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Period 3 Elements

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Chemistry

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.

Trends of period 3 elements

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 elements, periodic table period 3, StudySmarterPeriod 3 in the periodic table. Created using a StudySmarter Original by Olive Odagbu

Period 3 includes the following elements:

NameSymbolAtomic numberFact
SodiumNa11Sodium is the sixth most abundant element in the Earth's crust and is a vital component of table salt, NaCl.
MagnesiumMg12Magnesium 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.
AluminiumAl13Aluminium is the second most-produced metal in the world, beaten only by iron.
SiliconSi14Silicates 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.
PhosphorusP15The word phosphorus means 'light bearer', thanks to the fact that white phosphorus glows when exposed to oxygen.
SulphurS16Over 80 percent of the sulphur extracted today is turned into sulphuric acid, which in 2010 was the most produced organic chemical in the US.
ChlorineCl17Chlorine 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.
ArgonAr18The 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.

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.

Period 3 elements, melting point, StudySmarterMelting point of period 3 elements. Anna Brewer, StudySmarter Original

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.

Period 3 elements, first ionisation energy graph, StudySmarterFirst 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.

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.

Period 3 elements, electrical conductivity graph, StudySmarterElectrical conductivity of period 3 elements. Anna Brewer, StudySmarter Original

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:

  • 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.

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.

ElementConditionsProductObservationEquationOxidation state
NaHeatSodium oxideSodium peroxideOrange flame, white powder+1
MgHeatMagnesium oxideWhite flame, white powder+2
AlHeat, powdered aluminiumAluminium oxideWhite sparkles, white powder+3
SiHeat, pure oxygenSilicon dioxideWhite sparkles, white powder+4
P (white)Room temperaturePhosphorus(III) oxidePhosphorus(V) oxideYellow/white flame, white smoke+3+5
SHeat, pure oxygenSulphur dioxideBlue 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.

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.

ElementConditionsProductObservationEquationOxidation state
NaHeatSodium chlorideOrange flame, white powder+1
MgHeatMagnesium chlorideWhite flame, white powder+2
AlHeatAluminium chloridePale yellow solid+3
SiHeatSilicon tetrachlorideColourless liquid+4
P (white)Room temperaturePhosphorus(III) chloridePhosphorus(V) chlorideColourless liquidOff-white/yellow solid+3+5
SHeatDisulphur dichlorideOrange 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.

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.

ElementConditionsProductObservationEquationOxidation state
NaCold waterSodium hydroxide, hydrogenVigorous fizzing, colourless solution+1
MgCold waterHeat, steamMagnesium hydroxide, hydrogenMagnesium oxide, hydrogenSlow 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 - 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.

Period 3 Elements

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. 

Final Period 3 Elements Quiz

Question

Which of the following are period 3 elements?

Show answer

Answer

H

Show question

Question

Which of the following are period 3 elements?

Show answer

Answer

S

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Question

Period 3 elements have ____ electron shells.

Show answer

Answer

Two

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Question

Period 3 elements have ____ outer shell electrons.

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Answer

It depends

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Question

Atomic radius _____ as you move across period 3.

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Answer

Decreases

Show question

Question

Explain why atomic radius decreases as you move across period 3 in the periodic table.

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Answer

  • Atomic number increases and so nuclear charge increases.
  • Number of electron shells stays the same and so shielding stays the same.
  • Stronger attraction between the nucleus and outer electron shell. 

Show question

Question

First ionisation energy ____ as you move across period 3.

Show answer

Answer

Increases

Show question

Question

Explain why first ionisation energy increases as you move across period 3.

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Answer

  • Atomic number increases and so nuclear charge increases.
  • Shielding stays the same.
  • Stronger attraction between the outermost electron and the positive nucleus.

Show question

Question

Which element in period 3 has the highest electrical conductivity?

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Answer

Al

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Question

Which element in period 3 has the highest melting point?

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Answer

Si

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Question

Which element in period 3 has the lowest melting point?

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Answer

Ar

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Question

Describe and explain the trend in melting point as you move across from sodium to aluminium in period 3.

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Answer

Melting point increases. All three bond metallically. However, magnesium has a higher melting point than sodium because it has a smaller atomic radius and forms ions with a higher charge. Aluminium has a higher melting point than magnesium because it has an even smaller atomic radius and forms ions with an even higher charge.

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Question

Explain why melting point varies between sulphur, phosphorus and chlorine in period 3.

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Answer

All three have low melting points because they form simple covalent molecules, held together by weak intermolecular forces. However, sulphur has a higher melting point than phosphorus and chlorine because it forms larger molecules. Likewise, phosphorus has a higher melting point than chlorine because it also forms larger molecules.

Show question

Question

Explain why the last four elements in period 3 cannot conduct electricity.

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Answer

They are all either simple covalent molecules or monatomic gases, and therefore don't contain any charged particles that are free to move and carry a charge.

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Question

Reacting a period 3 element with oxygen produces ____.

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Answer

An oxide

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Question

When reacting a period 3 element with oxygen, the oxidation state of the period 3 element ____.

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Answer

Increases

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Reacting a period 3 element with chlorine produces ____.

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Answer

A chloride

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Question

Period 3 metal oxides have ____ melting points.

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Answer

High

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Question

True or false? All period 3 non-metal oxides have low melting points.

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Answer

False

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Question

Which of the following oxides bond covalently?

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Answer

Na2O

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Question

Which of the following oxides bond ionically?

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Answer

Na2O

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Question

Which of the period 3 oxides has the highest melting point? Justify your answer.

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Answer

SiO2

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Question

True or false? Period 3 metal oxides can conduct electricity when solid.

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Answer

False

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Question

When can non-metal period 3 oxides conduct electricity?

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Answer

They can't, no matter their state

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Question

What is the oxidation state of oxygen in all period 3 oxides?

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Answer

-2

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Period 3 metal oxides tend to be ____ in nature.

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Answer

Basic

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Question

Period 3 non-metal oxides tend to be ____ in nature.

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Answer

Acidic

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Question

Which of the following react with water to produce a hydroxide?

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Answer

Na2O

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Question

Which of the following are insoluble in water?

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Answer

Al2O3

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Question

True or false? Al2O3 is amphoteric in nature.

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Answer

True

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Question

Which of the following react with water to produce an acid?

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Answer

SO2

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Question

Period 3 metal oxides react with acids to form _____.

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Answer

A salt plus water.

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Question

Period 3 non-metal oxides react with bases to form ____.

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Answer

A salt plus water.

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Question

How do period 3 metal hydroxides react with acids and bases?

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Answer

NaOH and Mg(OH)2 react with acids to form a salt and water. Al(OH)3 also reacts with acids to form a salt and water. However, it also reacts with bases to form an aluminate.

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Question

Give the products when Al2O3 reacts with an acid and with a base.

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Answer

The reaction with an acid produces a salt and water. The reaction with a base produces an aluminate.

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Question

True or false? All period 3 metal chlorides have high melting points.

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Answer

False

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Question

Which of the following is true?

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Answer

Chlorine takes an oxidation state of -1 in all period 3 chlorides.

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Question

How do NaCl and MgCl2 react with water?

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Answer

They dissolve in water. NaCl produces a neutral solution whilst MgCl2 produces a slightly acidic solution with a pH of 6.

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Question

How do the period 3 non-metal chlorides react with water?

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Answer

They react vigorously, giving off fumes of HCl, and produce acidic solutions with a pH of 2.

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Question

How does AlCl3 react with water?

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Answer

It reacts vigorously, giving off fumes of HCl and forming a solution with a pH of about 3.

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