Group 17 elements could achieve stability by gaining one additional electron
WHAT ABOUT ELECTRON ORBITALS?

• Bohr models are incomplete

• Electrons are not in planet-like orbits
• Electron orbitals – complex shapes that
describe how electrons are spatially distributed
around the nucleus
• Quantum mechanics allows us to predict where
an electron might be at any given time
• This predicted area is known as the orbital
FIGURE 2.8 SUBSHELLS HAVE UNIQUE SHAPES

Within each Bohr model shell are subshells

Each subshell (s,p,d,f) has a specified number of orbitals containing
electrons
The s subshell is spherical and has one orbital
The p subshell is dumbbell shaped and has three orbitals
ORBITALS: ELECTRON CONFIGURATION OF
SCANDIUM

https://youtu.be/sMt5Dcex0kg
ELECTRONS DETERMINE
HOW ATOMS INTERACT

Chemical reactions are changes in

distribution of electrons between atoms
CHEMICAL REACTIONS

• Reactants – substances used at

beginning of reaction
• Products – substances formed at the end
of the reaction

2H2O2 2H2O + O2
Reactants Products
 CHEMICAL REACTION CAN BE
REVERSIBLE OR IRREVERSIBLE

2H2O2 2H2O + O2
Irreversible reaction: proceeds in one
direction until all the reactants are used up

HCO3- + H+ H2CO3
Reversible reaction: Reactants are
converted to products but some product can
be converted back to reactant
HOW DO ATOMS BOND TO FORM
MOLECULES?

Chemical bond: the attractive force that links

atoms together to form molecules
2.9 COVALENT BONDS – ELECTRONS ARE SHARED

Two or more atoms may bond with each other to form a

molecule.
When two hydrogens and an oxygen share electrons via
covalent bonds, a water molecule is formed.
MORE THAN ONE SET OF
ELECTRONS CAN BE SHARED

The oxygen atoms in an O2 molecule are joined by a

double bond.
IONIC BONDS – ATOMS GIVE UP OR GAIN
ELECTRONS

In the formation of an ionic compound, metals lose

electrons and nonmetals gain electrons to achieve an
octet.
POLAR BONDS–COVALENT BONDS IN WHICH THERE
IS UNEQUAL SHARING OF THE ELECTRONS

Polar Covalent Bonds

Electrons are unequally shared by
the atoms and attracted more to one
nucleus than the other

Water is a great example

Oxygen has higher
electronegativity than hydrogen
This means it wants the electrons
more than than hydrogen does
NON-POLAR BONDS–COVALENT BONDS IN WHICH
THERE IS EQUAL SHARING OF THE ELECTRONS

Non-Polar Covalent Bonds

Electrons are equally shared
by the atoms
Methane (CH4) is a great
example
Carbon shares electrons with
four hydrogens equally

Why is carbon dioxide a Both bond type and

nonpolar molecule even though molecular shape determine
the oxygen-carbon bond is a whether a molecule is polar or
polar covalent bond? nonpolar
AN EXAMPLE OF MIXING A POLAR COMPOUND WITH
A NONPOLAR COMPOUND

Oil and water do not mix. As this macro image of oil and water shows, oil
does not dissolve in water but forms droplets instead. This is due to it being a
nonpolar compound. (credit: Gautam Dogra).
 HYDROGEN BONDS AND
VANDER WAALS INTERACTIONS

• Ionic and Covalent bonds – require energy to break

Weaker bonds
Hydrogen bonds – interaction between the δ+ of hydrogen
and the δ- of a more electronegative atom on another
molecule – often occurs between water molecules

Van der Waals interactions – weak attractions or

interactions between two or more molecules (in close
proximity) due to changes in electron density