2.14. Atomic Structure: Electronegativity

Electronegativity is a fundamental concept in chemistry and is especially relevant for those preparing for the ENEM test. This concept refers to the ability of an atom in a molecule to attract electrons to itself. Electronegativity is a periodic property, which means it follows a predictable pattern as you move through the periodic table.

Before we dive into electronegativity, let's quickly recap atomic structure. An atom is composed of a nucleus, which contains protons and neutrons, and an electron cloud that orbits the nucleus. Electrons are organized into energy levels, or shells, and it is the outermost shell, or valence shell, that is involved in the formation of chemical bonds.

Electronegativity is a measure of an atom's tendency to attract a bonding pair of electrons. It was first proposed by Linus Pauling, who developed a scale to quantify electronegativity. On the Pauling scale, fluorine is the most electronegative element, with a score of 4.0, while francium is the least electronegative, with a score of 0.7.

Electronegativity is a function of atomic number (the number of protons in the nucleus of an atom) and atomic radius (the distance from the nucleus to the outermost electron). As you move from left to right across a period (row) of the periodic table, electronegativity generally increases. This is because the atomic number is increasing, which means there are more protons in the nucleus to attract the electrons. At the same time, the atomic radius is decreasing, which means that electrons are closer to the nucleus and therefore more strongly attracted to it.

As you move down a group (column) of the periodic table, electronegativity generally decreases. This is because the atomic radius is increasing, which means that electrons are further away from the nucleus and therefore less strongly attracted to it. In addition, there are more layers of electrons between the nucleus and the valence layer, which decreases the nucleus' attraction for the valence electrons.

Electronegativity is an important property because it influences the type of bond that will form between two atoms. If the difference in electronegativity between two atoms is large, an ionic bond will form, with the more electronegative atom "stealing" an electron from the less electronegative atom. If the electronegativity difference is small, a covalent bond will form, with the atoms sharing electrons.

In addition, electronegativity also influences the polarity of a molecule. If the atoms in a molecule have different electronegativities, the molecule will have an uneven distribution of charge and will be polar. If the atoms have similar electronegativities, the molecule will have an even distribution of charge and will be nonpolar.

In summary, electronegativity is a critical property of atoms that plays an important role in determining the type and nature of chemical bonds. Understanding electronegativity and how it varies on the periodic table is critical to success on the ENEM chemistry test.

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17Atomic Structure: Effective Nuclear Charge

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