O-Level Chemistry Question: Periodic Table & Group Trends

I received an email from Mahrukh, one of the reader of SimpleChemConcepts.com. She is a Chemistry student and asked me about an examination question regards to the topic of Periodic Table & Group Trends.

I have decided to take this opportunity to discuss this question by her with a blog post today so that more students can benefit from it.

Her question as follows:


Sulfur and selenium, Se, are in the same group of the Periodic Table. From this, we would expect selenium to form compounds having the formulae

A) Se2O, Na2Se and NaSeO4

B) SeO2, Na2Se and NaSeO4

C) SeO2, Na2Se and Na2SeO4

D) SeO3, NaSe and NaSeO4

Regarding your question, this should be covered under “Periodic Table & Group Trends” in most Chemistry Syllabus in different countries.

The concept that she is being asked is known as “Same Group Properties“.

Elements in the same Group of the Periodic Table will have the same number of valence or outermost shell electrons.

As such, elements in the same Group of the Periodic Table would tend to form compounds with similar chemical formula.

Sulfur is a Group VI element and has 6 valence electrons.

  • It forms SO2 – so we expect SeO2 to exist.
  • It forms Na2S – so we expect Na2Se to exist.
  • It forms Na2SO4 – so we expect Na2SeO4 to exist.
  • As such, the answer will be option (C).

    Hope the above explanation is useful to you.

    PS: If you are not sure why we have the following chemical formulae (SO2, Na2S, Na2SO4)  in the first place, you can revise via this blog post on Writing Chemical Formulae of Ionic Compounds and Covalent Substances.

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