This document provides an overview of key concepts related to the chemical basis of life, including: 1) Elements are the fundamental forms of matter that cannot be broken down further through normal means. The most common elements in the human body are oxygen, hydrogen, carbon, nitrogen, and calcium. 2) Atoms are made up of subatomic particles including protons, neutrons, and electrons. The number of protons determines the element, while the number of neutrons can vary between isotopes of the same element. 3) Electrons play an important role in how atoms interact, forming ions by gaining or losing electrons or bonding through electron sharing. The organization of the periodic table is based on electron configurations.

1. Life’s Chemical Basis
Chapter 2

2. Elements
• Fundamental forms of
matter
• Can’t be broken apart
by normal means
• 92 occur naturally on
Earth

3. Most Common Elements in Human
Body (98%)
Oxygen
Hydrogen
Carbon
Nitrogen
Calcium

4. What Are Atoms?
• Smallest particles that retain
properties of an element
• Made up of subatomic particles:
– Protons (+) located in the nucleus
– Neutrons (no charge) located in
the nucleus
– Electrons (-) located in space
around the nucleus

5. Atomic Number
(whole # on the periodic table)
• = the # of protons
• All atoms of a specific element have
the same atomic #
• Atomic # of hydrogen = 1
• Atomic # of carbon = 6
• Atomic # of helium = 2
• If you change the # of protons, you
change the atom into a different
element

6. Atomic Mass
(decimal # on the periodic table)
Number of protons
+
Number of neutrons
So why is it a decimal
number? You’ll soon
find out….

7. What is the role of neutrons?
• Since protons have a +
charge, they repel
each other
• Neutrons are neutral
and help to stabilize
the nucleus – think
police

8. The Organization of the Periodic Table:
Based on ELECTRONS!

9. Why Electrons Matter
• Atoms acquire, share, and
donate electrons
• Whether an atom will
interact with other atoms
depends on how many
electrons it has

10. Electrons
• Carry a negative charge
• Also repel one another
• Attracted to protons in the
nucleus
• Move in volumes of space
that surround the nucleus

11. Shell Model
• The shell model of electron orbitals
diagrams electron vacancies, filled from
inside out
• Each shell represents an energy level:
– 1st shell: (2 electrons)
– 2nd shell: (8 electrons)
– 3rd shell: (8 electrons)
• Atoms with vacancies in their outer shell CALCIUM
tend to give up, acquire, or share electrons 20p+ , 20e-

12. Electron Vacancies
• Unfilled levels make
atoms more likely to
react with other atoms
CARBON NITROGEN
• Hydrogen, carbon, and 6p+ , 6e- 7p+ , 7e-
nitrogen are examples of
elements with vacancies
in their outer levels
HYDROGEN
1p+ , 1e-

13. Ion Formation
• Atoms that lose electron(s),
become positively charged ions
(cations)
• Atoms that gain electron(s),
become negatively charged
ions (anions)
• Electronegativity:
– A measure of an atom’s
ability to pull electrons from
another atom

14. Atomic Structure: Helium
electron proton
- -
neutron
He

15. Atomic Structure: Nitrogen
N -
-
- -
- -
-

16. Atomic Structure of Hydrogen
HYDROGEN
electron
proton
neutron

17. Isotopes: What are They?
• Atoms of an element with the SAME # of
protons, but a DIFFERENT # of neutrons
• The atomic mass # on the periodic table is
an avg. mass of all the different isotopes
possible for the element
“Carbon 12” has 6
protons, 6 neutrons
• How do you figure out the number of
“Carbon 14” has 6
neutrons in an atom? protons, 8 neutrons
Atomic mass – atomic number = #
neutrons

18. Special Isotopes: Radioactive Isotopes
• Have an unstable nucleus
that emits energy or some
of the subatomic particles
(protons or neutrons)
• Some are used in research
and medical applications
(pacemakers, radiation
therapy, MRI)

19. Radioactive Isotopes Can be Used
as Tracers
• A tracer is a substance with a radioactive isotope attached
to it
• Emissions from the tracer can be detected with special
devices
• Following the movement of tracers is useful in many areas
of biology and medicine

20. Next Lecture…
Chemical Bonds!

21. Lab 3: Making Conversions Between Various
Measuring Systems
• Purpose of this lab:
To become familiar
with several systems of
measurement and
make conversions from
one unit to another as
well as learn to read
syringes and graduated
cylinders