2.6
共价键
当两个原子共用电子来完成它们的价电子层时，它们就会形成共价键。原子的电负性——共用电子被拉向原子的力——决定了电子如何共用。由共价键形成的分子可以是极性的，也可以是非极性的。电负性相似的原子形成非极性共价键；电子是平均分配的。具有不同电负性的原子不平等地共用电子，从而形成极性键。
共价键是由两个共享…
Covalent bonds share valence electrons, which fill each atom’s outer shell and create one stable molecule; in this case, with a single bond.
The number of covalent bonds can increase depending on how many electrons are shared. For example, when two oxygen atoms, each with six valence electrons, share two pairs of electrons, a double bond forms.
These bonds are nonpolar covalent because the electrons are shared evenly between the atoms. The atoms have the same attraction for the electrons in the covalent bond, meaning they have the same electronegativity.
If two different elements have similar electronegativity, such as carbon and hydrogen, they share electrons relatively equally, forming a nonpolar covalent bonds.
The more strongly an atom attracts the electrons in a covalent bond, the more electronegative the atom.
When two different elements with very different electronegativity bond together, they share electrons unequally, forming a polar covalent bond.
For example, water has one oxygen atom and two hydrogen atoms. The oxygen atom holds the negatively charged electrons most of the time, giving it a partial negative charge.
This situation leaves the hydrogen atoms with a partial positive charge, which makes the compound polar.
These covalent bonds usually form between nonmetals such as carbon, hydrogen, nitrogen, oxygen, and chlorine.
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Q1: What determines how many covalent bonds an atom can form?
The number of covalent bonds an atom can form is dictated by its valence electrons. Oxygen has six valence electrons and needs two more to become stable, so it forms two single bonds or one double bond. Carbon has four valence electrons and can form four covalent bonds, as in methane. Each atom shares electrons to complete its outer shell and achieve stability.
Q2: How does electronegativity affect whether a covalent bond is polar or nonpolar?
Electronegativity determines how electrons are shared in a covalent bond. When atoms have similar electronegativities, they share electrons equally, forming nonpolar bonds with no charge across the molecule. When atoms have very different electronegativities, one atom pulls electrons toward it more strongly, creating a polar bond with partial positive and negative charges on opposite sides.
Q3: Why is water considered a polar molecule?
Water is polar because oxygen is much more electronegative than hydrogen. Oxygen pulls the shared electrons toward itself most of the time, giving it a partial negative charge while the hydrogen atoms gain partial positive charges. This unequal electron distribution makes water polar, allowing it to interact with other charged molecules and dissolve many substances.
Q4: What is the difference between polar and nonpolar molecules in terms of water solubility?
Polar molecules are hydrophilic because their partial charges attract them to water and other charged molecules, making them soluble in water. Nonpolar molecules, such as fats containing long hydrocarbon stretches, are hydrophobic and do not dissolve in water. This difference is crucial for cells because water surrounds them and fills their cytoplasm, affecting how molecules are transported and used.
Q5: What happens to electrons when a covalent bond forms?
When a covalent bond forms, both atoms share a pair of electrons in a hybrid orbital that differs in shape from a normal orbital. The electrons orbit in a modified path around the nuclei of both atoms simultaneously. These covalent bonds are strong and cannot be broken by physical forces once formed, making them stable molecular connections.
Q6: Which elements typically form covalent bonds?
Covalent bonds usually form between nonmetals such as carbon, hydrogen, nitrogen, oxygen, and chlorine. These elements have valence electrons available for sharing and are positioned on the periodic table in ways that favor electron sharing over electron transfer. Their similar electronegativities allow them to form stable covalent bonds with each other.
Q7: How do single bonds and double bonds differ in covalent bonding?
A single bond forms when two atoms share one pair of electrons, as when oxygen bonds with two hydrogen atoms in water. A double bond forms when two atoms share two pairs of electrons, as when two oxygen atoms bond together. The number of shared electron pairs depends on how many valence electrons each atom needs to complete its outer shell.