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Lab 3: Chemistry
History of the atom
Democritus- 2500 years ago
If you take a stick of chalk and break it in half, is it still
chalk?
What if you break it in half again?
Thought there had to be a limit
Called this the ATOM
Means “unable to be cut”
Atom Anatomy
Protons and Neutrons
Protons- positively charged particles (+)
Neutrons- carry no charge (0)
Held inside the nucleus
Electrons
Electron- negatively charged particles (-)
In constant motion in the space surrounding the
nucleus
Elements and Isotopes
Elements
Pure substance that consists entirely of one type of atom
More than 100 named elements (92 found naturally)
Only about 2 dozen are found commonly in living
organisms
Represented by one- or two- letter symbols
C- Carbon
Na- Sodium
Hg- Mercury
Atomic Number
An element’s atomic number tells you a lot about the
element!
The atomic number tells you the number of protons
and electrons
The atomic mass tells you the total number of protons
and average number of neutrons
If an element gains or loses a proton, it is no longer the
same element!
Practice!
The element Tungsten is pictured below.
How many Protons does this element have?
How many Electrons?
How many Neutrons?
Isotopes
Sometimes elements have different numbers of
neutrons.
All atoms of Carbon have 6 protons and 6 electrons,
but some have 6 neutrons, or 7 neutrons.
When elements differ in the number of neutrons, they
are called isotopes
Carbon Isotopes
Isotope
Number of
Protons
Number of
Electrons
Number of
Neutrons
Carbon- 12
(nonradioactive)
6
6
6
Carbon- 13
(nonradioactive)
6
6
7
Carbon- 14
(radioactive)
6
6
8
Why Care About Isotopes?
Radioactivity
The nuclei of certain isotopes are unstable, and break
down into a more stable form
Some radiation the isotopes give off can be dangerous,
but radioactive isotopes have a number of important
uses
Dating
Geologists determine the ages of rocks and fossils by
analyzing the isotopes found in them
Bonding!
Most elements are found combined with other
elements!
A COMPOUND is a substance formed by the
combination of two or more elements
Example: Water (H2O)
Contains two Hydrogen atoms and one Oxygen atom
Types of Bonding
When compounds form, the chemical properties
usually change from their elemental properties
Example: Sodium Chloride (NaCl)
Sodium
Silver-colored metal that reacts violently with water
Chlorine
Poisonous yellow-green gas used in battles of WW1
Sodium Chloride
Table salt
Ionic Bonds
Formed when one or more electrons are transferred
from one atom to another
Ions
Atoms that gain or lose electrons
Atoms become negatively charged when they GAIN
electrons
Atoms become positively charged when they LOSE
electrons
Ionic Bonds
Positively and negatively charged ions are attracted to
each other
Ionic Bonding
Covalent Bonding
Electrons are shared instead of being transferred
Bond Number
Share two electrons Single bond
Share four electrons Double bond
Share six electrons Triple bond
Covalent Bond
Van der Waals Forces
Some atoms have a stronger attraction for electrons
Produces an uneven charge between atoms
Can hold molecules together (like water)
Polarity
Uneven electron sharing in covalent bonds
Creates partial positive and partial negative charges on
atoms
Hydrogen Bonding
Covalent bonds are very strong
When H atoms are partially positive, it allows them to
share attractions with other electronegative atoms.
This is called a Hydrogen bond.
Each H2O molecule can bond up to four times this way
Important in how proteins form their shapes
Cohesion
Hydrogen bonds last for only a short amount of time
before they switch molecules.
Cohesion occurs when molecules of the same type are
attracted to each other and “stick together”.
Water is incredibly cohesive because of its tendency to
be attracted to other water molecules
Cohesion
Importance
Transports water and nutrients from the roots to the
leaves
Evaporation from leaves pulls water from the roots
through the veins
Surface tension
Measure of how difficult it is to stretch or break the
surface of a liquid
Hydrogen bonds give water high surface tension
Water Striders take advantage
of cohesion
Adhesion
Water can “stick” to other molecules as well. This is
called adhesion
High Heat Capacity
Water heats slower than many metals
Hydrogen bonding resists temperature changes
When heat is added to the water, the hydrogen bonds
break, and water absorbs and stores that heat, while
slowly raising the temperature
When heat is removed, more hydrogen bonds form.
Heat energy is released and the cooling process is slowed
Density
Because of hydrogen bonding, Ice becomes less dense
than liquid water
When water freezes, the molecules move slower, giving
more opportunity for Hydrogen bonding to occur.
Causes the atoms to be held at “arms length” giving
less mass in an equivalent volume
Universal Solvent
Solution
Liquid consisting of a uniform mixture of two or more
substances
Dissolving agent is the solvent
The substance being dissolved is the solute
Water’s polarity helps it dissolve other polar and ionic
compounds
Salt in Water
NaCl is dissolved in water
+ charged sodium is attracted to the – charged oxygen
- charged chlorine is attracted to the + charged hydrogen
The ionic bond is broken and the salt dissolves!