Which of the following group 1a elements is the least reactive? a. cs (cesium) b. li (lithium) c. k (potassium) d. na (sodium)

B. Li (lithium) is the least reactive in group 1A elements

The periodic system of elements is an arrangement of elements classified based on similar properties and their atomic number order.

The periodic system is divided into groups (vertical order) and periods (horizontal order)

If we look at the atomic number of the elements in group 1A:

Shows that ,the element is getting down, the shell electron is increasing, so that in one group, from top to bottom, the radius of the atom tends to get bigger, according to the electron shell increase.

Ionization energy is the energy that an atom needs in the gas phase to release its electrons

In one group, from top to bottom, the first ionization energy tends to get smaller, because the addition of electron shells makes the attraction of the atomic nucleus to the outer electrons  (valence electron) weaker, so that it is easily released, forming cations +

Group 1A elements from top to bottom, the easier the elements release electrons, and the more reactive

For this reason, IA elements (also called alkali metal elements) are highly reactive metals.

This reactivity causes a very large reaction to water and air

From the properties above shows that the metal that is less reactive from group 1A is Lithium

electron affinity

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Identify the group number in the periodic table

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the charge on each ion in the compounds

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Keywords: element, group 1A, ionisation energy, valence electron