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Lesson Explainer: Ionic Bonds Science

In this explainer, we will learn how to describe the formation of ionic bonds in simple binary compounds.

One of the most familiar ionic compounds is sodium chloride, or table salt. People use table salt for adding flavor to food, preserving food, and melting snow. However, there are several other ionic compounds, like magnesium oxide and sodium sulfide.

Definition: Ionic Compound

An ionic compound is a neutral compound composed of positively and negatively charged ions.

Ionic compounds form when outer-shell electrons effectively leave reactant metal atoms and combine with nonmetal atoms. The metal atoms form positive ions because they lose negatively charged electrons. The nonmetal atoms form negative ions because they gain those negatively charged electrons. These oppositely charged ions make an ionic compound as they combine and form a lattice structure.

So, ionic compounds are the combination of oppositely charged ions. They are stable compounds because of the ionic bonds between their oppositely charged ions. Chemists state that ionic compounds have a giant lattice structure because they have a symmetrical arrangement of repeating ions and are much larger than ordinary covalent compounds.

Definition: Ionic Bond

An ionic bond is an electrostatic attraction between the positive and negative ions of a chemical compound.

Figure 1: Models for part of the giant sodium chloride lattice structure.

Example 1: Determining Which Two Types of Elements Form an Ionic Compound

Which two types of elements can react and form an ionic compound?

  1. A metal and another metal
  2. A metal and a nonmetal
  3. A nonmetal and another nonmetal

Answer

Ionic compounds are neutral compounds made up of positive and negative ions. They form when outer-shell electrons effectively leave metal atoms and combine with nonmetal atoms. The metal atoms become the positive ions of the ionic compound. The nonmetal atoms become the negative ions. Hence, option B must be the answer to this question.

Example 2: Describing the Attraction between the Ions of an Ionic Bond

What type of attraction exists between a positive ion and a negative ion in an ionic bond?

Answer

Ionic compounds form when electrons move between atoms during a chemical reaction process. The movement of outer-shell electrons makes complementary positive and negative ions.

The ionic compounds are the combination of these oppositely charged ions. They are stable because of the electrostatic attraction forces between their ions. So, the answer must be electrostatic attraction.

The compound sodium chloride forms when outer-shell electrons leave metallic sodium atoms and combine with nonmetal chlorine atoms. Each sodium atom turns into a positive Na+ ion when it loses a single outer-shell electron during the chemical reaction process.

All the reactant chlorine atoms turn into negative Cl ions as they gain these single outer-shell electrons. During the reaction, the electron number of sodium decreases by one and the electron number of chlorine increases by one.

Figure 2: The formation of sodium chloride from reactant sodium and chlorine atoms.

Example 3: Determining What Forms as a Sodium Atom Loses One Outer-Shell Electron

In the diagram, an atom of sodium loses one electron from the outer shell.

What is formed when this electron is lost?

Answer

Atoms have no overall electric charge because they have an equal number of protons and electrons. However, they can form ions when they lose electrons during a chemical reaction process. A sodium metal atom can form a positively charged Na+ ion when it loses a single outer-shell electron during a chemical reaction process. As such, the answer must be a positive sodium ion.

Magnesium oxide is another ionic compound that forms when outer-shell electrons move between atoms. The metal oxide forms as metallic magnesium atoms lose two outer-shell electrons and turn into Mg2+ ions. These electrons combine with reactant oxygen atoms that become O2 ions as they gain the lost electrons. The electron number of magnesium decreases by two during this reaction, and the electron number of oxygen increases by two.

Figure 3: The formation of magnesium oxide from reactant magnesium and oxygen atoms.

Definition: Oxide Compound

An oxide compound contains oxygen and another chemical element.

Example 4: Identifying the Electronic Configuration of a Metal Atom When It Reacts with Oxygen and Forms an Oxide

The following diagram is a representation of the electronic configuration of a metal atom.

This metal can react with the element oxygen to form an oxide. Which of the following diagrams would be a similar representation of the ion of the same metal formed during the reaction?

Answer

The electronic configuration diagram in question is for a magnesium atom, as it shows twelve electrons. Magnesium forms magnesium oxide when it reacts with oxygen. The reaction process occurs as two electrons move over from magnesium atoms to oxygen atoms. Each magnesium atom turns into an Mg2+ ion during the reaction, while reactant oxygen atoms become O2 ions. Option A shows the electronic configuration of the Mg2+ ion and must therefore be the answer.

Not all ionic substances have the same number of positive and negative ions. Some contain metal and nonmetal ions whose charge states are not equal and opposite. So, the compounds must have an unequal number of oppositely charged ions to have no overall electric charge and be stable.

Sodium oxide is a metal oxide and an ionic compound with an unequal number of positive and negative ions. It has twice as many sodium ions as oxygen ions since Na+ ions have the 1+ charge state and oxygen ions have the 2 charge state. The compound can only have no overall electric charge if it has two Na+ ions for each O2 ion. So, sodium oxide has the chemical formula NaO2.

Figure 4: Top view for a part of the compound sodium oxide.

All metal salts have an ionic character, but metal salts are not all equally soluble in polar substances like water. Metal salts like sodium chloride are highly soluble in water, and other salts like silver chloride and copper hydroxide are insoluble.

Table 1: The solubility of metal salts in water.

SolubleInsoluble
All nitrates
Most sulfates Lead sulfate, barium sulfate, and calcium sulfate
Most chlorides, bromides, and iodidesSilver chloride, silver bromide, silver iodide, lead chloride, lead bromide, and lead iodide
Sodium carbonate, potassium carbonate, and ammonium carbonateMost other carbonates
Sodium hydroxide, potassium hydroxide, and ammonium hydroxide Most other hydroxides

We can determine the charge state of a positive ion in an ionic compound from its periodic table group number. Metal ions ordinarily have a positive charge state that matches their group number. For example, Na+ ions have the 1+ charge state because sodium is a group one element. Mg2+ ions have the 2+ charge state because magnesium is a group two chemical element.

The nonmetal elements of ionic compounds ordinarily get a negative charge state equal to eight minus their number of outer-shell electrons. For example, S2 ions have the 2 charge state because sulfur has six outer-shell electrons, and eight minus six is two. Similarly, Cl and F ions have the 1 charge state because they have seven outer-shell electrons.

Figure 5: The standard charge of elements in ionic compounds.

Let us summarize what we have learned in this explainer.

Key Points

  • An ionic compound is a neutral compound composed of positively and negatively charged ions.
  • An ionic compound has a giant lattice structure.
  • An ionic bond is an electrostatic attraction between the positive and negative ions of a chemical compound.
  • Ionic compounds form as electrons move from metal atoms to nonmetal atoms.
  • An oxide compound contains oxygen and another chemical element.
  • Some ionic compounds have an unequal number of positive and negative ions.
  • Metal salts are not all equally soluble in polar substances like water.
  • The charge state of a positive ion in an ionic compound depends on its periodic table group number.
  • The charge state of a negative ion in an ionic compound depends on its periodic table group number.

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