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Seminar:
Elements, Compounds, and
Chemical Reactions
Freddie Arocho
SC155: Introduction to Chemistry
First Periodic Table
• Dmitri Mendeleev (18341907) was born in Tobolsk, in
Western Siberia.
• His chief contribution to
chemistry was the
establishment of the periodic
system of elements.
• Mendeleev was one of the
discoverers of the periodic
law in the 1860s. Other
contributors were Newlands,
Meyer, and De
Chantcourtois.
First Periodic Table
• The table and the
accompanying observations
were first presented to the
Russian Chemical Society
in March 1869.
• Some notes:
 “By ordering the
elements according to
increasing atomic weight
in vertical rows so that
the horizontal rows
contain analogous
elements”.
• Living things are mainly composed of
the elements carbon (C), hydrogen (H),
oxygen (O), and nitrogen (N).
Compounds
• Most elements do not exist by themselves.
• Elements combine with other elements.
• A compound is a pure substance made up of atoms
of two or more elements.
• Chemical formula shows the kind and proportion of
atoms of each element that occurs in a particular
compound.
Compounds
• Molecules are the simplest part of a substance that
retains all of the properties of the substance.
• Subscript after a symbol tell the number of atoms
of each element.
• H2O: 2 atoms of hydrogen and 1 atom of oxygen.
• Coefficients before a formula tell the number of
molecules.
• 2 H2O : 2 molecules of H2O.
Subscripts and Coefficients
Chemical Reactions
• A process in which at least one new substance
is produced as a result of chemical change.
• Atoms are rearranged to form one or more
different substances.
• Chemical reactions involve changes in matter,
the making of new materials with new
properties, and energy changes.
Chemical Equations
• Chemical equations show the conversion of
reactants (the molecules shown on the left of
the arrow) into products (the molecules shown
on the right of the arrow).
• “+” sign separates molecules.
• “->” is read as “yields”.
• Example:
C + O2 -> CO2
This reads “carbon plus oxygen react to yield carbon
dioxide”.
• The charcoal used in a grill is basically
carbon. The carbon reacts with oxygen to
yield carbon dioxide. The chemical equation
for this reaction, C + O2 -> CO2, contains the
same information as the English sentence but
has quantitative meaning as well.
Chemical Equations
Chemical Equations
Chemical Equations
• Law of Conservation of Matter: Matter is
not created nor destroyed; it is
transformed.
• Example: Decomposition of NaCl
Cl Na

Cl
+
Na
Chemical Equations
• Example: MgO + CaS
Mg
O
+
Ca
S

Mg
S
+
Ca
O
Mass Relations
• Atomic Mass vs. Molecular Mass:
 Atomic Mass: Mass of the atom of an element.
 Molecular Mass: Mass of a molecule; Sum of the atomic
masses of the atoms in a molecule.
 amu: atomic mass units.
• From the Periodic Table, we can obtain atomic masses:
C = 12.01 amu
Ca = 40.08 amu
H = 1.008 amu
K = 39.10 amu
P = 30.97 amu
S = 32.07 amu
O = 16.00 amu
Mass Relations
• Atomic masses:
C = 12.01 amu
Ca = 40.08 amu
H = 1.008 amu
K = 39.10 amu
P = 30.97 amu
S = 32.07 amu
O = 16.00 amu
• Calculate the molecular mass of H2O:
H: 2 atoms X 1.008 = 2.016
O: 1 atom X 16.00 = 16.00
Total: 18.016 ~ 18.02 amu
Mass Relations
• Atomic masses:
C = 12.01 amu
Ca = 40.08 amu
H = 1.008 amu
K = 39.10 amu
P = 30.97 amu
S = 32.07 amu
O = 16.00 amu
• Calculate the molecular mass of CO2:
C: 1 atom X 12.01 = 12.01
O: 2 atoms X 16.00 = 32.00
Total: 44.01 amu
Mass Relations
• Atomic masses:
C = 12.01 amu
Ca = 40.08 amu
H = 1.008 amu
K = 39.10 amu
P = 30.97 amu
S = 32.07 amu
O = 16.00 amu
• Calculate the molecular mass of CH4:
Mass Relations
• Atomic masses:
C = 12.01 amu
Ca = 40.08 amu
H = 1.008 amu
K = 39.10 amu
P = 30.97 amu
S = 32.07 amu
O = 16.00 amu
• Calculate the molecular mass of CH4:
C: 1 atom X 12.01 = 12.01
H: 4 atoms X 1.008 = 4.032
Total: 16.042 ~ 16.04 amu
Mass Relations
• Atomic masses:
C = 12.01 amu
Ca = 40.08 amu
H = 1.008 amu
K = 39.10 amu
P = 30.97 amu
S = 32.07 amu
O = 16.00 amu
• Calculate the molecular mass of K2SO4:
Mass Relations
• Atomic masses:
C = 12.01 amu
Ca = 40.08 amu
H = 1.008 amu
K = 39.10 amu
P = 30.97 amu
S = 32.07 amu
O = 16.00 amu
• Calculate the molecular mass of K2SO4:
K: 2 atoms X 39.10 = 78.20
S: 1 atom X 32.07 = 32.07
O: 4 atoms X 16.00 = 64.00
Total: 174.27 amu
Mass Relations
• Atomic masses:
Na = 22.99 amu
Cl = 35.45 amu
• Decomposition of NaCl:
Cl Na
Total Mass:
58.44 amu

=
Cl
+
Na
35.45 amu + 22.99 amu
Some Notes on Ions
• When atoms lose or gain electrons, they
become ions.
 Cations are positive and are formed by elements
on the left side of the Periodic Table.
 Anions are negative and are formed by elements
on the right side of the Periodic Table.
Common Cations
Common Anions