1. Ch.2 “Chemical Context of Life”

2. Concept 2.1: Matter consists of chemical
elements in pure form and in combinations
called compounds
• Matter
– “anything that takes up space and has mass”
– 3 Forms
• Solid
• Liquid
• Gas
– Composed of elements

3. • Element
– A substance that cannot be broken down to other
substances by chemical reactions
– Composed of atoms
• Atom: smallest component of an element, contains
nucleus of neutrons and protons, and electron cloud

4. • Compound
– A substance consisting of two or more different
elements combined in a fixed ratio.
• Example: NaCl (table salt)
– 1:1 ratio of Sodium (Na) to Chloride (Cl)
• Example: H20 (water)
– 2:1 ratio of Hydrogen (H) to Oxygen (O)

5. Elements-Table 2.1
• 92 Natural Elements (118 total)
– Essential elements: 20-25% of natural elements are
needed by organisms to live a healthy life and
reproduce.
• Vary amongst organisms
• 4 elements: Oxygen, Carbon, Hydrogen, and Nitrogen (96%)
• Remaining elements: Calcium, Phosphorus, Potassium, etc.
– Trace elements: required by an organism in small
quantities
• Iron (Fe) needed by all forms of life, others only needed by
certain organisms

6. Concept 2.2: An element’s properties
depend on the structure of its atoms
• Atom: smallest unit of matter that still retains
the property of an element (no chemical
change)
• Subatomic particles: form the atom
– Neutrons (neutral), protons (+), and electrons (-)

7. Figure 2.5
Cloud of negative
charge (2 electrons)
Electrons
Nucleus
(a)
(b)

8. • Neutrons=Protons= ~1.7x10-24 g
• Very small unit to measure
– Assigned term “Dalton”
– Neutron=Proton=1 Dalton
– Electron=1/2000 Dalton **Not used when
calculating atomic mass because it’s so small**

9. Atomic Number and Atomic Mass
• Atomic number: number of protons
– Unique to each element
– Charge is considered neutral on each atom
– Number of protons=number of electrons
• Mass number: sum of neutrons and protons in
the nucleus of the atom. ~22.99