Ionic Bond Formation

For a negative ion (anion) to form, the atom must gain electrons. Conversely, for a positive ion (cation) to form, the atom must lose electrons and find an atom to accept these electrons. The process involves electron transfer from the cation to the anion.

An infographic illustrating the Formation of an Ionic Bond in three steps. Step 1 shows two separate atoms with a dashed arrow indicating the Transfer of an Electron from the yellow atom to the blue atom. Step 2 displays the result of the electron transfer: a Positive Ion shown in yellow with a plus sign and a Negative Ion shown in blue with a minus sign, along with arrows pointing towards each other to denote Attraction. Step 3 presents the IONIC BOND, where the positive and negative ions are combined, depicting the complete ionic bond with the electron now orbiting the blue ion.

Let’s look at the formation of ions using sodium and chlorine as an example:

A chlorine atom gains an electron to complete its outer shell and becomes a negatively charged chlorine anion. Simultaneously, a sodium atom loses an electron, becoming a positively charged sodium cation. This electron transfer typically occurs when a sodium atom and a chlorine atom are close under the right conditions, leading to both atoms becoming ions.

An illustration showing the electron transfer process during the formation of an ionic bond between a sodium atom and a chlorine atom. The top section shows a sodium atom with an electron configuration of 2.8.1 and a chlorine atom with a configuration of 2.8.7. One electron is transferred from sodium to chlorine, depicted by an arrow. Below, the resulting ions are shown: Na+ with a stable electron configuration of 2.8, mirroring the noble gas neon, and Cl- with a configuration of 2.8.8, mirroring argon. The phrase 'Strong electrostatic forces of attraction between ions of opposite charge form the ionic bond' explains the bond's nature. Both ions are shown with full outer shells of electrons, signifying stability.

After the electron transfer, the opposite charges of the chlorine anion and sodium cation result in a strong electrostatic attraction between them, forming an ionic bond.

Dot and Cross Diagrams

Dot and cross diagrams are tools to visualise the transfer of electrons, where dots represent electrons from one atom, and crosses represent electrons from another. They illustrate how ions change their electron configuration to achieve full outer shells.

Let’s look at two examples.

Sodium chloride

A dot and cross diagram for a sodium atom and a chlorine atom would look like this:

A diagram featuring two separate atoms: sodium (Na) on the left and chlorine (Cl) on the right. The sodium atom is illustrated with three concentric circles representing electron shells, with 11 electrons depicted as red dots. The chlorine atom also has three electron shells, with 17 electrons represented by blue crosses. The diagram is set against a white background and is used to visually describe the electron distribution in each atom before the formation of an ionic bond.

After they form an ionic bond, they would look like this:

Two atomic diagrams within square brackets indicating ions. On the left is a sodium ion (Na+) with two electron shells and ten electrons represented as red dots, indicating it has lost an electron and become a cation. On the right is a chlorine ion (Cl-) with three shells and eighteen electrons represented as blue crosses, indicating it has gained an electron and become an anion. A plus sign is shown next to the sodium ion, and a minus sign is next to the chlorine ion, representing their respective charges after electron transfer.

You can see that one of sodium’s electrons is now in the chlorine ion’s outermost electron shell. Both ions now have full electron shells.

Magnesium oxide

Let’s look at how a magnesium atom (Mg) and an oxygen atom (O) form an ionic bond:

  • Magnesium, located in Group 2 of the periodic table, will lose two electrons, while oxygen, in Group 6, will gain two electrons. As a result, magnesium becomes a cation with a +2 charge, and oxygen becomes an anion with a -2 charge.
  • The transfer of the two electrons from magnesium to oxygen results in both atoms achieving full outer electron shells, leading to a stable ionic compound.
  • We can draw this using dots to represent electrons originally with magnesium, and crosses for electrons originally with oxygen.

Before ionic bonding:

A diagram showing two separate atoms: magnesium (Mg) on the left with three electron shells and twelve electrons depicted as red dots, and oxygen (O) on the right with two electron shells and eight electrons shown as blue crosses. The magnesium atom has its outer shell filled with two electrons, while the oxygen atom has six electrons in its outer shell, indicating their valence electrons prior to bonding.

After ionic bonding:

A diagram within square brackets showing two ions: a magnesium ion (Mg2+) on the left with two electron shells and ten electrons, indicated by red dots, and an oxygen ion (O2-) on the right with two electron shells and ten electrons, shown as blue crosses. Two additional electrons, represented by red dots, are on the oxygen ion, indicating it has gained two electrons. The magnesium ion is labelled with a '+2' indicating a 2+ charge, and the oxygen ion is labelled with a '-2' indicating a 2- charge, representing their charges after the electron transfer.

The Lattice Structure of Ionic Compounds

Ionic compounds typically form an ionic lattice, not isolated molecules. This structure arises from the electrostatic forces that attract cations to anions and vice versa, arranging them in a regular, repeating three-dimensional pattern. This alternating arrangement of anions and cations creates a stable structure.

Take sodium chloride (NaCl) for example. There is a repeating organisation between the anions and cations. It is this structure that you are looking at when you see crystals of table salt: an ionic lattice of millions of sodium and chlorine ions, bound together by ionic bonds.

A visual representation of the formation of solid sodium chloride (NaCl) from sodium and chlorine atoms. On the left, a neutral sodium atom with 11 protons and 11 electrons loses an electron, becoming a Na+ ion with 10 electrons. Similarly, a neutral chlorine atom with 17 protons and 17 electrons gains an electron to become a Cl- ion with 18 electrons. An arrow labelled 'electron transfer' shows the movement of an electron from the sodium atom to the chlorine atom. On the right, a three-dimensional lattice structure represents solid sodium chloride, with alternating purple spheres (sodium ions) and green spheres (chloride ions) arranged in a repeating cubic pattern, with '+' and '-' signs indicating their charges.

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