The second one is endothermic as endothermic reactions require energy for the reaction to take place.
Valence electrons is an outer shell electron that is associated with an atom, and that can participate in the formation of a chemical bond if the outer shell is not closed. In a single covalent bond, both atoms in the bond contribute one valence electron in order to form a shared pair.
The presence of valence electrons can determine the elements chemical properties, such as its valence—whether it may bond with other elements and, if so, how readily and with how many. In this way, a given element's reactivity is highly dependent upon its electronic configuration. For a main group element, a valence electron can exist only in the outermost electron shell; in a transition metal, a valence electron can also be in an inner shell.
An atom with a closed shell of valence electrons (corresponding to an electron configuration s2p6 for main group elements) tends to be chemically inert. Atoms with one or two valence electrons more than a closed shell are highly reactive due to the relatively low energy to remove the extra valence electrons to form a positive ion. An atom with one or two electrons less than a closed shell is reactive due to its tendency either to gain the missing valence electrons and form a negative ion, or else to share valence electrons and form a covalent bond.
Similar to a core electron, a valence electron has the ability to absorb or release energy in the form of a photon. An energy gain can trigger the electron to move (jump) to an outer shell; this is known as atomic excitation. Or the electron can even break free from its associated atom's shell; this is ionization to form a positive ion. When an electron loses energy (thereby causing a photon to be emitted), then it can move to an inner shell which is not fully occupied.
When forming ions, elements typically gain or lose the minimum number of electrons necessary to achieve a full octet. For example, fluorine has seven valence electrons, so it is most likely to gain one electron to form an ion with a 1- charge.
The shape of the molecule depends on how many electrons and bonds you have in the outer shell of the central atom.
For NH3, N is the central atom. It has 3 covalent bonds and a lone pair. Thus the repulsion of the lone pair electrons and the bond electrons will force them to arrange at the corners of a tetrahedral. The molecule will look as a trigonal pyramid (since the lone pair is not "visible" in the structure.
<span>For BH3, B is the central atom and has no lone pair apart from the three bonds. So the repulsion of the bond electrons will force them to be in a planar trigonal arrangement.
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Answer:
i)
1 is Argon
2 is Kalium
3 is Calcium
ii)
Y belongs to alkaline metals
Z belongs to alkaline earth metals
iii)
Kalium (Y) is alkaline metal
In some plants such as Cactuses, the stem is flattened and green and carries out the function of the leaf. Such a stem, adapted for the manufacture of food is called phylloclade. The leaves here are reduced or modified into spines to lessen the transpiring surface.
