Download General Structure of the Periodic Table

Periodic Table Reading and Questions
A Short History of the Periodic Table
In 1789, building upon the work of precursors and contemporaries alike, the French chemist Antoine
Laurent Lavoisier first defined an element as a fundamental substance that could not be broken down by any
chemical means then known. In the same Treatise on Chemical Elements, he compiled a list of 33 elements (a
number of which were not actually elements but turn out to be compounds) and devised a naming system for
the discovery of new elements.
Lavoisier's definition and list of elements helped spur an attempt by chemists to systematize and
understand the elements. In 1803, the English chemist John Dalton used the general scientific recognition
that elements combined with each other according to different ratios by weight to create an atomic theory
that claimed all elements were built out of variable numbers of hydrogen atoms. As a part of this theory,
Dalton created a scale of atomic weight based on the hydrogen atom (the weight of hydrogen was set equal
to 1). In 1869, the Russian chemist Dmitry Mendeleyev organized the elements in a table according to their
atomic weights (the German chemist Julius Lothar Meyer independently struck upon the same organization in
1870).
In the sixty-seven years from Dalton's formulation of atomic weight to Mendeleyev's periodic table
many scientists had tried to create a working organizational structure for the elements. Mendeleyev
succeeded where others failed because he realized that there existed a number of as yet unknown elements
with atomic weights between the weights of already known elements. By leaving vacancies for those elements
he believed were undiscovered, he hit upon an organizational scheme that seemed to vertically group
elements with similar properties. Among elements with low atomic weights, he found that similar chemical
characteristics recurred every seven elements. Among heavier elements, he found that characteristics
resurfaced every seventeen elements. This phenomenon in which physical and chemical characteristics of
elements are periodic functions of their atomic weight is called the periodic law (and gives the periodic table
its name). In 1879, Mendeleyev's periodic table received a powerful boost in general acceptance when it
predicted the existence of the elements gallium, germanium, and scandium.
Through time, Mendeleyev's periodic table has undergone some small changes. Many, many new
elements have been added. The discovery of the inert gases raised the number of elements between similar
elements to eight for the lighter elements and eighteen for the darker elements. In a few instances,
scientists have discovered that organization along atomic weights does not coincide with vertical similarities.
In such instances, as in the case of tellurium (Te) and iodine (I), similarity wins out over atomic weight in
determining organization.
General Structure of the Periodic Table
As stated last section, the periodic table organizes the elements according to general patterns of
similarity. Below is a very small image of the periodic table. It is basically unreadable in terms of specific
information, but it allows us to easily look at the periodic tables structure general trends.
The vertical columns of the periodic are called groups. The horizontal rows are called periods. There are 18
groups and 7 periods. Elements in similar groups or periods have properties and/or characteristics in
commom. An example would be the fact that all the elements in group 18 (the last column) are all noble or
inert gases which do not react easily with other elements. In discussing the periodic table from here on out
we will use the terms group and period. Down a group means moving from top to bottom; across a period
means moving from left to right.
Reading the Periodic Table: Carbon
To describe the information contained within each individual box we will use a specific example: carbon.
Figure 2: Description of Carbon on the Periodic Table
Element Name
The purpose of the element name is obvious. However, many Periodic Tables do not include element names.
For those situations you must memorize the symbols that accord to each element name.
Element Symbol
Each element has a specific one or two letter symbol that is used interchangeably with its name. These
should be memorized. Most of the time, symbols quite clearly accord to the name of the element they
represent, as C accords to carbon. Occasional, however, an element's name and symbol have little relation.
For example, the symbol for mercury is Hg.
Atomic Number
As you move across a period the atomic number increases. Similarly, as you move down a group the atomic
number increases. In this way, the atomic number represents exactly where in the periodic table an element
stands.
More importantly, and the reason why the ordering of the elements according to atomic number yields
elements in groups with similar chemical and physical properties, the atomic number is the same as the
number of protons in the nucleus of an atom of an element, and also the same as the number of electrons
surrounding the nucleus in a neutral state. Carbon, for example, has six protons and six electrons.
Atomic Mass
Along with protons, an atom also contains neutrons in its nucleus. The atomic mass (also called atomic weight)
of an element is the combined number of protons and neutrons in the nucleus.
Atoms of particular elements generally have different "versions," meaning that elements have atoms with
different numbers of neutrons in their nucleus. These different versions are called isotopes. The atomic
weight displayed is actually the weighted average of the mass numbers of the various isotopes. The atomic
weight for Carbon is 12.01 because around 99% of all carbon is the carbon-12 isotope.
Simple Periodic Trends
Atomic Number
The Atomic number increases from the top left to the bottom right. It ascends sequentially across each
period.
Atomic Weight
Weight The atomic weight of the elements generally increases as you move down a group and across a period.
Hydrogen, at the top left of the table, is the lightest element. The unnamed element 112 is the heaviest.
There are some instances when this rule does not hold true, however. For instance, because it has a high
percentage of isotopes with many neutrons, the atomic weight of tellurium (Te) is higher than that for iodine
(I), even though iodine has a higher atomic number.
Types of Elements
Elements can be organized by group or period, but they also can be placed into three distinct groups: metals,
semi-metals, and non-metals.
Metals
Metals are on the left side of figure 1. Metals are generally lustrous solids, often deformable (though
mercury (Hg) is a liquid at room temperature). Metals are highly conducive to both heat and electricity.
Nonmetals
Nonmetals are the boxes on the upper right hand of the periodic table. More than half of the non-metals are
gaseous at normal temperatures.
Metalloids
Metalloids are the boxes between the metals and nonmetals on the periodic table. As their transitory name
and placement on the periodic table suggest, they exist in between the distinctions of metals and nonmetals.
Metals and Nonmetals
Most chemical compounds are formed by the interactions between metals and non- metals.
Questions: Answer the following questions in complete sentences on a separate sheet of notebook paper.
1.
Why was Mendeleyev successful in designing the periodic table that was used by the scientific
community?
2.
What is the significant of groups and periods?
3.
Explain what the atomic number and the atomic mass tell you about an element. How are the two
numbers related?
4.
Which two elements exist in liquid form at room temperature?
5.
Describe the trends in the periodic table.