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2.4 Compounds, Atoms, and Ions

2.4 Compounds, Atoms, and Ions . (7.1 pg 169-174. Compounds are formed when two or more atoms bond together; modern atomic theory suggests the tendency of atoms to bond is based on the arrangement of their e (protons/neutrons not involved.).

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2.4 Compounds, Atoms, and Ions

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  1. 2.4 Compounds, Atoms, and Ions (7.1 pg 169-174

  2. Compounds are formed when two or more atoms bond together; modern atomic theory suggests the tendency of atoms to bond is based on the arrangement of their e (protons/neutrons not involved.)

  3. Ironically, to understand bonding we look to the family of elements that are totally un-reactive (Noble Gases). • One theory of bonding assumes there is an underlying tendency for elements to acquire an outer electron shell similar to the nearest noble gas.

  4. An ion = an atom that has either lost or gained electrons. • We represent the charge of an ion with a the charge written as a superscript (above) to the right • e.g. a Nitrogen atom that has gained 3 e is written as N3-. The type of charge (+ or -) is always present.

  5. An atom may lose or gain e to become more stable. The most stable atoms are those with full outer e shells (group 8 noble gases). • The outermost electron shell on each atom is called the valence shell and the electrons that are in it are valence electrons.

  6. Nitrogen has 5 valence electrons. It would be easier for it to gain 3 more electrons than to lose 5, so it become an ion with a charge of 3-. • This e configuration would match that of Neon, the nearest noble gas. • The tendency to lose / gain e to get a full valence shell is called the Octet rule (elements want a full outer shell, which usually means a shell with 8e).

  7. The diagram below shows how 2 atoms on either side of the Noble gas Neon form ions (Fig.4 p.171). • Fluorine (F) has 7 valence electrons in is outer shell.It would be more stable with 8. It has the tendency to gainone e. Since electrons are negative,this gives F a negative charge, F-

  8. Sodium (Na) has 1 electron in its outer shell. It would take too much energy to gain 7 more electrons. Instead, it has a tendency to lose an electron.Its outer shell is now the 2ndshell, and is ‘full’ with 8 e.

  9. Both of these ions end up withelectron shell configurations that are the same as the nearest noble gas, Neon.

  10. There are two kinds of ions: • Anions = A negative ION (remember the first letter of each); always have a negative charge. • Cations= A positive ION (remember that CATS are positive); always have a positive charge • An ion’s charge is easy to figure out. Add the number of protons to the number of electrons on the ion. Then write the charge in the upper right hand corner. • E.g. Sulfur ion = 16p + 18e- = 16 + 18(-) = 2- = S2- • Another method is described on TABLE 1 p.173 of your text; you may prefer this method of determining the charge of ions.

  11. Please note: Metals form positive ions, non-metals form negative ions. • Groups (chemical families) tend to have the same ion charge. • Hydrogen is unique – it can become H+ or H- (no family).

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