Chemical Reactions
Chemical reactions happen when atoms give electrons to
another atom, take electrons from another atom, or share
electrons.
Electrons orbit the nucleus of an atom in energy shells. For the
most stable atom, the outer shell needs to be full of electrons. To
get a full shell, atoms will give, take or share electrons with other
atoms. This causes a chemical reaction and forms a chemical
bond between the combined atoms. The new substance(s)
formed will have different properties.
Here are some examples:
Sodium (Na) has 11 electrons:
2 filling its first shell, 8 more filling its second shell, and one all alone in its’ 3rd shell.
To become more stable, sodium will easily give up that single electron so it will have a full shell. The
extra electron will go to another atom that needs an electron to have a full shell.
Chlorine has 17 electrons, and needs another electron to complete its 3rd energy shell to become
stable. If it takes one electron from sodium it forms a bond between the atoms of sodium and
chlorine, and creates a new compound; sodium chloride, or table salt.
If lithium gives
one valence
electron to florine,
they stabilize one
another and form
lithium floride.
If one
calcium atom
gives an
electron to each of
2 chlorine atoms, it
stabilizes all,
3 and creates
calcium
chloride
Chemical bonds formed when electrons are transferred from one atom to another are called ionic
bonds. When an atom donates an electron, it becomes positively charged, because it now has more positively
charged protons than negatively charged electrons. The atom that accepts the electron becomes negatively
charged. These charged particles are called ions. Since opposite electrical charges attract, the ions bond to
each other in a bond called an ionic bond. Ionic bonding generally takes place between a metal and a
nonmetal.
Sometimes, rather than exchange electrons, atoms simply share electrons. Chemical bonds formed when
electrons are shared between atoms are covalent bonds. The prefix co- means together/jointly (as in
cooperate ,copilot,costar) , so co-valent bonds are bonds from joined/shared valences.)
Oxygen is missing 2 electrons in its outer shell. By
sharing electrons with another oxygen atom, it creates
O2 and becomes stable.
Hydrogen atoms
lack one valence
electron. Oxygen
lacks 2. If one
oxygen atom combines
with 2 hydrogen atoms,
they stabilize one another,
and form a water molecule.
When atoms change their electrons, it causes chemical reactions which create new substances with
different properties. They are not just a physical change.
Physical Change
Molecules stay the same. The size, shape or phase of a
substance may change, but a new chemical is not
formed.
Ex: * water is still H2O if you freeze it.
* paper is still paper if you crunch it up, or cut it
into tiny pieces.
Here are some changes that are physical:
*Changes in size or shape
*Changes in phase
*Changes in density (for example, food coloring
dropped in water becomes less dense, but the
molecules of the food coloring and watert
stay the same)
Can be caused by cutting, grinding, bending, freezing,
melting, mixing (if mixture parts of the mixture stay
molecularly separate)
Chemical Reaction
Molecules change and a new substance with different
properties is formed—The substance(s) present at the
beginning of the reaction are no longer there at the
end.
Ex. *water that reacts with carbon dioxide becomes
carbonic acid, and is not water or carbon dioxide
any more.
* paper isn’t paper if it has burned to ash
Here are some signs that a chemical reaction may be
happening:
*Temperature changes from the reaction
*Sound, light or odor is produced
*Gas is produced (fizzes or bubbles)
*Precipitate forms (solid formed from 2 liquids)
*Color change (this may also happen in some
physical changes)
Here is a memory tool that may help you”
Chemical reactions may leave you AGAPE.
__Altered molecules
__Gas formed
__Aroma, color changes
__Precipitate formed
__Energy released/absorbed
Can be caused by burning, rusting, cooking,
fermentation,