What do the salt you sprinkle over your chips, porcelain, the chlorophyll in leaves, and water treatment facilities have in common?
They all feature period 3 elements, the eight elements found in the third row of the periodic table.
- This article is about period 3 elements.
- Once we've looked at which elements are part of period 3, we'll look at some of their properties.
- You'll be able to explain trends in properties using your knowledge of structure and bonding.
- We'll then explore how period 3 elements react with oxygen, chlorine, and water.
Period 3 elements in the periodic table
Period 3 elements refer to the chemical elements that are found in the third row (or period) of the periodic table (thus also known as third row elements). They are in period 3 because their outer electrons are in 3-level orbitals.
In total, there are eight elements in period 3. Starting from left to right, they are:
- Sodium (Na)
- Magnesium (Mg)
- Aluminum (Al)
- Silicon (Si)
- Phosphorus (P)
- Sulfur (S)
- Chlorine (Cl)
- Argon (Ar)
These elements belong to different groups and have varying properties. Sodium and magnesium are metals, while aluminium is also a metal but exhibits some properties of a metalloid. Silicon is a metalloid, and phosphorus, sulfur, chlorine, and argon are nonmetals. These elements show a gradual transition in properties as you move from left to right across the period, ranging from metallic to nonmetallic characteristics.
Here's a copy of the periodic table. One of the rows (which are also known as periods) is highlighted in pink. This is period 3, and the eight elements within it are the period 3 elements.
Fig. 1 - Period 3 in the periodic table
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 includes the following elements:
| Table 1. Properties of period 3 elements. |
|---|
| Name | Symbol | Atomic number | Fact |
| Sodium | Na | 11 | Sodium is the sixth most abundant element in the Earth's crust. Sodium is a soft, silver-white metal that is highly reactive. It has a low melting point and reacts vigorously with water, releasing hydrogen gas. Sodium is an important element in many biological processes and is commonly found in table salt (sodium chloride, NaCl). |
| Magnesium | Mg | 12 | Magnesium is a light, silvery-white metal that is relatively reactive. It has a higher melting point than sodium and is less reactive with water. Mg ions are essential to over 300 enzymes in the human body. They're also a part of chlorophyll. This metal is produced in large, ageing stars when three helium nuclei are added to a carbon nucleus. |
| Aluminium | Al | 13 | Aluminium is the second most-produced metal in the world, beaten only by iron. It is a lightweight, silvery metal that is corrosion-resistant due to the formation of a thin oxide layer on its surface. It is a good conductor of heat and electricity. |
| Silicon | Si | 14 | Silicates and other minerals containing silicon make up over 90% of the Earth's crust. Silicon is used to not only make porcelain, but also the semiconductor chips in most electronics. It has both metallic and non-metallic properties. It is a hard and brittle solid that is a poor conductor of electricity in its pure form. However, it becomes a semiconductor when combined with other elements. |
| Phosphorus | P | 15 | Phosphorus exists in various forms, including white phosphorus (a waxy, flammable solid), red phosphorus (a less reactive form), and black phosphorus (a layered structure with semiconducting properties). The word phosphorus means 'light bearer', thanks to the fact that white phosphorus glows when exposed to oxygen. |
| Sulphur | S | 16 | Sulfur is a yellow, brittle solid that can form compounds with a wide range of colours and smells. It is used in fertilisers, sulfuric acid, rubber, etc. Over 80% of the sulphur extracted today is turned into sulphuric acid, which in 2010 was the most produced organic chemical in the US. |
| Chlorine | Cl | 17 | Chlorine is a pale green gas with a pungent odour. It is highly reactive and is commonly used as a disinfectant and in the production of various chemicals, including plastics, solvents, and pesticides. Chlorine has the third highest electronegativity value in the periodic table, exceeded only by oxygen and its fellow halogen fluorine. It is commonly used in sewage treatment plants. |
| Argon | Ar | 18 | The word argon is derived from the Greek word for 'lazy', thanks to the fact that this noble gas is an inert gas, i.e. extremely unreactive. It is colourless, odourless, and tasteless. Argon is the most abundant noble gas in the Earth's atmosphere and is used in various applications, such as filling incandescent light bulbs and as a shielding gas in welding. |
Trends 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
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.
First ionisation energy is the energy needed for one mole of gaseous atoms to each lose their outermost electron, forming one mole of gaseous cations.
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.
You'll notice that there is a dip between groups 2 and 3, and 5 and 6. This is because of electron sub-shells and orbitals; we look at this in more detail in Trends in 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.
You can learn about metalloids in Periodic Table.
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:
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.
Check out Redox for more about oxidation states and redox reactions.
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.
| Element | Conditions | Product | Observation | Equation | Oxidation state |
| Na | Heat | Sodium oxideSodium peroxide | Orange flame, white powder | \(4Na(s) + O_2(g) \rightarrow 2Na_2O(s)\) | +1 |
| Mg | Heat | Magnesium oxide | White flame, white powder | \(2Mg(s) + O_2(g) \rightarrow 2MgO(s)\) | +2 |
| Al | Heat, powdered aluminium | Aluminium oxide | White sparkles, white powder | \(4Al(s) +3O_2(g) \rightarrow 2Al_2O_3(s)\) | +3 |
| Si | Heat, pure oxygen | Silicon dioxide | White sparkles, white powder | \(Si(s) + O_2(g) \rightarrow SiO_2(s)\) | +4 |
| P (white) | Room temperature | Phosphorus(III) oxidePhosphorus(V) oxide | Yellow/white flame, white smoke | \(P_4(s)+ 3O_2(g) \rightarrow P_4O_6(g)\)\(P_4(s) + 5O_2(g) \rightarrow P_4O_{10}(g)\) | +3+5 |
| S | Heat, pure oxygen | Sulphur dioxide | Blue flame, colourless gas | \(S(s) + O_2(g) \rightarrow SO_2(g)\) | +4 |
Aluminium reacts extremely rapidly with oxygen in the air. However, we are able to use aluminium in sectors such as construction, transportation, and food packaging because it forms a protective layer of aluminium oxide on the surface, preventing the metal underneath from reacting any further.
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.
| Element | Conditions | Product | Observation | Equation | Oxidation state |
| Na | Heat | Sodium chloride | Orange flame, white powder | \(2Na(s) + Cl_2(g) \rightarrow 2NaCl (s)\) | +1 |
| Mg | Heat | Magnesium chloride | White flame, white powder | \(Mg(s) + Cl_2(g) \rightarrow MgCl_2 (s)\) | +2 |
| Al | Heat | Aluminium chloride | Pale yellow solid | \(2Al(s) + 3Cl_2 (g) \rightarrow 2AlCl_3(s)\) | +3 |
| Si | Heat | Silicon tetrachloride | Colourless liquid | \(Si(s) + 2Cl_2 (g) \rightarrow SiCl_4(l)\) | +4 |
| P (white) | Room temperature | Phosphorus(III) chloridePhosphorus(V) chloride | Colourless liquidOff-white/yellow solid | \(P_4(s) + 6Cl_2(g) \rightarrow 4PCl_3(l)\) \(P_4(s) + 10Cl_2(g) \rightarrow 4PCl_5(s)\) | +3+5 |
| S | Heat | Disulphur dichloride | Orange liquid | \(2S(s)+ Cl_2(g) \rightarrow S_2Cl_2 (l)\) | +1 |
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 | \(2Na(s) + 2H_2O(l) \rightarrow 2NaOH (aq) + H_2(g)\) | +1 |
| Mg | Cold waterHeat, steam | Magnesium hydroxide, hydrogenMagnesium oxide, hydrogen | Slow fizzing, colourless solutionWhite flame, white powder | \(Mg(s) + 2H_2O(l) \rightarrow Mg(OH)_2(aq) + H_2(g)\)\(Mg(s) + H_2O(l) \rightarrow MgO(s) + H_2(g)\) | +2+2 |
Sodium hydroxide is strongly alkaline; a universal indicator added to the solution will turn purple. Magnesium hydroxide is less alkaline because it is only sparingly soluble. In fact, it often forms a thin layer on the surface of the metal, preventing a further reaction.
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.
Period 3 Elements - Key takeaways
- Period 3 is the third row in the periodic table. It contains the elements sodium, magnesium, aluminium, silicon, phosphorus, sulphur, chlorine, and argon.
- Period 3 shows trends in atomic properties:
- Ionic radius decreases across the period.
- First ionisation energy increases across the period.
- Both melting points and electrical conductivity vary across the period.
- Period 3 elements react with oxygen to form oxides, and chlorine to form chlorides. Some period 3 elements also react with water to form hydroxides.
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