All the elements in their normal state are reactive and unstable.
In order to attain stability i.e. a state of minimum energy, they tend to form ions by loosing or gaining electrons. By doing so, they achieve stable electronic configuration or noble gas configuration.
However, some elements do not form ions. Instead of that, two such atoms share electrons with each other and achieve this noble gas configuration.
At the end, the main reason for attaining a noble gas configuration is to attain max. stability and min. energy.
Krypton is a noble gas and need not lose electrons. It is already stable.
Magnesium (Mg) has 12 electrons. To attain a noble gas configuration, Mg would need to lose 2 electrons to have the same electron configuration as a noble gas (like neon). This would result in Mg forming a +2 ion.
Seven electrons should be added to attain the higher noble gas configuration.
Nitrogen has 5 valence electrons. It needs three more to attain noble gas configuration (of that of neon)
A species (element, cation or anion) should have eight valence electrons to have a noble gas electronic configuration. However element upto atomic number 4 may have 2 valence electrons and attain the electronic configuration of helium noble gas.
All halogen family elements need to gain 1 electron to attain a noble gas configuration. This is because noble gases have a full outer shell of electrons, which is the most stable configuration in terms of electron arrangement.
Because it is energetically favorable
Phosphorus should gain 3 electrons and form P3- ion to attain a noble gas configuration
elements in group 17 need one more electron to attain the nearest noble gas configuration, hence they are more reactive
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Krypton is a noble gas and need not lose electrons. It is already stable.
chlorine would need only one electron to attain an octet structure.
Magnesium (Mg) has 12 electrons. To attain a noble gas configuration, Mg would need to lose 2 electrons to have the same electron configuration as a noble gas (like neon). This would result in Mg forming a +2 ion.
An atom of silicon needs to gain 4 electrons in its 3p sublevel to attain the noble gas electron configuration of argon, the noble gas in period 3 of the periodic table.
They should gain 2 electrons to attain noble gas electronic configuration. (it doesn't become a noble gas as the number of protons are not same)
Seven electrons should be added to attain the higher noble gas configuration.
Nitrogen has 5 valence electrons. It needs three more to attain noble gas configuration (of that of neon)