How To Write Chemical Symbols?

Updated: May 4, 2020



Chemical symbols are used to represent an atom or elements. For example, the chemical symbol for hydrogen is H.


Symbols For Common Ions are as follows:



The valency of an element is needed to be able to write the formulae of a compound. Recall that valency of an element is the number of valence electrons the element has. In ionic compounds, the valency of an ion is equal to its charge.

*Tip: Valence electrons means outermost electrons or the electrons at the outermost shell


Writing Formula For Ionic Compounds

Ionic compounds are made up of positive and negative ions. In an ionic compound, the total positive charges must equal the total negative charges → the total charge on the ionic compound must be zero.

*Refer to Ionic Bond and Covalent Bonds notes to clearly understand how each of the ions is formed and combined to be the following compounds.


Make use of the table of symbols for common ions to write out the formula for ionic compounds


Examples:

Sodium chloride

  • Relevant ions are: Na+ and Cl−

  • Simplest ratio to obtain zero total charge: 1 : 1

  • Formula: NaCl

Calcium Nitrate

  • Relevant ions are: Ca2+ and NO3−

  • Simplest ratio to obtain zero total charge: 1:2 → this means 1 of calcium ion and 2 of nitrate ions to form calcium nitrate

  • Formula: Ca(NO3)2

  • Note: The brackets are used to indicate that there are two NO3 ions in the ionic compound

Sodium Sulphate

  • Relevant ions are: Na+ and (SO4) 2−

  • Simplest ratio to obtain zero total charge: 2:1

  • Formula: Na2SO4




Writing Formula For Covalent Compounds

The way to write the formula for covalent compounds is slightly different from that of ionic compounds. You will need to know the number of valence electrons that each of the elements has.

*Refer to Ionic Bond and Covalent Bonds notes to clearly understand how each of the ions is formed and combined to be the following compounds.


Examples:

Hydrogen Chloride

  • Relevant elements: H and Cl

  • Valence electrons: 1 for H and 7 for Cl

  • 1 covalent bond is formed

  • Formula: HCl

Water

  • Relevant elements: H and O

  • Valence electrons: 1 for H and 6 for O

  • O requires 2 electrons to achieve stable noble gas configuration. H provides 1 electron

  • Hence, 2 H will be required for O to achieve a stable noble gas configuration

  • 2 covalent bonds are formed

  • Formula: H2O

Carbon Dioxide

  • Relevant elements: C and O

  • Valence electrons: 4 for C and 6 for O

  • C provides 4 electrons but O only requires 2 electrons to achieve a stable noble gas configuration

  • Hence, 2 O will be required for C to achieve a stable noble gas configuration

  • 2 covalent bonds are formed

  • Formula: CO2



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