Ionic bonding: causes, charges and diagrams
Bonding, structure, and the properties of matter • Chemical bonds
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Definition of ionic bonding
Ionic bonding is the strong electrostatic attraction between positively charged ions (cations) and negatively charged ions (anions). Ionic bonding forms when one atom transfers one or more electrons to another atom, producing charged particles that attract each other. The resulting structure commonly forms a regular three-dimensional lattice of alternating ions.
Cause: transfer of electrons
Transfer of electrons occurs because atoms seek a full outer shell of electrons (often an inert gas configuration). Metals have few electrons in their outer shell and lose them easily to form cations. Non-metals have almost full outer shells and gain electrons to form anions. The net result is electron transfer from metal to non-metal, producing ions with stable electron arrangements.
Cause: electrostatic forces
Electrostatic forces are the attraction between opposite electric charges. After electron transfer, positive and negative ions attract each other strongly. That electrostatic attraction is the ionic bond and operates in all directions, producing high melting and boiling points and rigid lattice structures. Strength of ionic bonding depends on ion charges and sizes: larger charges and smaller ions result in stronger attraction.
Predicting ion charges from group numbers (limit to Groups 1, 2, 6, 7)
Simple rules apply for the specified groups. Metals in Group 1 lose one electron to form +1 ions (e.g., Na → Na+). Metals in Group 2 lose two electrons to form +2 ions (e.g., Mg → Mg2+). Non-metals in Group 7 gain one electron to form -1 ions (e.g., Cl → Cl-). Non-metals in Group 6 gain two electrons to form -2 ions (e.g., O → O2-). These rules apply only to the specified groups and to main-group chemistry.
Diagrams that show ionic compounds
Dot-and-cross diagrams show the outer electrons of atoms before and after transfer: one symbol (dot) for electrons from one atom and another symbol (cross) for electrons from the other. Ionic lattice diagrams show repeating patterns of cations and anions in two-dimensional slices or simplified ball-and-stick models. A structure that shows alternating positive and negative ions in a repeating arrangement indicates an ionic compound.
Deducing empirical formula from models or diagrams
Empirical formula reflects the simplest whole-number ratio of ions in the lattice. Count the numbers of each type of ion shown in the unit cell or repeated section of the diagram and reduce to the smallest whole-number ratio. Examples: a 1:1 ratio of Na+ to Cl- gives NaCl; two Mg2+ for one O2- would show a 2:1 ratio and give Mg2O (empirical formula MgO after balancing charges). Always balance total positive and negative charges when deriving the formula.
Drawing dot-and-cross diagrams (Groups 1/2 with 6/7)
Represent only outer-shell electrons. Show electrons as dots or crosses and indicate the transfer from metal to non-metal. After transfer, draw the resulting ions with square brackets and the correct charge. Limit diagrams to metals in Groups 1 or 2 combining with non-metals in Groups 6 or 7. For example, show Li (1 outer electron) transferring one electron to F (7 outer electrons) to produce Li+ and F-; show Mg (2 outer electrons) transferring two electrons to O (6 outer electrons) to produce Mg2+ and O2-.
Key notes
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