3.7. Periodic Classification of Elements: Atomic Radius

The Periodic Classification of Elements is one of the most essential parts of Chemistry and is one of the most important topics for the ENEM test. This topic is based on the concept of atomic radius and how it varies on the periodic table. Understanding this concept is crucial to understanding the properties of elements and how they react with each other.

Atomic radius is the measure of the distance from the center of an atom's nucleus to its outermost electron. It is usually measured in picometers (pm) or angstroms (Å). The atomic radius is not a fixed measurement, as it varies depending on the state of the atom and its surroundings. However, we can identify general trends in the periodic table that help us predict the behavior of elements.

To understand how the atomic radius varies in the periodic table, it is useful to divide it into groups and periods. Groups are the vertical columns in the periodic table, while periods are the horizontal rows. The size of the atomic radius increases as we go down a group and decreases as we move from left to right through a period.

The reason for this trend is the charge of the nucleus and the number of energy levels occupied by electrons. As we move down a group, the number of energy levels increases, which means that the electrons are further away from the nucleus. This causes the atomic radius to increase. On the other hand, as we move from left to right in a period, the charge on the nucleus increases, but the number of energy levels remains the same. This means that electrons are more strongly attracted to the nucleus, causing the atomic radius to decrease.

It is important to note that this is a simplification and that there are exceptions to these trends. For example, transition elements show very little change in atomic radius despite the increase in nuclear charge. This is because the transition electrons are added at an internal energy level, which does not increase the size of the atom much.

In addition, elements in groups 12 through 18 show a decrease in atomic radius despite an increase in the number of electrons. This is because additional electrons are added to an already occupied energy level, which increases the repulsion between electrons and causes the atomic radius to decrease.

In short, the Periodic Classification of Elements and the concept of atomic radius are fundamental to understanding Chemistry. They help us to predict the behavior of elements and their reactions. Therefore, it is essential to understand these concepts to be successful in the ENEM test.

We hope that this e-book course will provide you with a clear and detailed understanding of these concepts and help you to effectively prepare for the ENEM exam. Remember, understanding the periodic table and atomic radius is not just about memorization, but about understanding trends and the reasons behind them.

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