Atoms, Ions & Compounds

Atoms and Ions

Atoms are the basic building blocks of matter. They consist of a nucleus containing protons (positively charged) and neutrons (neutral), surrounded by electrons (negatively charged) in shells.

Ions are charged particles formed when atoms lose or gain electrons. This happens because atoms tend to have a full outer shell of electrons, which is more stable.

- When an atom loses electrons, it becomes a positive ion (cation) because there are now more protons than electrons.

- When an atom gains electrons, it becomes a negative ion (anion) because there are more electrons than protons.

Ions are different in size compared to their original atoms:

• Positive ions are smaller than their atoms because they lose electrons, reducing electron repulsion and allowing the remaining electrons to be pulled closer to the nucleus.
• Negative ions are larger than their atoms because gaining electrons increases repulsion between electrons, expanding the electron cloud.

For example, a sodium atom (Na) loses one electron to form a Na⁺ ion, while a chlorine atom (Cl) gains one electron to form a Cl⁻ ion.

Ionic Bonding

Ionic bonding occurs between metals and non-metals. Metals lose electrons to form positive ions, and non-metals gain electrons to form negative ions.

This transfer of electrons creates ions with full outer shells, which are then strongly attracted to each other by electrostatic forces, forming an ionic bond.

These ions arrange themselves in a regular, repeating pattern called a giant ionic lattice. This structure is very stable due to the strong attractions between oppositely charged ions.

Properties of ionic compounds:

• High melting and boiling points because a lot of energy is needed to break the strong ionic bonds in the lattice.
• They conduct electricity only when molten or dissolved in water because the ions are free to move and carry charge.
• In solid form, ionic compounds do not conduct electricity because the ions are fixed in place.

For example, sodium chloride (NaCl) is an ionic compound formed from Na⁺ and Cl⁻ ions.

The transfer of electrons can be shown as:

$$\mathrm{Na} \rightarrow \mathrm{Na}^{+} + e^{-}$$

$$\mathrm{Cl} + e^{-} \rightarrow \mathrm{Cl}^{-}$$

For instance, sodium (Na) has 1 electron in its outer shell and chlorine (Cl) has 7. Sodium transfers its 1 electron to chlorine, resulting in Na⁺ and Cl⁻ ions that attract each other to form NaCl.

Covalent Bonding

Covalent bonding happens between non-metal atoms. Instead of transferring electrons, atoms share pairs of electrons to achieve full outer shells.

Each shared pair of electrons forms a covalent bond. This sharing allows each atom to count the shared electrons towards its outer shell.

Covalent bonds usually form simple molecular structures, where small groups of atoms are held together by covalent bonds, but the forces between molecules are weak.

Properties of covalent compounds:

• Low melting and boiling points because only weak intermolecular forces between molecules need to be overcome.
• They do not conduct electricity because they have no free charged particles.

Dot and cross diagrams are used to show covalent bonding. Electrons from different atoms are shown as dots or crosses to illustrate sharing.

For example, in a water molecule (H₂O), each hydrogen shares one electron with oxygen, forming two covalent bonds.

The oxygen atom shares one electron with each hydrogen atom, completing the outer shell for all atoms.

For instance, in a chlorine molecule (Cl₂), two chlorine atoms share one pair of electrons to form a single covalent bond.

Metallic Bonding

Metallic bonding occurs between metal atoms. Metals have atoms arranged in a giant metallic lattice.

In this lattice, the outer electrons of metal atoms become delocalised — they are free to move throughout the structure rather than being attached to a single atom.

The metal atoms become positive ions, and the delocalised electrons create strong electrostatic forces of attraction between the positive ions and negative electrons. This is the metallic bond.

Properties of metals due to metallic bonding:

• Good electrical conductivity because delocalised electrons can move and carry charge.
• Good thermal conductivity as vibrations and free electrons transfer heat energy.
• High melting and boiling points because metallic bonds are strong and require lots of energy to break.
• Malleability and ductility because layers of metal ions can slide over each other without breaking the metallic bonds.

For example, copper has a giant metallic lattice with delocalised electrons that allow it to conduct electricity and be shaped without breaking.

For instance, in a piece of iron, the iron atoms lose their outer electrons to form positive ions in a lattice. The delocalised electrons move freely, holding the structure together and allowing conductivity.