0620 · 2.4Ions and ionic bonds

Ion formation and ionic bonding

0:00Solid sodium chloride is a poor electrical conductor, but sodium chloride dissolved in water conducts electricity well. The substance contains charged particles in both cases. The difference is whether those particles can move. Understanding that difference begins with how ions form and how they attract one another. In this video, we will connect electron transfer to the structure and properties of ionic compounds. We will build dot-and-cross diagrams for sodium chloride, magnesium oxide and calcium chloride.

0:28Each diagram will show where the electrons came from and which ions formed. Those ideas will then explain melting points and electrical conductivity. An atom contains positively charged protons and negatively charged electrons. A neutral atom has equal numbers of these particles, so their charges balance. Electrons occupy shells around the nucleus. For ionic bonding, we focus on the electrons in the outer shell because these are the ones transferred between atoms.

0:55An ion is an atom or group of atoms with an electrical charge. Here, we will consider ions formed from individual atoms. An atom becomes charged when it loses or gains electrons. Its number of protons stays unchanged, so it remains the same element even though its charge changes. When an atom loses electrons, it forms a positive ion, known as a cation. Sodium provides a useful example because its atom contains eleven protons and eleven electrons.

1:21Losing one electron leaves eleven positive charges but only ten negative charges. The resulting sodium ion has a 1+ charge because one positive charge is no longer balanced. When an atom gains electrons, it forms a negative ion, known as an anion. A chlorine atom contains seventeen protons and seventeen electrons. Gaining one electron gives it eighteen electrons while the proton number remains seventeen. The resulting chloride ion has a 1− charge because there is one extra negative charge.

1:51The direction of the charge change can initially seem surprising. Losing a negatively charged electron makes an atom more positive, not more negative. Gaining an electron makes it more negative because electrons carry negative charge. Think about the balance between protons and electrons rather than treating the words losing and gaining as charge signs. Sodium is in Group I and has one electron in its outer shell. Chlorine is in Group VII and has seven outer electrons.

2:18A sodium atom transfers its outer electron to a chlorine atom. This produces a sodium ion with a 1+ charge and a chloride ion with a 1− charge. The sodium atom begins with an electron arrangement of two, eight, one. After losing its outer electron, the sodium ion has an arrangement of two, eight. Chlorine begins with two, eight, seven and becomes two, eight, eight after gaining that electron. Both ions now have full outer shells, like the nearby noble gases.

2:47Full outer shells help us recognise the ions these elements commonly form. They are not a sign that atoms consciously want or need electrons. Electron transfer produces charged particles, and opposite charges attract. That attraction is the central idea behind an ionic bond, rather than a shared pair of electrons. An ionic bond is a strong electrostatic attraction between oppositely charged ions. Electrostatic means the attraction arises from electrical charges.

3:14The positive sodium ion attracts the negative chloride ion. Electron transfer explains how the ions form, while their attraction explains the bond between them.

Sodium chloride dot-and-cross diagrams

3:23A dot-and-cross diagram makes the electron transfer visible. Use a cross for sodium’s original outer electron and dots for chlorine’s seven outer electrons. Move the cross to chlorine, giving it eight electrons in the diagram. Then put each ion inside square brackets and write its charge outside the brackets at the upper right. The chloride ion should contain seven dots and one cross around its symbol. The sodium ion is shown in brackets with a 1+ charge and no original outer electron remaining.

3:51This does not mean the sodium ion has no electrons at all. Its remaining full shell is simply omitted from this outer-shell-only diagram. Dots and crosses identify where the electrons originally came from. They do not represent two different kinds of electron. Once transferred, sodium’s electron belongs to the chloride ion rather than sitting between the ions. Keeping that distinction clear prevents an ionic diagram from accidentally becoming a diagram of shared electrons.

4:17The same reasoning applies to other combinations of Group I and Group VII elements. A Group I atom loses one electron and forms a cation with a 1+ charge. A Group VII atom gains one electron and forms an anion with a 1− charge. Equal numbers of these ions give an electrically neutral compound. Ionic compounds have high melting points and boiling points. They are good electrical conductors when molten or aqueous, but poor conductors when solid.

4:44Molten means melted into a liquid by heating. Aqueous means dissolved in water, which is a different way of allowing the ions to move. Not every ionic compound dissolves readily, so aqueous does not simply mean any solid placed in water. It describes the dissolved substance in a solution. For an ionic compound that does dissolve, the solution contains ions. We will connect their movement to electrical conduction after looking more closely at the structure.

Giant ionic lattice structure

5:09This next part is for the Extended paper. Ionic compounds have a giant lattice structure, which is a regular arrangement of alternating positive and negative ions. Giant means that the structure contains very many ions in a repeating pattern. The pattern extends in three dimensions, rather than forming separate pairs of particles. Our drawing shows only a small section through the sodium chloride lattice. Each positive ion has negative neighbours, and each negative ion has positive neighbours.

5:38Attractions act throughout the structure, not just between one sodium ion and one chloride ion. A salt crystal is therefore not a collection of separate sodium chloride molecules. The formula of sodium chloride tells us the simplest ratio of its ions. There is one sodium ion for every chloride ion throughout the lattice. It does not mean that the crystal contains only two ions. The equal amounts of positive and negative charge make the whole compound electrically neutral.

Magnesium oxide and calcium chloride

6:04Magnesium oxide provides a worked example involving a different electron transfer. A magnesium atom has the electron arrangement two, eight, two, so it has two outer electrons. An oxygen atom has the arrangement two, six, leaving space for two more electrons in its outer shell. One magnesium atom can therefore supply both electrons gained by one oxygen atom. First, magnesium loses two electrons and forms a magnesium ion with a 2+ charge.

6:32Its remaining electron arrangement is two, eight. Next, oxygen gains those two electrons and forms an oxide ion with a 2− charge. The oxide ion has an electron arrangement of two, eight, giving it a full outer shell. For the diagram, use crosses for magnesium’s two outer electrons and dots for oxygen’s six outer electrons. Transfer both crosses to oxygen, so the oxide ion shows six dots and two crosses.

6:58Enclose each ion in square brackets with the correct charge outside. The ions do not share any electrons, so no bonding pairs appear between them. One magnesium ion has a 2+ charge, which balances one oxide ion with a 2− charge. The ion ratio is therefore one to one, despite the transfer of two electrons. The formula shown on screen expresses that ratio. The strong electrostatic attraction between these oppositely charged ions is the ionic bond.

7:25Calcium chloride shows that the ratio of ions is not always one to one. A calcium atom has two outer electrons, while each chlorine atom needs one electron to complete its outer shell. Calcium loses both electrons, but each goes to a different chlorine atom. One calcium atom therefore transfers electrons to two chlorine atoms. The calcium ion has a 2+ charge, and each chloride ion has a 1− charge.

7:51Two chloride ions provide a total charge of 2−. This balances the calcium ion’s 2+ charge, giving an electrically neutral compound. The formula therefore shows two chloride ions for every calcium ion. In this diagram, each chloride ion needs seven dots and one cross, giving eight outer electrons altogether. The calcium ion is enclosed in brackets with a 2+ charge. Both chloride ions have separate brackets and a 1− charge each.

8:17Showing only one chloride ion would leave the charges unbalanced and lose one transferred electron. These examples give a reliable method for constructing an ionic diagram. Identify the outer electrons, then determine how many each metal atom loses and each non-metal atom gains. Use enough atoms for every transferred electron to be accounted for. Finally, check the full outer shells, the individual ion charges and the overall charge balance.

Ionic properties and conduction

8:42The giant lattice explains the high melting points and boiling points of ionic compounds. There are many strong electrostatic attractions between oppositely charged ions throughout the structure. A large amount of energy is needed to overcome these attractions. Strong bonding therefore means substantial heating is required for melting or boiling. Do not explain melting in terms of weak forces between molecules, because an ionic compound contains no molecules. The ions have already formed, and heating does not turn them back into neutral atoms.

9:12During melting, the rigid arrangement breaks down sufficiently for ions to move past one another. They remain charged particles in the molten substance. Electrical conduction requires charged particles that are free to move. A solid ionic compound contains ions, but they are held in fixed positions within the lattice. They can vibrate around those positions but cannot travel through the solid to carry charge. This is why the solid is a poor electrical conductor.

9:38When an ionic compound is molten, its ions are free to move and carry charge through the liquid. When a soluble ionic compound dissolves, its ions can move through the aqueous solution. Both states therefore allow electrical conduction. The important difference is ion mobility, not whether charged particles exist in the first place. A common examination mistake is to explain this conduction using free electrons. In molten and aqueous ionic compounds, moving ions carry the electrical charge.

10:07Another mistake is to say that the ions become charged only after melting. They are already charged in the solid, but their fixed positions prevent them from carrying charge through it.

Exam answers and recap

10:16An examination may ask you to describe ion formation, draw a dot-and-cross diagram or explain a compound’s properties. The command word explain requires a reason, not just a statement of the property. Consider the question shown about magnesium oxide. Build the answer from its structure, the attractions between ions and whether those ions can move. Magnesium oxide has a giant lattice of alternating positive and negative ions. There are many strong electrostatic attractions between oppositely charged ions.

10:46A large amount of energy is needed to overcome these attractions, so magnesium oxide has a high melting point. Each sentence connects a feature of the structure to the observed property. For the conductivity part, give the movement explanation directly. In solid magnesium oxide, the ions are held in fixed positions and cannot move through the lattice to carry charge. In molten magnesium oxide, the ions are free to move and carry charge.

11:11This explains why the molten substance conducts but the solid is a poor conductor. For a diagram question, check more than the total number of electrons. The dots and crosses must show the correct origin of transferred electrons. Every ion needs square brackets and its own charge outside them. The numbers of cations and anions must also give an electrically neutral compound overall. The central connection is electron transfer, followed by attraction between the ions formed.

11:38Losing electrons produces positive ions called cations, while gaining electrons produces negative ions called anions. An ionic bond is a strong electrostatic attraction between oppositely charged ions. Dot-and-cross diagrams show the electron transfer rather than electron sharing. Those ions form a giant lattice with many strong attractions, explaining high melting points and boiling points. Solid ionic compounds conduct poorly because their ions cannot move through the structure. Molten and aqueous ionic compounds conduct because their ions are free to move and carry charge.

12:12Structure, attraction and movement explain the properties together.