Formation of Ions

We know that ions are formed when an atom loses or gains electrons to ensure that its outermost shell is filled. But how do we know which atoms lose electrons and which gain them?

The formation of ions depends on whether the atom is a metallic or non-metallic element.

Metallic Elements

  • They form cations by losing electrons, resulting in positively charged ions.
  • For elements in Groups 1, 2 and 3, the number of electrons lost matches the group number.
Number of electrons lostMetallic element in a neutral stateIon
1 electron (Group 1)This image has an empty alt attribute; its file name is image-2.png
2 electrons (Group 2)
3 electrons (Group 3)

The electron loss usually results in a change in the electron configuration to achieve a more stable, often noble gas, configuration. However, it does not change the number of electron shells themselves. The electron shells are a feature of the atom’s structure and do not change unless there is a nuclear reaction.

Non-metallic Elements

  • They form anions by gaining electrons, resulting in negatively charged ions.
  • Group 6 elements gain 2 electrons, and Group 7 elements gain 1 electron.
  • Group 0 elements do not form ions due to their already full outer electron shell.
  • The number of shells remains the same; the outer shell gains the necessary electrons to be filled.
Number of electrons gainedNon-metallic element in a neutral stateIon

Using the Periodic Table

If somebody asks us to identify the properties of the ion of a certain element, we can find out all the following properties using just the periodic table

The periodic table can help us identify the properties of an ion:

  • Charge: Determined by its group number
  • Number of protons: Found by its atomic number
  • Number of electrons: Calculated from the atomic number plus or minus the ion’s charge
A detailed periodic table of elements showing the organisation of all known elements. The table is categorised by colour-coded groups, including alkali metals, alkaline earth metals, transition metals, metalloids, non-metals, halogens, noble gases, lanthanoids and actinoids. Each element is represented by its atomic number, symbol, name and relative atomic mass. The table is also marked with vertical columns indicating groups 1 through 8 and horizontal rows indicating periods, from 1 to 7, and horizontal groups indicating groups, from 1 to 8.. The lanthanoids and actinoids are displayed separately at the bottom. Arrows and annotations explain the information provided for each element, such as atomic mass and chemical symbol, using Hydrogen as an example.


Let’s try to identify the properties of a calcium ion:

  • Charge: Calcium is in Group 2, so it loses 2 electrons, resulting in a +2 charge.
  • Number of protons: Calcium’s atomic number is 20 so it has 20 protons.
  • Number of electrons: the neutral atom has 20 (the same as the atomic number/number of protons). As the calcium ion has lost 2 electrons (giving it a +2 charge), the ion must therefore have 18 electrons in total.

And for a fluorine ion:

  • Charge: Fluorine is in Group 7, meaning its outer electron shell has 7 out of 8 electrons. Therefore it will gain one electron to fill this outer shell, giving it a charge of -1.
  • Number of protons: its atomic number is 9 so it has 9 protons.
  • Number of electrons: A neutral fluorine atom has 9 electrons. As it has gained 1 electron, the ion will have 10 electrons in total.

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