The concept of valence was developed in the last half of the 19th century and was successful in explaining the molecular structure of many organic compounds. The quest for the underlying causes of valence lead to the modern theories of chemical bonding, including Lewis structures , valence bond theory , molecular orbitals , valence shell electron pair repulsion theory and all the advanced methods of quantum chemistry.
The combining power or affinity of an atom of an element was determined by the number of hydrogen atoms that it combined with.
In methane, carbon has a valence of 4; in ammonia, nitrogen has a valence of 3; in water, oxygen has a valence of two; and in hydrogen chloride, chlorine has a valence of 1. Chlorine, as it has a valence of one, can be substituted for hydrogen, so phosphorus has a valence of 5 in phosphorus pentachloride, PCl 5.
Valence diagrams of a compound represent the connectivity of the elements, lines between two elements, sometimes called bonds, represented a saturated valency for each element. The most reactive nonmetals are the halogens, e. They have one less electron configuration than a noble gas, so they require only one additional valence electron gain an octet.
To form an ionic bond , a halogen atom can remove an electron from another atom in order to form an anion e. To form a covalent bond , one electron from the halogen and one electron from another atom form a shared pair. For example, in "H—F" , the dash represents a shared pair of valence electrons, one from "H" and one from "F".
How do valence electrons determine chemical properties? Be able to indicate valence electrons when given the electron configuration for an atom.
What makes a particular element very reactive and another element non-reactive? Summary Valence electrons are the outer-shell electrons of an atom. Valence electrons determine the reactivity of an atom. Review Define valence electron. Define inner shell electron. Today's periodic tables place the elements in order of their atomic number, which is the number of protons in the nucleus. In Mendeleev's time, protons had not yet been discovered.
Mendeleev wrote that "arrangement according to atomic weight corresponds to the valence of the element and to a certain extent the difference in chemical behavior. Mendeleev combined the order of atomic weight with common valences to organize the elements in a table. That is, he organized the elements in groups according to their chemical characteristics.
Because those properties repeat every so often, the result was a periodic table in which each vertical column, called a group, contains elements with similar characteristics, and each horizontal row, called a period, arranges the elements by weight, increasing from left to right and top to bottom.
About 50 years after Mendeleev's first periodic table, scientists discovered the atom was built around a nucleus with positively charged protons and neutral neutrons -- both of which are relatively heavy. The positively charged nucleus is surrounded by a cloud of negatively charged electrons. The number of protons -- also called the atomic number -- typically matches the number of electrons.
It turns out that the number of electrons an element has largely determines its chemical properties.
0コメント