Formation of Ions and Ionic Compounds

Learning Objective

In this lesson we will learn how atoms can gain or lose electrons to form ions, and subsequently ionic compounds.

Learning Outcomes

By the end of this lesson you will be able to:

  • Describe the most stable type of electron configuration.

  • Describe how metals lose electrons to form positive ions, and non-metals gain electrons to form negative ions.

  • Describe how positive and negative ions combine to form ionic compounds.

  • Correlate the reactivity of an element with its location on the periodic table and the type of ion it forms.

  • Differentiate between monatomic and polyatomic ions.

 

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Introduction

  • The electron configuration of atoms determines how stable they are.
    This in turn determines how chemically reactive an element is.
  • Chemical stability specifically relates to the number of valence electrons in atoms; in other words, the number of electrons in the valence shell (outer shell).
  • The most stable electron configurations are those where atoms have full valence shells.
    Atoms that do not have full valance shells can gain or lose electrons to acquire full valence shells and become more stable.

 

Electron Configuration

  • A full valence shell contains eight electrons, except when the valence shell is the only shell – here a full valence shell contains two electrons.
    The only elements that have atoms with full valence shells are the noble gases.
    These all have eight valence electrons, except helium which has two.
    Noble gases are very unreactive and rarely form compounds.
  • All other elements are composed of atoms that do not have full valence shells.
    However, most of these are able to attain full valence shells by gaining or losing electrons.
  • Example 1
    Chlorine has the following electron configuration:
    • 1st shell: 2 electrons
    • 2nd shell: 8 electrons
    • 3rd shell: 7 electrons
    Having 7 electrons in the valence shell makes a chlorine atom unstable.
    To have a full outer shell and thus be more stable, it readily gains 1 electron.

 
formation of negative chloride ion gain electrons

Chlorine atoms can gain an electron to attain a full valence shell.

 

  • Example 2
    Sodium has the following electron configuration:
    • 1st shell: 2 electrons
    • 2nd shell: 8 electrons
    • 3rd shell: 1 electron
    Having 1 electron in the valence shell makes a sodium atom unstable.
    To have a full outer shell and thus be more stable, it readily loses 1 electron.

 
formation of positive sodium ion lose electrons

Sodium atoms can lose an electron to attain a full valence shell.

 

  • When atoms attain full valence shells by losing or gaining electrons, they attain a noble gas electron configuration.
    In the above examples, chlorine attains the electron configuration of argon and sodium attains the electron configuration of neon.

 

Formation of Ions

  • When an atom loses or gains electrons, the number of electrons no longer equals the number of protons.
    It is therefore no longer electrically neutral – it is now charged.
  • An atom that has become charged by gaining or losing electrons is called an ion.
  • Atoms that form ions gain or lose the number of electrons that result in them have full valence shells.
    For most atoms, a full valence shell consists of eight electrons.
    For the smallest atoms and ions (hydrogen, helium, lithium and beryllium), a full valence shell consists of two electrons.
  • When an atom loses electrons, it forms a positive ion.
    This is because there are now more positively charged protons than negatively charged electrons.
    A positive ion is also called a cation.
  • When an atom gains electrons, it forms a negative ion.
    This is because there are now more negatively charged electrons than positively charged protons.
    A negative ion is also called an anion.

 

charge negative ion protons electrons        charge atom neutral protons electrons        charge positive ion protons electrons

Atoms have equal numbers of protons and electrons.
Negative ions have more electrons than protons.
Positive ions have more protons than electrons.

 

  • The charge of an ion is referred to as its valency.
    The size of the charge is equal to the difference in the numbers of protons and electrons.
    If an atoms gains one electron, it will have one more electron than protons; subsequently, the resulting ion will have a charge of –1. Similarly, if an atom gains two electrons, the resulting ion will have a charge of –2, and if an atom gains three electrons, the resulting ion will have a charge of –3.
    If an atoms loses one electron, it will have one more proton than electrons; subsequently, the resulting ion will have a charge of +1. Similarly, if an atom loses two electrons, the resulting ion will have a charge of +2, and if an atom loses three electrons, the resulting ion will have a charge of +3.

 
oxygen ion charge -2 protons electrons

Oxygen ions have a charge of –2 as there are two more electrons than protons.

 

Ions and the Periodic Table

  • Since the layout of the periodic table reflects electron configurations of the elements, there is a relationship between the location of an element on the periodic table and the type of ion it forms, or if it doesn’t form ions.

 

Groups 1, 2 and 13

  • Atoms of all elements in group 1 of the periodic table have 1 valence electron.
    When forming ions, these atoms lose 1 electron to attain full valence shells.
    Therefore they all form positive ions with a charge of +1.
  • Atoms of all elements in group 2 of the periodic table have 2 valence electrons.
    When forming ions, these atoms lose 2 electrons to attain full valence shells.
    Therefore they all form positive ions with a charge of +2.
  • Atoms of all elements in group 13 of the periodic table have 3 valence electrons.
    When forming ions, these atoms lose 3 electrons to attain full valence shells.
    Therefore they all form positive ions with a charge of +3.

 

Groups 15, 16 and 17

  • Atoms of all elements in group 15 of the periodic table have 5 valence electrons.
    When forming ions, these atoms gain 3 electrons to attain full valence shells.
    Therefore they all form negative ions with a charge of –3.
  • Atoms of all elements in group 16 of the periodic table have 6 valence electrons.
    When forming ions, these atoms gain 2 electrons to attain full valence shells.
    Therefore they all form negative ions with a charge of –2.
  • Atoms of all elements in group 17 of the periodic table have 7 valence electrons.
    When forming ions, these atoms gain 1 electron to attain full valence shells.
    Therefore they all form negative ions with a charge of –1.

 

Groups 3-12

  • Atoms of elements in groups 3-12 of the periodic table (known as the transition metals) have either 1 or 2 electrons in their valence shells.
    When forming ions, these atoms usually lose 1 or 2 electrons to form positive ions with a charge of +1 or +2.
  • However, the numbers of valence electrons of these elements and the charges of the ions they form are not readily predictable from their location in the periodic table.
    Also, some of these elements can form ions with a charge of +3, and many can form more than one type of ion.
    For example, chromium can form ions with a charge of either +2 or +3.
  • The only way to determine the ions formed by transition metals (and post transition metals such as tin and lead) is to consult a valency table.

 

Group 18

  • Atoms of all elements in group 18 of the periodic table (known as the noble gases) have 8 electrons in their valence shells, except helium which has 2.
    In other words, they have full valence shells.
    This makes them very stable, and as a result, they do not form ions.

 

Other Elements that Do Not Form Ions

  • Apart from the noble gases, there are some other elements that do not form ions, even though they do not have full valence shells.
    These include the elements boron, carbon and silicon.
    These elements still react to form compounds, but do so by a process that involves sharing of electrons between atoms, rather than transfer of electrons. (We will learn about these types of compounds next year.)

 
ions valency periodic table elements groups

The types of ions formed by elements in different groups of the periodic table.

 

Chemical Reactivity of Elements

  • We have seen that the most stable elements are the noble gases (group 18), as they are composed of atoms with full valence shells.
    We have also seen that elements in groups 1, 2 and 13 react to form positive ions, and that elements in group 15, 16 and 17 react to form negative ions.
    For these elements that form ions, there is a relationship between electron configuration (and therefore group number) and how reactive they are.
  • The elements whose atoms are the closest to having full valence shells are the most reactive.
    This is because for them to change from an atom to an ion requires the least energy.
    That is, it requires less energy to gain one electron than it does to gain two, and it requires less energy to lose one electron than it does to lose two.
  • Therefore, the most reactive metals (which form positive ions) are in group 1, as these only need to lose one electron to have a full valence shell.

 
reactivity metal groups

Reactivity of metals

 

  • Similarly, the most reactive non-metals (which form negative ions) are in group 17, as these only need to gain one electron to have a full valence shell.

 
reactivity non-metal groups

Reactivity of non-metals

 

Monatomic and Polyatomic Ions

  • As we have seen above, atoms can gain or lose electrons to form ions.
    These ions are known as monatomic ions, as they are derived from single atoms.
  • Ions can also form when certain molecules gain or loses electrons.
    These ions are known as polyatomic ions, as they are derived from multiple atoms.
  • In polyatomic ions, the charge is shared among the atoms within them.
    For example, hydroxide ions consist of a hydrogen atom and an oxygen atom, with an overall charge of –1.
  • Other common polyatomic ions include ammonium, nitrate, sulfate and carbonate.
    The symbols and charges of polyatomic ions can be found on a valency table.

 
monatomic polyatomic ions

Monatomic ions are derived from single atoms.

 
monatomic polyatomic ions

Polyatomic ions are derived from molecules.

 

Ionic Compounds

  • Atoms cannot simply gain or lose electrons to attain full valence shells. They can only form ions when they combine with other atoms that can donate or accept electrons.
    When atoms form positive ions, their valence electrons are transferred to other atoms that need these electrons to attain full valence shells, and which subsequently form negative ions.
    Thus the formation of positive and negative ions happens simultaneously.
  • Once positive ions and negative ions are formed, they combine to form ionic compounds.
    These compounds are held together by strong electrostatic forces called ionic bonds, which form between the oppositely charged positive and negative ions.
  • The ions in ionic compounds are arranged in a three-dimensional lattice structure.
    Because of their lattice structure, ionic compounds are hard, crystalline solids at room temperature.
    Many, such as sodium chloride, are soluble in water; others, such as calcium carbonate, are insoluble in water.

 
ionic compound lattice 3d structure

Ionic compound lattice.

(Image: Benjah-bmm27, Wikimedia Commons)

 

Metals and Non-Metals

  • When metals form ions, they always form positive ions.
    When non-metals form ions, they always form negative ions.
  • When ionic compounds are composed only of monatomic ions, they will therefore consist of positive metal ions and negative non-metal ions.
    Because metals always form positively charged ions, two metals cannot form an ionic compound.
    Similarly, because non-metals always form negatively charged ions, two non-metals cannot form an ionic compound.

 

sodium metal positive ion        chlorine non-metal negative ion        sodium chloride ionic compound salt

Sodium (metal) and chlorine (non-metal) react to form the ionic compound sodium chloride (salt).

(Images: Dnn87, Wikimedia Commons; Larenmclane, Wikimedia Commons; Chemicalinterest, Wikimedia Commons)

 

Summary

  • The most stable type of electron configuration for atoms is to have a full valence shell.
    Noble gases have full valence shells, and therefore are very unreactive.
    Most other elements are able to attain full valance shells by gaining or losing electrons.
  • When atoms gain or lose electrons, they form ions, which are charged particles due to the uneven numbers of protons and electrons.
    If an atom gains electrons, it forms a negative ion (anion), as there are more electrons than protons.
    If an atom loses electrons, it forms a positive ion (cation), as there are more protons than electrons.
    The size of the charge of an ion is equal to the difference in the numbers of protons and electrons.
  • Atoms of group 1 elements have 1 valence electron, therefore lose 1 electron to form ions with a charge of +1.
    Atoms of group 2 elements have 2 valence electrons, therefore lose 2 electrons to form ions with a charge of +2.
    Atoms of group 13 elements have 3 valence electrons, therefore lose 3 electrons to form ions with a charge of +3.
    Atoms of group 17 elements have 7 valence electrons, therefore gain 1 electron to form ions with a charge of –1.
    Atoms of group 16 elements have 6 valence electrons, therefore gain 2 electrons to form ions with a charge of –2.
    Atoms of group 15 elements have 5 valence electrons, therefore gain 3 electrons to form ions with a charge of –3.
  • For elements that form ions, the closer their atoms are to having full valence shells, the more reactive they are.
    Therefore, group 1 metals are more reactive than group 2 metals, and group 2 metals are more reactive than group 13 metals.
    Similarly, group 17 non-metals are more reactive than group 16 non-metals, and group 16 non-metals are more reactive than group 15 non-metals.
  • Monatomic ions are ions derived from single atoms.
    Polyatomic ions are ions derived from multiple atoms.
  • The formation of positive and negative ions occurs simultaneously.
    Metal atoms donate electrons to non-metal atoms, resulting in the formation of positive metal ions and negative non-metals ions.
    Positive and negative ions combine to form ionic compounds, which are lattice structures held together by ionic bonds between positive and negative ions.

 
formation of ions and ionic compounds

The formation of ions and ionic compounds.

 

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