Elements and Compounds. The Periodic Law.

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Lecture 17

• Elements and Compounds. The Periodic Law.

• Classes of Matter
• Molecules
• Groups of Metals and Nonmetals
• The Periodic Law

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Physics and Chemistry
There is vague boundary between physics and
chemistry.
Processes like motion and nuclear reactions are
in the physics domain.
Processes of changing molecules (called chemical
reactions) are in the chemistry domain.
Chemistry rose from alchemy, which searched for
ways to change ordinary metals into gold.
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Chemical Change

• Example two different substances form a new one
  under certain conditions (for example, heating).
• The result is a new substance with new properties.

Mixing of the same initial substances will result
in a heterogeneous substance with varying
properties from one particle to another. There is
no chemical change.
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Three Classes of Matter
First class Elements (chemical
elements). Examples hydrogen, oxygen, carbon,
iron.
Second class Compounds (two or more elements
joined together by chemical reactions). Examples
zinc sulfide, water
Third class Mixtures (solution is a homogeneous
mixture of two or more components). Examples
seawater, air
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Atoms and Molecules
John Dalton of England proposed that All the
atoms of each element are the same, But different
from atoms of other elements.
Compounds consist of atoms of different
elements. Each compound has a fixed ratio of
atoms of the involved elements.
Most compounds consist of molecules, which are
made of atoms of the elements in the compound
(H2O). There are also some elemental substances
(O2).
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Non-Molecular Compounds
Liquids and solids are usually assembled from
individual atoms.
Others are composed from ions, atoms or groups of
atoms with positive or negative electric
charges. Example NaCl ?? Na Cl?
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Chemical Activity
Chemical elements are usually divided into metals
and nonmetals. Metals iron, copper,
aluminum. Nonmetals oxygen, chlorine, fluorine.
Chemical activity refers to ability of metals and
nonmetals to form compounds. Active ones combine
with others readily. Inactive ones have little
tendency to combine.
Activity can be determined by measuring amounts
of heat given off in similar chemical
reactions. The more heat is given off, the more
active is the element.
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Families of Elements
There are a few families of elements which
exhibit similar properties.
Halogens (chlorine, bromine, fluorine,
iodine) Highly chemically active Dissolve in
water to form acids
Alkali Metals (lithium, sodium, potassium) Soft
and very chemically active Have low temperature
melting points
Inert (Noble) Gases (helium, neon, argon) Very
chemically inactive, exist in elemental form
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The Periodic Table
Halogens are followed in atomic number by noble
gases, then by alkali metals. This is suggestive
of a periodical law in properties of chemical
elements.
The Periodic Law Elements (listed in order of
atomic number) with similar chemical and physical
properties appear at regular intervals.
The periodic law was first formulated by Dimitri
Mendeleev of Russia in 1869. Predictions unknown
elements Unexpected success noble gases
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Groups and Periods
Elements in vertical columns are called
groups. The horizontal rows are called periods.
In periods, there is a steady change from an
active metal to a noble gas through less active
metals, weakly active nonmetals, and active
nonmetals.
In groups, activity of alkali metals increases
from top to bottom, while activity of halogens
decreases in this direction.
Periods 4 5 contain transition metals which
resemble one another, but not those in the
numbered groups. Period 6 ? rare-earth metals
period 7 actinides.
3D Periodic Table
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Summary
Substances react with each other forming new ones
by chemical reactions There are three general
classes of substances elements, compounds, and
mixtures Both chemical and physical properties
of elements show periodic behavior if arranged in
the order of increasing atomic numbers.