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La tabla periódica de Dmitri Mendeleev

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Why do you think Mendeleev made a periodic table? Many elements remained to be discovered in Mendeleev's time. The periodic table helped predict the properties of new elements.

Compare the modern periodic table with Mendeleev's table. What do you notice? Mendeleev's table didn't have very many elements, did it? He had question marks and spaces between elements, where he predicted undiscovered elements would fit.

The periodic table helps predict some properties of the elements compared to each other. Atom size decreases as you move from left to right across the table and increases as you move down a column. The energy required to remove an electron from an atom increases as you move from left to right and decreases as you move down a column. The ability to form a chemical bond increases as you move from left to right and decreases as you move down a column.

Introduction | Periods & Groups | More about Groups | Review Questions | Quiz

There are two sets of groups. The group A elements are called the representative elements. The group B elements are the nonrepresentative elements.

Introduction | Periods & Groups | More about Groups | Review Questions | Quiz

Elements are classified according to their properties. The major categories of elements are the metals, nonmetals, and metalloids.

You see metals every day. Aluminum foil is a metal. Gold and silver are metals. If someone asks you whether an element is a metal, metalloid, or non-metal and you don't know the answer, guess that it's a metal.
What are Properties of Metals?
Metals share some common properties. They are lustrous (shiny), malleable (can be hammered), and are good conductors of heat and electricity. These properties result from the ability to easily move the electrons in the outer shells of metal atoms.
What are the Metals?
Most elements are metals. There are so many metals, they are divided into groups: alkali metals, alkaline earth metals, and transition metals. The transition metals can be divided into smaller groups, such as the lanthanides and actinides.

The alkali metals are located in Group IA (first column) of the periodic table. Sodium and potassium are examples of these elements. Alkali metals form salts and many other compounds. These elements are less dense than other metals, form ions with a +1 charge, and have the largest atom sizes of elements in their periods. The alkali metals are highly reactive.

The alkaline earths are located in Group IIA (second column) of the periodic table. Calcium and magnesium are examples of alkaline earths. These metals form many compounds. They have ions with a +2 charge. Their atoms are smaller than those of the alkali metals.

The transition metals include most of the elements, so they can be categorized into smaller groups. The lanthanides and actinides are classes of transition elements. Another way to group transition metals is into triads, which are metals with very similar properties, usually found together.

The iron triad consists of iron, cobalt, and nickel. Just under iron, cobalt, and nickel is the palladium triad of ruthenium, rhodium, and palladium, while under them is the platinum triad of osmium, iridium, and platinum.

The actinides are in the row below the lanthanides. Their atomic numbers follow actinium. All of the actinides are radioactive, with positively charged ions. They are reactive metals that form compounds with most nonmetals. The actinides are used in medicines and nuclear devices.
Groups 13-15: Not all Metals
Groups 13-15 include some metals, some metalloids, and some nonmetals. Why are these groups mixed? The transition from metal to nonmetal is gradual. Even though these elements aren't similar enough to have groups contained within single columns, they share some common properties. You can predict how many electrons are needed to complete an electron shell. The metals in these groups are called basic metals.

Elements that don't have the properties of metals are called nonmetals. Some elements have some, but not all of the properties of the metals. These elements are called metalloids.

The nonmetals are poor conductors of heat and electricity. Solid nonmetals are brittle and lack metallic luster. Most nonmetals gain electrons easily. The nonmetals are located on the upper right side of the periodic table, separated from metals by a line that cuts diagonally through the periodic table. The nonmetals can be divided into classes of elements that have similar properties. The halogens and the noble gases are two groups of nonmetals.

The halogens are located in Group VIIA of the periodic table. Examples of halogens are chlorine and iodine. You find these elements in bleaches, disinfectants, and salts. These nonmetals form ions with a -1 charge. The physical properties of the halogens vary. The halogens are highly reactive.

The noble gases are located in Group VIII of the periodic table. Helium and neon are examples of noble gases. These elements are used to make lighted signs, refrigerants, and lasers. The noble gases are not reactive. This is because they have little tendency to gain or lose electrons.

Elements that have some properties of metals and some properties of nonmetals are called metalloids. Silicon and germanium are examples of metalloids. The boiling points, melting points, and densities of the metalloids vary. The metalloids make good semiconductors. The metalloids are located along the diagonal line between the metals and nonmetals in the periodic table.

Introduction | Periods & Groups | More about Groups | Review Questions | Quiz

Test your comprehension of this periodic table lesson by seeing if you can answer the following questions:

1. The modern periodic table isn't the only way to categorize the elements. What are some other ways you could list and organize the elements?

2. List the properties of the metals, metalloids, and nonmetals. Name an example of each type of element.

3. Where in their group would you expect to find elements with the largest atoms? (top, center, bottom)

4. Compare and contrast the halogens and noble gases.

5. What properties can you use to tell the alkali, alkaline earth, and transition metals apart?

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Learn Chemistry with this Periodic Table Study Guide

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Why do you think Mendeleev made a periodic table? Many elements remained to be discovered in Mendeleev's time. The periodic table helped predict the properties of new elements. Compare the modern periodic table with Mendeleev's table. What do you notice? Mendeleev's table didn't have very many elements, did it?

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La tabla de los elementos

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Los átomos son eléctricamente neutros. Están conformados de la misma cantidad de cargas positivas y negativas (protones y electrones). Cuando no son neutros, se conocen como iones.