3.4. Periodic Classification of Elements: Periods and Groups

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The Periodic Classification of Elements is a crucial tool for the study of Chemistry, being a topic of great importance for the Enem test. This organizational system allows students to understand the relationships between different chemical elements, their properties and behaviors. The table is divided into periods and groups, each with different characteristics.

The periods of the periodic table are the seven horizontal lines, numbered from 1 to 7. Each period corresponds to the number of the highest energy level of an atom in the ground state. The first period is the shortest, containing only two elements: hydrogen and helium. The second and third periods have eight elements each, while periods 4 through 7 are much longer due to the inclusion of d and f orbitals.

Groups, on the other hand, are the 18 vertical columns of the periodic table, also known as families. Elements in each group have the same number of electrons in the valence shell (the outermost electron shell), which results in similar chemical properties. For example, all Group 1 elements (except hydrogen) are alkali metals, which are highly reactive due to the single electron in their valence shell.

Groups in the periodic table are subdivided into groups A and B. Groups A are known as representative groups and include the alkali metals (Group 1A), alkaline earth metals (Group 2A), chalcogens (Group 6A), halogens (Group 7A) and noble gases (Group 8A). These elements follow the octet principle, which means that their valence shells are complete with eight electrons, with the exception of the first period.

The B groups are known as transition groups and include the transition metals (Groups 3B to 12B). These elements have electrons in their d or f shells, and their chemical properties are less predictable than those of representative groups.

It is important to note that the periodic table is also divided into blocks (s, p, d and f) according to the energy sublevel being filled. The s block contains groups 1A and 2A, the p block contains groups 3A to 8A, the d block contains groups 3B to 12B, and the f block includes the lanthanides and actinides.

Understanding the organization and trends of the periodic table is critical to success on the Enem. Element properties, such as atomic radius, ionization energy, electron affinity, and electronegativity, vary predictably across the table. These trends are influenced by the electronic structure of the atoms and are a reflection of the forces of attraction between the electrons and the nucleus.

In short, the Periodic Classification of Elements is a powerful tool for understanding the world of Chemistry. By studying this topic in depth, students will be well prepared to answer any question about the periodic table that may appear on the ENEM test.

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333.5. Periodic Classification of Elements: Periodic Properties