Seminar:Elements, Compounds, and

Chemical Reactions

Freddie Arocho

SC155: Introduction to Chemistry

First Periodic Table

• Dmitri Mendeleev (1834-1907) 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

SOMg Ca

+O S

Mg Ca+

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 amuCa = 40.08 amuH = 1.008 amuK = 39.10 amuP = 30.97 amuS = 32.07 amuO = 16.00 amu

Mass Relations• Atomic masses:

C = 12.01 amuCa = 40.08 amuH = 1.008 amuK = 39.10 amuP = 30.97 amuS = 32.07 amuO = 16.00 amu

• Calculate the molecular mass of H2O:H: 2 atoms X 1.008 = 2.016O: 1 atom X 16.00 = 16.00

Total: 18.016 ~ 18.02 amu

Mass Relations• Atomic masses:

C = 12.01 amuCa = 40.08 amuH = 1.008 amuK = 39.10 amuP = 30.97 amuS = 32.07 amuO = 16.00 amu

• Calculate the molecular mass of CO2:C: 1 atom X 12.01 = 12.01O: 2 atoms X 16.00 = 32.00

Total: 44.01 amu

Mass Relations• Atomic masses:

C = 12.01 amuCa = 40.08 amuH = 1.008 amuK = 39.10 amuP = 30.97 amuS = 32.07 amuO = 16.00 amu

• Calculate the molecular mass of CH4:

Mass Relations• Atomic masses:

C = 12.01 amuCa = 40.08 amuH = 1.008 amuK = 39.10 amuP = 30.97 amuS = 32.07 amuO = 16.00 amu

• Calculate the molecular mass of CH4:C: 1 atom X 12.01 = 12.01H: 4 atoms X 1.008 = 4.032

Total: 16.042 ~ 16.04 amu

Mass Relations• Atomic masses:

C = 12.01 amuCa = 40.08 amuH = 1.008 amuK = 39.10 amuP = 30.97 amuS = 32.07 amuO = 16.00 amu

• Calculate the molecular mass of K2SO4:

Mass Relations• Atomic masses:

C = 12.01 amuCa = 40.08 amuH = 1.008 amuK = 39.10 amuP = 30.97 amuS = 32.07 amuO = 16.00 amu

• Calculate the molecular mass of K2SO4:K: 2 atoms X 39.10 = 78.20S: 1 atom X 32.07 = 32.07O: 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:

Total Mass:

58.44 amu = 35.45 amu + 22.99 amu

Cl Na Cl + Na

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