Metals, on the left side of the periodic table, tend to form compounds with non-metals, on the right side of the table.

• The alkali metals of group 1A, for instance, react with the halogens of group 7A to form a variety of compounds.

Atoms are electrically neutral because they contain equal numbers of protons and electrons. By gaining or losing one or more electrons, however, an atom can be converted into a charged particle called an ion.

The loss of one or more electrons from a neutral atom gives a positively charged ion called a cation.

The gain of one or more electrons by a neutral atom gives a negatively charged ion called an anion.

Having eight valence electrons (filled s and p subshells) leads to stability and lack of chemical reactivity.

• Main group elements frequently combine in such a way that each winds up with eight valence electrons, called an electron octet.

OCTET RULE :The tendency of atoms to gain or lose electrons to achieve a stable, noble gas configuration, that is, a completely filled subshell containing eight electrons.

• Main group metals lose electrons to form cations and attain an electron configuration like that of the noble gas just before them in the periodic table.

• Main group non-metals gain electrons to form anions and an electron configuration like that of the noble gas just after them in the periodic table.

GROUP TENDENCIES:

• The metals of groups 1A–3A, form cations with charges identical to their group number (i.e., 1A elements form cations with +1 charge, etc.).

  • The ions of these elements all have noble gas configurations as a result of electron loss from their valence s subshells.

• Elements in Group 4A would have to gain or lose too many electrons to achieve an octet.

  • Therefore, the first three elements in groups 4A (C, Si, Ge) and 5A (N, P, As) do not ordinarily form cations or anions.

• The group 6A elements, oxygen and sulphur, form ions having noble gas configurations, achieved by gaining two electrons.

• Group 7A,halogens, form ions having noble gas configuration by gaining one electron.

  • Transition metals lose electrons to form cations. The charges of transition metal cations are not as predictable as those of main group elements, however, because many transition metal atoms can lose one or more d electrons in addition to losing valence s electrons.

  • Transition metal cations generally do not have noble gas configurations because they would have to lose all their d electrons.

Ionic charges of main group elements can be predicted using the group number and the octet rule.

• For 1A and 2A metals: cation charge = group number

• For non-metals in groups 5A, 6A, and 7A: anion charge = 8 - (group number)

The ease with which an atom loses an electron to form a positively charged cation is measured by a property called the atom’s ionization energy, defined as the energy required to remove one electron from a single atom in the gaseous state.

the ease with which an atom gains an electron to form a negatively charged anion is measured by a property called electron affinity, defined as the energy released on adding an electron to a single atom in the gaseous state.

The period table begins with small ionization energies for the 1A elements and a gradual increase as we move across a row, ending with very large ionization energies for the noble gases.

• Because ionization energy measures the amount of energy that must be added to pull an electron away from a neutral atom, the small values for alkali metals (Li, Na, K) and other elements on the left side of the periodic table mean that these elements lose an electron easily.

• Conversely, the large values shown for halogens (F, Cl, Br) and noble gases (He, Ne, Ar, Kr) on the right side of the periodic table mean that these elements do not lose an electron easily.

Electron affinities, measure the amount of energy released when an atom gains an electron.

• Although electron affinities are small compared to ionization energies, the halogens nevertheless have the largest values and, therefore, gain an electron most easily, whereas metals have the smallest values and do not gain an electron easily.

Naming the monoatomic ions of elements is done in two methods:

• The first is an old method that gives the ion with the smaller charge the word ending -ous and the ion with the larger charge the word ending -ic.

• The second is a newer method in which the charge on the ion is given as a Roman numeral in parentheses right after the metal name.

Ions that are composed of more than one atom are called polyatomic ions. Most polyatomic ions contain oxygen and another element, and their chemical formulas include subscripts to show how many of each type of atom are present.

The atoms in a polyatomic ion are held together by covalent bonds.

A polyatomic ion is charged because it contains a total number of electrons different from the total number of protons in the combined atoms.

Ionic bonds are a type of linkage formed from the electrostatic attraction between oppositely charged ions in a chemical compound. Compounds having ionic bonds are called ionic compounds.

Ionic compounds are named by citing first the cation and then the anion, with a space between words. There are two kinds of ionic compounds, and the rules for naming them are slightly different.

• Type I: Ionic compounds containing cations of main group elements (1A, 2A, aluminum). Since the charges on these cations do not vary, we do not need to specify the charge on the cation.

• Type II: Ionic compounds containing metals that can exhibit more than one charge. Since some metals, including the transition metals, often form more than one ion, we need to specify the charge on the cation in these compounds. Either the old (-ous, -ic) or the new (Roman numerals) system can be used.

ionic compounds are usually crystalline solids. Different ions vary in size and charge; therefore, they are packed together in crystals in different ways. The ions in each compound settle into a pattern that efficiently fills space and allows for maximum interaction with adjacent ions of opposite charge.

ionic compounds are good conductors of electricity in a solution. Ionic compounds dissolve in water if the attraction between water and the ions overcomes the attraction of the ions for one another.

Ionic compounds also have high melting and boiling points. The attractive force between oppositely charged cations and anions is extremely strong, so the ions need to gain a large amount of energy to overcome the attractions between one another.

Ionic solids shatter if struck sharply. A blow disrupts the orderly arrangement of cations and anions, forcing particles of like electrical charge closer together. The proximity of like charges creates repulsive energies that split the crystal apart.

Since a hydrogen atom contains one proton and one electron, a hydrogen cation is simply a proton because it has lost its single electron. A hydroxide anion (OH-), is a polyatomic ion in which an oxygen atom is covalently bonded to a hydrogen atom.

The H+ cation and the OH- anion is that they are fundamental to the concepts of acids and bases.

• Acid is a substance that provides H+ ions when dissolved in water.

• Base is a substance that provides OH- ions when dissolved in water.