Chemistry Lesson 8

cation (positive ion)

forms when a neutral atom loses one or more electrons from its valence shell

anion (negative ion)

forms when a neutral atom gains one or more electron in its valence shell

Ionic compounds (or salts)

composed of ions called ionic compounds (or salts), held together by ionic bonds.

Ionic Bonds

electrostatic forces of attraction between oppositely charged cations and anions. (positive and negative).

Ionic properties (solids)

Ionic solids: high melting and boiling points. Have a crystalline structure can be rigid and brittle.

Ionic solids: are poor conductors of electricity for the same reason that the strength of the ionic bond prevents ions (+,-) them from moving freely in the solid state.

Ionic solids: will dissolve in water readily or melted ionic compounds become excellent conductors of heat and electricity because ions can move freely.

Neutral atoms (single elements)

Neutral atoms like Na (Sodium) has different properties than its compound state such as sodium chloride NaCl₂. For example Na is a silver white metal that burns vigoursly in air and reacts explosively with water. Cl a yellow green gas is corrosive to metal and poisonous to animals and plants.

Compounds

Na and Cl alone are harmful but when the sodium cations and clorine anions become a compound they are essential to life.

Binary Ionic compounds

Composed of just two elements a metal and non metal. Metal (has cations) nonmetal (anions). Example NaCl is a binary compound. Metals have low ionization potentials lose electrons easily. Nonmetals gain electrons easily and have high electron affinities.

Formation ionic compounds

substances must be electrically neutral there must be equal negative charges (anions) as positive charges (cations). For example: aluminum oxide, Al₂O₃, indicates that this ionic compound contains two aluminum cations, Al³⁺, for every three oxide anions, O^2- [thus, (2 +3) + (3 –2) = 0].

Exception to Ionic Formation

Ionic formation is not all one wall of positive ions and negative ions. The attractive forces between ions are isotropic—the same in all directions—meaning that any particular ion is equally attracted to all of the nearby ions of opposite charge. This results in the ions arranging themselves into a tightly bound, three-dimensional lattice structure.

Forming a cation (positive ion)

tend to lose electrons example group 1 (alkali mentals) 2 (alkaline earth metals), equal the number of valence electrons and charges formed when valence electrons are removed.

calcium is a group 2 element whose neutral atoms have 20 electrons and a ground state electron configuration of 1s²2s²2p⁶3s²3p⁶4s². When a Ca atom loses both of its valence electrons, the result is a cation with 18 electrons, a 2+ charge, and an electron configuration of 1s²2s²2p⁶3s²3p⁶. The Ca2+ ion is therefore isoelectronic with the noble gas Ar.

Forming cations (group 13 - 17)

the group numbers exceed the number of valence electrons by 10 Thus, the charge of a cation formed by the loss of all valence electrons is equal to the group number minus 10. For example, aluminum (in group 13) forms 3+ ions (Al3+).

forming cations (exceptions)

some elements in the bottom of groups like 13-17 will lose 10 electrons which would give Tl+3, Sn+4, Pb+4, and BI+5. BUT sometimes there are partial loss of electrons that can lead to cations Tl+, Sn+2, Pb+2 and Bi+3. This is known as inert pair effect. low energy of valence s-electron.

Mercury (g12) can have Hg₂ ²⁺ diatomic ion formed from two mercury atoms with Hg-Hg bond. or Monotomic ion Hg²⁺ formed with one mercury atom.

forming cations (inner and transition elements)

Most inner and transition metals have 2⁺ or 3⁺ that result from losing s outer valence electrons first followed by 1 or 2 d electrons next outer most shell. Example For example, iron (1s²2s²2p⁶3s²3p⁶3d⁶4s²) forms the ion Fe2+ (1s²2s²2p⁶3s²3p⁶3d⁶) by the loss of the 4s electrons and the ion Fe3+ (1s²2s²2p⁶3s²3p⁶3d⁵) by the loss of the 4s electrons and one of the 3d electrons. (see bold compared to charges)

forming cations (aufbau principle)

even though with the building up of electron configurations start with the inner electrons closest to the nucleus and then fill outward to the valence electrons. Not in the formation of cations though the first lost are the valence electrons or the outer most s, d, and f electron.