Lecture 17

Elements and Compounds. The Periodic Law.

• Classes of Matter

• Molecules

• Groups of Metals and Nonmetals

• The Periodic Law

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.

Chemical ChangeExample: 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.

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

Atoms and MoleculesJohn 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).

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

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.

Families of Elements

There are a few families of elements which exhibit similar properties.

Halogens (chlorine, bromine, fluorine, iodine):Highly chemically activeDissolve in water to form acids

Alkali Metals (lithium, sodium, potassium):Soft and very chemically activeHave low temperature melting points

Inert (Noble) Gases (helium, neon, argon):Very chemically inactive, exist in elemental form

The Periodic TableHalogens 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 LawElements (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 elementsUnexpected success: noble gases

Groups and PeriodsElements 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

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.