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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.
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.
Period 3 in the periodic table. Created using a StudySmarter Original by Olive Odagbu
Period 3 includes the following elements:
| Name | Symbol | Atomic number | Fact |
| Sodium | Na | 11 | Sodium is the sixth most abundant element in the Earth's crust and is a vital component of table salt, NaCl. |
| Magnesium | Mg | 12 | Magnesium 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. |
| Silicon | Si | 14 | Silicates and other minerals containing silicon make up over 90 percent of the Earth's crust. Silicon is used to not only make porcelain, but also the semiconductor chips in most electronics. |
| Phosphorus | P | 15 | The word phosphorus means 'light bearer', thanks to the fact that white phosphorus glows when exposed to oxygen. |
| Sulphur | S | 16 | Over 80 percent 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 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 extremely unreactive. |
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.
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 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 varies as you move across period 3 in the periodic table. This is all to do with structure and bonding.
Melting point of period 3 elements. Anna Brewer, StudySmarter Original
Here's why.
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.
First ionisation energy of period 3 elements. Anna Brewer, StudySmarter Original
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.
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.
Electrical conductivity of period 3 elements. Anna Brewer, StudySmarter Original
Note the following:
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.
We'll look at the reactions of period 3 elements with three different species:
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 |  | +1 |
| Mg | Heat | Magnesium oxide | White flame, white powder |  | +2 |
| Al | Heat, powdered aluminium | Aluminium oxide | White sparkles, white powder |  | +3 |
| Si | Heat, pure oxygen | Silicon dioxide | White sparkles, white powder |  | +4 |
| P (white) | Room temperature | Phosphorus(III) oxidePhosphorus(V) oxide | Yellow/white flame, white smoke |  | +3+5 |
| S | Heat, pure oxygen | Sulphur dioxide | Blue flame, colourless gas |  | +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.
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 |  | +1 |
| Mg | Heat | Magnesium chloride | White flame, white powder |  | +2 |
| Al | Heat | Aluminium chloride | Pale yellow solid |  | +3 |
| Si | Heat | Silicon tetrachloride | Colourless liquid |  | +4 |
| P (white) | Room temperature | Phosphorus(III) chloridePhosphorus(V) chloride | Colourless liquidOff-white/yellow solid |  | +3+5 |
| S | Heat | Disulphur dichloride | Orange liquid |  | +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.
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 |
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 show trends in atomic properties. Atomic radius decreases across the period, whilst first ionisation energy increases across the period. Melting points and electrical conductivity both vary across the period.
The period 3 element with the highest melting point is silicon. This is because of its giant covalent structure.
Period 3 elements are called typical elements because each period 3 element has general properties that are representative of the properties of the other elements within their group. For example, the properties of sodium are very similar to the properties of the other group 1 elements, such as potassium.
There are eight elements in period 3: sodium, magnesium, aluminium, silicon, phosphorus, sulphur, chlorine, and argon.
Period 3 elements show trends in atomic properties. Atomic radius decreases across the period, whilst first ionisation energy increases across the period. Melting points and electrical conductivity both vary across the period.
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