Period 3 Elements

An element's period tells us how many electron shells it has. We can therefore correctly infer that period 3 elements all have three electron shells. However, they have different numbers of electrons. An element's atomic number tells us about the number of protons in the nucleus of one of its atoms; for neutral atoms, this is equal to its number of electrons. Atomic number increases one by one as you move across the period, from sodium to argon. This also means that the number of electrons increases one by one as you move across the period; each element has one more electron than the element before it. This will become important when we look at the properties of period 3 elements, which we'll move on to next.

Properties of period 3 elements

Period 3 in the periodic table is a great example of periodicity. In chemistry, this means the repetition of trends in properties after a certain interval of atomic number. In simple terms, periodicity tells us that there are clear patterns when it comes to particular atomic properties. These repeat with every new period in the periodic table. In this article, we'll explore four such properties.

• Atomic radius
• Melting point
• First ionisation energy
• Electrical conductivity

Atomic radius

Atomic radius decreases as you move across period 3 in the periodic table. To understand why, we need to go back to the atomic structure of period 3 elements.

Remember how atomic number increases as you move across a period in the periodic table? This means that the number of protons increases. Each element has the same number of protons as it does electrons, so the number of electrons also increases. These electrons are found orbiting the nucleus in shells. (Check out Periodic Trends for more information.)

However, although elements in the same period have different numbers of electrons, they have the same number of electron shells. In period 3, all of the elements have three electron shells. This also means that they have the same number of inner electron shells. The inner electron shells shield the outer electron shell from some of the charge of the nucleus. The attraction between the remaining charge of the nucleus and the outer electron shell determines atomic radius.

So, as you go across a period, atomic number increases - each element has one more proton and one more electron than the element before it. It means that the charge of the nucleus increases. However, the number of inner shells stays the same, so all period 3 elements experience the same shielding of the nuclear charge. This means that as you go across a period, the outer electron shell feels a larger overall nuclear charge. The outermost electron shell experiences a stronger attraction to the positively charged nucleus, so the negative electrons are pulled in closer to the nucleus in the centre of the atom. This decreases atomic radius.

Melting point

Melting point varies as you move across period 3 in the periodic table. This is all to do with structure and bonding.

Fig. 2 - Melting point of period 3 elements

Here's why.

• Sodium (Na), magnesium (Mg), and aluminium (Al) have medium-high melting points. This is because they are metals. Magnesium has a higher melting point than sodium because it both has a smaller atomic radius, and it forms ions with a higher charge: sodium forms 1+ ions, whereas magnesium forms 2+ ions. Aluminium has a higher melting point than both sodium and magnesium because, once again, it has a smaller atomic radius and forms ions with an even higher charge.
• Silicon (Si) has a very high melting point. This is because it exists as a giant covalent macromolecule held together by lots of covalent bonds. In order to melt silicon, you need to overcome these covalent bonds, which requires a lot of energy.
• Phosphorus (P), sulphur (S), and chlorine (Cl) have low melting points. This is because they are simple covalent molecules. Although there are strong covalent bonds within the molecules, the only forces between molecules are weak intermolecular forces. These don't require much energy to break. Sulphur forms larger molecules than phosphorus, which in turn forms larger molecules than chlorine. This increases the strength of the intermolecular forces, raising the melting point of the element.
• Argon (Ar) has a very low melting point. This is because it is a monoatomic gas; it doesn't form molecules. The intermolecular forces between its atoms are very weak and require hardly any energy to overcome at all.

Ionisation energy

In general, first ionisation energy increases as you move across period 3 in the periodic table. As with atomic radius, this is due to the number and arrangement of protons and electrons in the element.

Fig. 3 - First ionisation energy of period 3 elements

As you move across the period, each element has one more proton and one more electron than the element before it. This means that the nuclear charge increases. However, all of the elements in period 3 have the same number of electron shells. This means that any inner shells' shielding of the nuclear charge remains the same. An increased nuclear charge but the same levels of shielding results in a stronger attraction between the nucleus and the outermost electron, increasing first ionisation energy.

Electrical conductivity

The last trend we'll look at is electrical conductivity. It varies across the period. We've shown all values relative to the conductivity of aluminium, which is the best conductor out of the lot.

Fig. 4 - Electrical conductivity of period 3 elements

Note the following:

• Sodium (Na), magnesium (Mg), and aluminium (Al) have high conductivities. This is because they are metals, made from a lattice of positive metal ions in a sea of delocalised electrons. The electrons are free to move and carry a charge, enabling them to conduct electricity. Magnesium is a better conductor than sodium because it has more delocalised electrons per positive metal ion. Aluminium is a better conductor still because once again, it has more delocalised electrons per positive metal ion.
• Silicon (Si) has a low conductivity. This is because it is a metalloid, and so is a semi-conductor.
• Phosphorus (P), sulphur (S), chlorine (Cl), and argon (Ar) don't conduct electricity. This is because they form covalent molecules, or in the case of argon, monoatomic gases. There are no charged particles free to move around and carry a charge.

That's it for the trends in properties of period 3 elements. Let's now move on to our next topic - some of their reactions.

Reactions of period 3 elements

We'll look at the reactions of period 3 elements with three different species:

• Oxygen
• Chlorine
• Water

Reaction with oxygen

All period 3 elements (with the exception of chlorine and argon) react with oxygen (either in the air or in pure oxygen gas, if stated otherwise) to produce an oxide. The oxidation state of the period 3 element increases, and the oxidation state of oxygen decreases. This makes the reaction a redox reaction.

Here's a handy table comparing the reactions of period 3 elements with oxygen. We've included the element, conditions, product, observation, equation, and the final oxidation state of the period 3 element.

Reaction with chlorine

Now it is time for the reactions of period 3 elements with chlorine. They all form chlorides. As before, we've made a table to help you out.

You'll notice that we've missed out chlorine and argon once again. Argon doesn't react with chlorine, thanks to its noble gas status, and it doesn't make sense to talk about chlorine reacting with chlorine!

Disulphur dioxide is a nasty-smelling liquid used to prepare mustard gas in the Levinstein process. This involves reacting disulphur dioxide with ethene at 60°C.

Reaction with water

Lastly, let's explore how period 3 elements react with water. Luckily for you, you only need to know about the reactions involving sodium and magnesium. They form hydroxides, but magnesium also reacts with steam to form an oxide. Here's how the reactions compare.

Element	Conditions	Product	Observation	Equation	Oxidation state
Na	Cold water	Sodium hydroxide, hydrogen	Vigorous fizzing, colourless solution		+1
Mg	Cold waterHeat, steam	Magnesium hydroxide, hydrogenMagnesium oxide, hydrogen	Slow fizzing, colourless solutionWhite flame, white powder		+2+2

By now you should be able to describe and explain the trends in properties of period 3 elements, as well as describe how they react with oxygen, chlorine, and water. If you want to learn more about period 3 oxides and chlorides, we'd recommend reading Period 3 Oxides, which will tell you everything you need to know about these compounds.