Valence Electrons same number for all elements in (with the exception of )

The same group; hydrogen.

Explanation:

Valence electrons can be defined as the number of electrons present in the outermost shell of an atom. Valence electrons are used to determine whether an atom or group of elements found in a periodic table can bond with others. Thus, this property is typically used to determine the chemical properties of elements.

Hence, valence electrons is same number for all elements in the same group of the periodic table (with the exception of hydrogen).

Hydrogen is the simplest chemical element that exists. The symbol for the chemical element Hydrogen is "H" and it is a colourless, tasteless, odorless, and highly flammable gas.

Hydrogen is a chemical element found in group (1) of the periodic table and as such it has one (1) electrons in its outermost shell. Therefore, Hydrogen has an atomic number of one (1) and a single valence electrons because it has only one proton and one electron in its nucleus.

Based on the octet rule which states that atoms of chemical elements gain, lose or share electrons so as to have eight (8) electrons in their valence shell. Therefore, atoms of chemical elements bond in order to attain the electronic configuration of a noble gas i.e a full valence shell which comprises of eight (8) electrons.

However, the chemical element "Hydrogen" is an exception to the octet rule because it is only able to hold a maximum of two (2) valence electrons in its outermost shell to become full.

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a- all cells come from pre-existing cells