How do valence electrons affect reactivity?

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The valence electrons are the electrons in the outermost electron shell of an atom.

The number of electrons in an atom's outermost valence shell governs its bonding behaviour. That is why elements whose atoms have the same number of valence electrons are grouped together in the Periodic Table. Generally, elements in Groups 1, 2, and 13 to 17 tend to react to form a closed shell, corresponding to the electron configuration s²p⁶. This tendency is called the octet rule, because the bonded atoms have eight valence electrons.

METALS

The most reactive kind of metallic element is a metal from Group 1 (e.g., sodium or potassium). An atom in Group 1 has only a single valence electron. This one valence electron is easily lost to form a positive ion with an s²p⁶ configuration (e.g., Na⁺ or K⁺). A metal from Group 2 (e.g., magnesium) is somewhat less reactive, because each atom must lose two valence electrons to form a positive ion (e.g., Mg²⁺) with an s²p⁶ configuration.

Within each group of metals, reactivity increases as you go down the group. The valence electrons are less tightly bound and easier to remove, because they are farther away from the nucleus of the atom.

NONMETALS

A nonmetal tends to attract additional valence electrons to attain a full valence shell. Either it can share electrons with a neighboring atom to form a covalent bond) or it can remove electrons from another atom to form an ionic bond.

The most reactive kind of nonmetal is a halogen such as fluorine or chlorine. It has an s²p⁵ electron configuration, so it requires only one additional valence electron to form a closed shell.

To form an ionic bond, a halogen atom can remove an electron from another atom in order to form an anion (e.g., F⁻, Cl⁻, etc.).

To form a covalent bond, one electron from the halogen and one electron from another atom form a shared pair (e.g., in H–F, the dash represents a shared pair of valence electrons, one from H and one from F).

Within each Group of nonmetals, reactivity decreases from top to bottom, because the valence electrons are at progressively higher energies and the atoms do not gain much stability by gaining electrons. In fact, oxygen (the lightest element in Group 16) is more reactive than chlorine, even though it is not a halogen, because the valence electrons of oxygen are closer to the nucleus (at a lower energy).

Therefore, a metal from the bottom of Group 1 (like potassium) and a nonmetal from the top of Group 17 (like fluorine) will react violently, because they both benefit greatly from the reaction. K loses one electron to F and forms the ionic compound potassium fluoride, K⁺F⁻.

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    The number of electrons in an atom's outermost valence shell governs its [bonding](http://socratic.org/chemistry/bonding-basics/bonding) ...
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    Student from Canada This really helped :) thanks!
    The number of electrons in an atom's outermost valence shell governs its [bonding](http://socratic.org/chemistry/bonding-basics/bonding) ...
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    The most reactive kind of metallic element is a metal from Group 1 (e.g., sodium or potassium). An atom in Group 1 has only a single ...
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    The number of electrons in an atom's outermost valence shell governs its [bonding](http://socratic.org/chemistry/bonding-basics/bonding) ...
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    The [valence electrons](http://socratic.org/chemistry/the-periodic-table/valence-electrons-and-the-periodic-table) are the electrons in the ...
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    GameChanger100 I changed the number of Protons to the new number or 51 instead of 50.5.
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    Meave60 GameChanger100. You should investigate a contributor's credentials before you imply that an answer is incorrect. Ernest Z is a retired Chemistry professor with 33 years of teaching experience. If you found an error in his answer, you should have edited it by correcting the error.
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    Meave60 GameChanger100. There was nothing in this answer about 50.5 protons. Your update was a statement that answers on this site are unreliable. I believe you made up the reason for your update to make it look like there was a serious error. However, anyone can check your updates by clicking on update. I found no such edit in your update. This answer is about valence electrons and reactivity. There is nothing about protons. This answer is completely correct. I also taught Chemistry for many years.
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Feb 4, 2015

The electrons on the outer shell determine the bonding behavior.

On the periodic table the group numbers give the charge of the atom.

Group 1, +1 group 2, +2 etc.

The intensity of reaction increases as you move down the group; group one is the most reactive. (This is for metals.)

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