Ionic Bonding

Since opposite charges attract, the Na⁺ and Cl⁻ ions are attracted to each other (think of static cling!).Intro to Physics and Chemistry Index

General Chemistry Index

Where are we going with this? This page will give the ability to use laboratory observations and data to compare and contrast ionic, covalent, network, metallic, polar, and non-polar substances with respect to constituent particles, strength of bonds, melting and boiling points, and conductivity; provide examples of each type.

Ionic Bonding
When ions bond… 

Ionic bonding occurs when oppositely charged ions are attracted to each other after one or more electrons have been transferred from one atom to another. This usually occurs between a metal and a nonmetal.

So, many metals have three or fewer electrons in their outermost energy level. Other elements, especially nonmetals, need only a few more electrons to fill their valence shell.

Compounds formed between metals and nonmetals often involve ionic bonding. In ionic bonding, one or more valence electrons are transferred from the metal atom to the nonmetal atom.

Story (sort of) time…

So, for instance, sodium (Na) meets up with chlorine (Cl). Sodium has that one electron stuck out there by itself in the last set of orbitals (in the 3s orbital) and chlorine is missing one electron in its last set of orbitals (in the 3p sublevel). 

Sodium transfers its electron to chlorine, giving both particles stable valence-shell electron arrangements. BUT, since each particle now has a different number of protons and electrons, each particle has a net electrical charge. (Thus, they are now considered ions.) They have opposite charges, in fact. Since opposite charges attract, the Na⁺ and Cl⁻ ions are attracted to each other (think of static cling!).

Now, we need a fancy word…

Electronegativity is a measure of how strongly an atom attracts electrons toward itself when it is bonded to another atom.

Atoms with high electronegativity pull strongly on shared electrons. Atoms with low electronegativity do not pull as strongly.

In general, electronegativity increases from left to right across the periodic table and increases as you move upward within a group.

Basically: electronegativity tells us how hard an atom is pulling on the electrons in a bond.

This is ionic bonding…

When the electronegativity difference between two atoms is large, one or more valence electrons may be transferred from the atom with lower electronegativity to the atom with higher electronegativity. This creates positively and negatively charged ions that attract each other.

A little more… 

Depending on the charge, ions get names…

Cation: a positively charged ion

Anion: a negatively charged ion

 

And then there's this…

Unlike molecular compounds, ionic compounds do not usually exist as separate molecules. Instead, positive and negative ions form large, repeating crystal structures called ionic lattices. The chemical formula shows the simplest whole-number ratio of ions in the compound.

 

How ionic compound formulas are determined…

In a neutral ionic compound, the charges of all the ions add up to zero.

The ratio of ions in the formula must produce an overall net charge of zero.

It is usually straightforward to predict the formulas of ionic compounds when the charges of the ions are known.

Example:

Mg commonly forms an Mg²⁺ ion.

S commonly forms an S²⁻ ion.

Thus, one Mg²⁺ ion combines with one S²⁻ ion, producing a 1:1 ratio:

+2 + (−2) = 0

The formula is MgS.

Li commonly forms a Li⁺ ion.

S still commonly forms an S²⁻ ion.

Therefore, two Li⁺ ions are needed for every one S²⁻ ion to produce a net charge of zero:

+1 + 1 + (−2) = 0

The formula is Li₂S.

Writing them out…

EXAMPLES:

If you had:

Na⁺ 

O²⁻

You get Na₂O.

_____________

If you have polyatomic ions, you can think of each one as a single “chunk.” Then, apply the charge-balancing process to the entire chunk.

If you had:

Zn²⁺

PO₄³⁻

You get Zn₃(PO₄)₂.

NOTE: If the subscript of a polyatomic ion is 1, do NOT use parentheses.

For example:

Na⁺ + NO₃⁻ → NaNO₃, not Na(NO₃).

Using the "criss-cross" method:

• the absolute value of the positive ion's charge becomes the subscript of the negative ion.
• the absolute value of the charge of the negative ion becomes the subscript for the positive ion

NOTE: Reduce the subscripts to the lowest whole-number ratio. For example, if the criss-cross method gives

Mg₂S₂

reduce the 2:2 ratio to 1:1 and write

MgS