The diagram shows a small part of a sodium chloride (NaCl) crystal, made up of sodium ions and chloride ions electrostatically attracted to each other - Leaving Cert Chemistry - Question 5 - 2015

In a sodium atom, there are a total of 11 electrons. The electron configuration can be illustrated as follows:

• The first shell holds 2 electrons.
• The second shell holds 8 electrons.
• The third shell has 1 electron.

Visually, this is represented by:

    1s²
   2s² 2p⁶
   3s¹

Where each 's' indicates an electron subshell, and the superscripts represent the number of electrons in those subshells.

A sodium atom becomes a sodium ion by losing its one valence electron from the third shell. This loss of an electron gives the sodium ion a positive charge, resulting in Na⁺, as it now has 11 protons and only 10 electrons.

In a chlorine atom, there are a total of 17 electrons. The electron configuration is as follows:

• The first shell holds 2 electrons.
• The second shell holds 8 electrons.
• The third shell holds 7 electrons.

Visually, this is represented by:

    1s²
   2s² 2p⁶
   3s² 3p⁵

This indicates the distribution of electrons across the different energy levels.

A chlorine atom becomes a chloride ion by gaining one additional electron, which completes its third shell, resulting in Cl⁻. This gain of an electron gives the chloride ion a negative charge, leading to 17 protons and 18 electrons.

Chlorine atoms are smaller than sodium atoms because they have fewer electrons in their outer shells and higher effective nuclear charge due to the presence of more protons. This stronger attraction between the protons in the nucleus and the electrons draws the electron cloud closer to the nucleus, making the atomic radius smaller.

Electronegativity is a measure of the tendency of an atom to attract a bonding pair of electrons. It is expressed on a scale with higher values indicating a stronger ability to attract electrons.

Sodium chloride can dissolve in water because the polar water molecules have positive and negative ends that interact with the charged ions. The positive end attracts the chloride ions (Cl⁻) while the negative end attracts the sodium ions (Na⁺), effectively pulling the ions apart from the crystal lattice.

Sodium chloride solution can conduct electricity because when it dissolves in water, it dissociates into free-moving sodium (Na⁺) and chloride (Cl⁻) ions. These ions are charged particles that enable the flow of electrical current when an electric field is applied.