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Atoms, Molecules and Ions Study Guide (page 2)

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Updated on Sep 24, 2011

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Elements usually form +1 cations in group 1,+2 cations in group 2, and +3 cations in group 3.

Nonmetals usually form –1 anions in group 7,–2 cations in group 6, and –3 anions in group 5.

Ions

As noted earlier, the periodic table is organized in octaves (Groups 1A to 8A). The octet rule states that atoms form ions and are bond by surrounding themselves with eight (octet) outer electrons. Notable exceptions are hydrogen (two electrons; duet rule) and group 3 elements (six electrons). They tend to acquire the stability of their closest noble gases in the periodic table either by losing (metals), gaining (nonmetals), or sharing electrons in their valence shell. The valence shell contains the electrons in the outermost energy level.

An anion is a negatively charged ion formed when an atom gains one or more electrons. Most anions are nonmetallic. Their names are derived from the elemental name with an -ide suffix. For example, when chlorine (Cl) gains an electron, a chloride ion (Cl) is formed. Because chlorine is in group 7, it only needs to gain one electron to achieve the octet structure of argon (Ar). An oxygen atom (O) acquires two electrons in its valence shell to form an oxide ion (O2) that has the same stable electron configuration as neon (Ne).

Table 2.1 Isotopes of Hydrogen

A cation is a positively charged ion formed when an atom loses one or more electrons. Most cations are metallic and have the same name as the metallic element. For example, when lithium (Li) loses an electron, a lithium ion (Li+) is formed. Lithium is in group 1 and needs to lose one electron to acquire the noble gas electron structure of helium (He), the closest noble gas.

Molecules

Ionic compounds are compounds formed by combining cations and anions. The attractive electrostatic force between a cation and an anion is called an ionic bond.

A molecular compound is formed when a nonmetal and metal combine to form a covalent bond. Covalent bonds are the type of bonds formed when two atoms share one or more pairs of electrons to achieve an octet of electrons. A polar covalent bond is formed when the atoms unequally share paired electrons.

Electronegativity is the ability of an atom (in a bond) to attract the electron density more than the other atom(s). Electronegativity increases from left to right and from the bottom to the top of the periodic table. Thus, fluorine (F) is the most electronegative element of the periodic table, with the maximum value of 4.0 in the Pauling scale of electronegativity. Metals are electropositive. See Lesson 11 for more on electronegativity.

Formulas and Nomenclature

An essential step in learning chemistry is understanding chemical formulas and how to name compounds. Compounds can be divided into four classifications:

1. Type I: binary ionic compounds
2. Type II: binary ionic compounds with the metal possessing more than one type of cation
3. Type III: binary covalent compounds
4. Organic compounds
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