This document discusses the chemical level of organization in living things. It describes the structure of atoms and the six main elements found in biological molecules. Atoms contain protons, neutrons and electrons. Elements are substances made of one type of atom. There are three main types of chemical bonds: ionic bonds form when atoms gain or lose electrons to become ions; covalent bonds form when atoms share electrons; and hydrogen bonds form weak attractions between molecules. These bonds allow atoms to combine into molecules and compounds that make up living things.

1. Chemistry of Life
Unit 1 Lesson 4

2. Chemistry of Life
Students will be able to:
• Describe the structure of atom and name the six elements found
in biological molecules.
• Describe the three main types of chemical bonding.
Key Vocabulary:
Anion, Atom, Cation, Chemical level, Compound, Covalent bond,
Electron, Element, Hydrogen bond, Ionic bond, Levels of
organization, Molecule, Neutron, Nucleus, Periodic table of
elements, Proton,

3. Chemistry of Life
• An organism can be broken down into
smaller levels of organization.
• These levels are pictured to the right.
• The smallest level of organization is the
chemical level where atoms join together
to form small molecules.

4. Chemistry of Life
The Chemical Level
• Atoms are the smallest units which
make up matter - this is the case in
both living and non-living things.
• The main components of the atom
are shown in the diagram to the right:

5. Chemistry of Life
The Atom
Atoms contain two regions:
1. The nucleus containing positively charged particles known as
protons and neutral, uncharged, particles known as neutrons.
2. Electron shells or orbitals contain negatively charged particles.
Electrons orbit the nucleus in different energy levels.

6. Chemistry of Life
The Atom
• The forces which operate between the oppositely charged
particles - protons and electrons - hold the atom together. Most
atoms contain all three types of subatomic particles. Hydrogen
atom (H) is an exception because it has one proton and one
electron and no neutrons.
• The number of protons in the nucleus determines the kind of the
element an atom is, this is known as the atomic number, while the
number of electrons determines how the atom reacts.

7. Chemistry of Life
Atomic Number
The Atom
• The number of protons in the nucleus
determines the kind of the element an
atom is, this is known as the atomic
number.
• Carbon has an atomic number of 6.
This means that it has 6 protons and 6
electrons

8. Chemistry of Life
Comprehension Check...
How many protons do each of these atoms
have?
20 7 13 11 9
Ca N Al Na F
40 14.007 26.982 22.99 18.98
Calcium Nitrogen Aluminum Sodium Fluorine

9. Chemistry of Life
Comprehension Check...
Answers
20 7 13 11 9
Ca N Al Na F
40 14.007 26.982 22.99 18.98
Calcium Nitrogen Aluminum Sodium Fluorine
20 7 13 11 9

10. Chemistry of Life
Elements
• An element is a substance which is made up of one type of atom.
For this reason, elements are called pure substances.
• There are 94 naturally occurring elements known to scientists.
These have been arranged on a chart called the periodic table
(shown on the next slide).

11. Chemistry of Life

12. Chemistry of Life
The main elements found in living things are:
• Carbon (C)
• Hydrogen (H)
• Oxygen (O)
• Sulfur (S)
• Nitrogen (N)
• Phosphorous (P)
C
O
H
N
P
S

13. Chemistry of Life
• It is the bonding between the atoms of these six elements that
makes all life possible.
• Chemical bonding involves atoms gaining, losing or sharing
electrons in order to form larger structures.
• This allows them to complete any partially full electron shells
and become more stable in the process.

14. Chemistry of Life
Molecules vs Compounds:
• When atoms bond they form molecules and compounds.
• Molecules are any group of atoms which are chemically
bonded together.
• Compounds are two or more different atoms bonded
together.
H H H
O
H
A hydrogen molecule Water is both a compound and a molecule

15. Chemistry of Life
Comprehension Check
Decide if the following or molecules, compounds or both.

16. Chemistry of Life
Answers...
Molecules
Molecules AND compounds

17. Chemistry of Life
Types of bonds:
• Atoms can form three different types of chemical bonds -
• ionic,
• covalent, and
• hydrogen bonds.

18. Chemistry of Life
1. Ionic bonds
• Ionic bonds are formed when
atoms gain or lose electrons and
become ions.
• When an atom donates an
electron from its outer shell, it
forms a positive ion called a
cation.
• Atoms which accept electrons
form negatively charged ions
known as anions.

19. Chemistry of Life
1. Ionic bonds
• Since opposite charges attract, the positive and negative ions
are pulled together and form an ionic bond.
• These bonds are strong and therefore require large amounts
of energy in order to be broken.
• An example of the exchange of electrons between a sodium
and chlorine atom can be seen in the diagram below.

20. Chemistry of Life
1. Ionic Bonds
• Generally, the donation of an electron by one atom cannot
happen without an electron recipient, so these two
processes of electron donation and electron acceptance
must occur at the same time.
• Certain ions such as sodium, potassium and calcium are
referred to electrolytes in biology. These ions are essential
for the function of the nervous system, muscle contraction
and water balance in the body.

21. Chemistry of Life
Comprehension Check...
Can you...
✓ Name the three types of bonding?
✓ Define ionic bond?
✓ Draw a diagram to show how an ionic bond forms?

22. Chemistry of Life
Comprehension Check: Answers
✓ Name the three types of bonding:
Ionic, covalent and hydrogen bonding
✓ Define ionic bond
A bond which forms through the loss and gain of
electrons
✓ Draw a diagram to show how an ionic bond form
See diagram to the right

23. Chemistry of Life
2. Covalent Bonds
• The covalent bonds form when
electrons are shared between atoms.
• This is commonly seen in carbon-based
molecules and also in water, an essential
molecule found in all living things.

24. Chemistry of Life
2. Covalent Bonds
• In the water molecule, each hydrogen
atom shares one electron with the
oxygen atom and the oxygen atom
shares two electrons (one electron
with each hydrogen atom).
• The shared electrons complete each
atom’s outermost (valence) shell,
making the water molecule more
stable than its component atoms.

25. Chemistry of Life
3. Hydrogen Bonds
• Hydrogen bonds are relatively
weak bonds which require
little energy to break.
• These bonds occur between
two molecules as a result of
the electrostatic attraction
between a proton in one
molecule and an electronegative
atom in the other.

26. Chemistry of Life
3. Hydrogen Bonds
• Both covalent and hydrogen
bonds can be seen in the section
of DNA molecule, shown below,
• The covalent bonds hold the
backbone of the DNA molecule
together, whereas the hydrogen
bonds act to stabilize the two
strands allowing it to form a
double helix.
Hydrogen Covalent
bond bond

27. Chemistry of Life
Comprehension Check
Can you…
✓ Define covalent bond?
✓ Name the main atom which forms covalent bonds?
✓ Describe how a hydrogen bond differs to a covalent
bond?

28. Chemistry of Life
Comprehension Check Answers:
✓ Define covalent bond:
A bond formed when electrons are shared
✓ Name the main atom which forms covalent bonds?
Carbon
✓ Describe how a hydrogen bond differs to a covalent bond?
Covalent bonds are strong whereas hydrogen bonds are weak