Forming ionic compounds

We have learnt that atoms want to achieve a full electron shell and in the process, they either want to give up electrons, share electrons or take electrons.

Elements can be grouped in 2 ways:

  • The big group is – whether an element is metal or non-metal
  • Smaller group – depending on the number of electrons in the outer shell

The differences between metal and non-metal is really big (usually opposite to each other), its like comparing girls with boys!






The difference is huge!

  • Metals prefer to give up electrons
  • Non-metals prefer to share electrons or take electrons.


Generally metals have less electrons (like Sodium in Group I has 1 outer shell electron, Magnesium in Group II has 2, and Aluminium in Group III has 3) in the outer shell, so they are comfortable with losing them rather than gaining a lot more electrons to make up a full shell.

Conversely, non-metals have more electrons in their outer shells so they usually lack a couple more electrons to make up a full shell!

Do you notice that metals and non-metals appear to be made for each other? One really wants to give up electrons, the other really delights in gaining electrons!


After that, you have your smaller differences in the vertical grouping elements.

For Metals,

  • Group I metals, also known as Alkali Metals are so light they can float on water, while the transition elements like iron are so dense they sink to the bottom.
  • Group I metals also a lot more reactive than other metals!

For non-metals,

  • you have group 7 that is known as the halogens (F, Cl, Br, I) which are used in light-bulbs
  • the group called noble gases that have full electron shells and are unreactive!

Points to note: Elements can grouped into metal/non-metals, and then based on number of outer shell electrons

Also, metals prefer to give electrons away, non-metals prefer to take electrons

Now that we known what the elements are like, lets zoom in on what happens when an atom loses/gains electrons.

Its something like a marriage you know, because when an atom loses electrons, it loses electrons to another party who now holds its electrons. Once that happens, you have a positive charge on one and negative charge on the other which really binds them together!

“I pronounce you man and wife”

translated in chemistry terms become

“I pronounce you ions”

Atoms that have married each other by mutual exchange of electrons are no longer called atoms (because they have an imbalance of electrons compared to protons) and are called ions!

In some sense, atoms love to become ions. Take Sodium for example, Sodium is a reactive element, its atoms go out and wreck havoc and have to be stored in oil to prevent it from reacting with oxygen in the air, but once it achieved its dream of losing an electron and transform into an ion, it happily, quietly, and usually stays in its ion form forever and ever and ever…

Ions come in 2 flavours,

  • Positively charged ion
  • Negatively charged ion

Coincidentally, the metal atom usually end up as the positively charged ion, and non-metal atom as a negatively charged ion as per our discussion on their respective taste for electrons above.

Notice the imbalance in the number of protons and electrons that is gives the ion an overall charge so that behaves differently than when it was an atom.

The difference in behavior is stark: ions are bounded forever and ever to ions of the opposite charge! Because the electrostatic force is just that attractive and strong at the atomic level!

Do take note: ions are atoms which have gained an overall charge after losing or gaining electrons

Remember that elements vertically arranged in periodic table are like cousins because they have same number of outer shell electrons?

This means that the ion they form have the same overall charge, because they all lose/gain the same number of electrons!

Transition metals can have different number of outer shell electrons, so they form ions of different charge!


remember the marriage between metal and non-metal atoms, another word for it is “ionic bonding”. Doesn’t it sound so apt?

There are 3 simple steps to follow:

Step 1: Deduce the charge of the ion formed

  • Key question: What group is the element in? Group I – +1 charge, Group VII, -1 charge

Step 2: Make sure that the number of electrons lost = number of electrons gained

  • Find the lowest common multiple if confused, your Magnesium Bromide gives you Mg2+ ion and Br ion? Charges of +2 and -1, so ignore the signs – LCM of 1 and 2 is 2, you need 2/1 = 2 Br ions and just 2/2 =1 Mg2+ ion, so formula is MgBr2
  • How about the big tricky Aluminium Oxide? Aluminium atom wants to lose 3 electrons to become Al3+, oxygen wants to gain 2 electrons to become O2-, LCM of 2 and 3 is 6, number of Al3+ ions = 6/3 = 2, number of O2- ions = 6/2 = 3, so formula is Al2O3

Step 3: write the ionic compound formula / draw the electronic configuration of the ions formed

  • Take note of the formula in bold below, and drawing of electronic configuration with (1) brackets to indicate its an ion and (2) charge at top right hand corner.

This is an example of the ionic compound Sodium Chloride or NaCl

This is an example of the ionic compound Magnesium Fluoride or MgF2

If you are stuck, no worries!

This 100,000 plus views video will guide you along!

Take note of the common mistakes part below:

common mistakes

Next topic on:

Forming Covalent Bonds! 


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