3. Periodic Classification of Elements

The Periodic Classification of Elements is one of the most important topics in chemistry, being fundamental for understanding the atomic structure and chemical properties of elements. In this chapter of our ENEM Chemistry e-book, we will explore in detail the periodic table, its history, its structure and the importance of its groups and periods.

1. History of the Periodic Table

The periodic table as we know it today is the result of centuries of study and discoveries. In the 19th century, scientist Dmitri Mendeleev noticed that, by arranging known elements in order of increasing atomic mass, certain sets of properties repeated periodically. Mendeleev then created a table that reflected this periodicity, which allowed him to predict the existence of elements not yet discovered.

With the development of quantum theory in the 20th century, the periodic table was revised to reflect the electronic structure of atoms. Today, elements are arranged in order of increasing atomic number, which is equal to the number of protons in the nucleus of the atom.

2. Structure of the Periodic Table

The modern periodic table is divided into groups (columns) and periods (rows). The groups are numbered from 1 to 18 and the periods from 1 to 7. The elements of the same group have the same electronic configuration in the last energy level (valence layer) and, therefore, similar chemical properties. Elements of the same period have the same number of energy levels.

In addition, the periodic table is divided into blocks, according to the electronic subshell being filled. The s block includes groups 1 and 2, the p block includes groups 13 to 18, the d block includes groups 3 to 12, and the f block includes lanthanides and actinides.

3. Importance of Groups and Periods

Each group and period of the periodic table has unique characteristics that are important for understanding the properties of elements.

For example, Group 1 is known as the alkali metals, and includes elements such as lithium, sodium, and potassium. These elements are very reactive and have an electron in the valence shell. Group 17 is known as the halogens, and includes elements such as fluorine, chlorine, and bromine. These elements are very reactive and have seven valence electrons.

Period 2 includes elements such as carbon, nitrogen, and oxygen, which are fundamental to life as we know it. These elements have two energy levels and are capable of forming a wide variety of compounds.

In summary, the Periodic Classification of Elements is a powerful tool for understanding the chemical properties of elements and predicting their behavior in chemical reactions. By studying the periodic table, you will be well prepared to answer ENEM chemistry questions involving the atomic structure and properties of elements.

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29Periodic Classification of Elements: History of Periodic Classification

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