2.S: Chemical Bonds (Summary)

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California Standards Chemistry

Biological, chemical, and physical properties of matter result from the ability of atoms to form bonds from electrostatic forces between electrons and protons and between atoms and molecules. As a basis for understanding this concept:

Students know atoms combine to form molecules by sharing electrons to form covalent or metallic bonds or by exchanging electrons to form ionic bonds.
Students know chemical bonds between atoms in molecules such as H² , CH⁴ , NH³ , H²CCH² , N² , Cl² , and many large biological molecules are covalent.
Students know salt crystals, such as NaCl, are repeating patterns of positive and negative ions held together by electrostatic attraction.
Students know the atoms and molecules in liquids move in a random pattern relative to one another because the intermolecular forces are too weak to hold the atoms or molecules in a solid form.
Students know how to draw Lewis dot structures.
*Students know how to predict the shape of simple molecules and their polarity from Lewis dot structures.
*Students know how electronegativity and ionization energy relate to bond formation.
*Students know how to identify solids and liquids held together by Van der Waals forces or hydrogen bonding and relate these forces to volatility and boiling/melting point temperatures.

Summary

Compounds are pure substances that are composed of more than one element. Ionic compounds are held together by electrostatic attractions between cations and anions. Most cations are metal elements, which have electron configurations with three or fewer valence electrons, and most anions are nonmetal elements, which have five to seven valence electrons. The charge of cations is positive and the number of electrons lost and the charge of anions is negative and the number of electrons gained. The number of electrons gained or lost leads to a stable, noble gas electron configuration. After electrons are transferred the ions combine to form a crystal lattice. The number of cations and anions is not always one-to-one in the chemical formula, but must be in a ratio that makes the number of electrons lost and gained be equal. This chemical formula of an ionic compound shows the ratio of elements using subscripts to denote the number of elements.

Another type of compound is covalent compounds or molecular compounds. These compounds are composed of molecules, which are individual groups of atoms bonded with covalent bonds. The molecules form when electrons are shared between elements that are usually nonmetals. Elements in covalent compounds share electrons until they have an octet of electrons around them. This may require double bonds or triple bonds that have 2 pairs of electrons or 3 pairs of electrons shared between two elements. Molecules can be either polar or nonpolar and the shape of the molecule is one factor that determines this property. The shape of a molecule can be determined by using VSEPR theory, which holds that both bonding and nonbonding pairs of electrons will tend to move as far apart as possible because of the electron-electron repulsions. If a shape is symmetrical then the molecule is nonpolar. If the shape of a molecule is asymmetrical and the molecule has bonds that have electronegativity differences of 1.8 to 0.4 then the molecule will be polar.

Polar molecules have intermolecular forces called dipole-dipole attractions. These intermolecualr forces hold molecules next to each other and result in the substance being a solid or liquid (compared to a nonpolar compound of the same molecular mass). Nonpolar molecules also exhibit a weak intermolecular attraction called Van der Waal’s forces or London forces. But these weak forces do not hold molecules together strongly so the compounds are gases or easily boiled or melted.

Polyatomic ions are molecules with added or removed electrons which makes them anions or cations respectively. They act like other ions, but the chemical formula of polyatomic ions is listed in a table or needs to be memorized.

Metallic compounds are combinations of metal elements as cations with the valence electrons free to move among the cations. These shared valence electrons form a bond for metals called a “sea of electrons”, which is responsible for many of the properties of metals.

Contributors

Kenneth Pringle and Curriki. This content is licensed under a Creative Commons Attribution Share-Alike 3.0 License.