پيشنمايش
Bonds
The atoms within these molecules are held together by strong attractive forces called chemical bonds , which are formed via the interaction of the valence electrons of the combining atoms. The chemical and physical properties of the resulting compound are usually very different from those of the constituent elements. For example, elemental sodium, an alkali metal, is so reactive that it can actually produce fire when reacting with water because the reaction is highly exothermic. Diatomic chlorine gas is so toxic that it was used for chemical warfare during World War I. However, when sodium and chlorine react, the biologically important compound NaCl (table salt) is produced.
For many molecules, the constituent atoms bond according to the octet rule, which states that an atom tends to bond with other atoms so that it has eight electrons in its outermost shell, thereby forming a stable electron configuration similar to that of the noble gases. However , there are elements that can be exceptions to the rule than those that follow the rule. These “exceptional” elements include hydrogen, which can only have two valence electrons (achieving the configuration of helium); lithium and beryllium, which bond to attain two and four valence electrons, respectively; boron, which bonds to attain six valence electrons; and all elements in period 3 and greater, which can expand the valence shell to include more than eight electrons by incorporating d-orbitals. For example, in certain compounds, chlorine can form seven covalent bonds, thereby holding 14 electrons in its valence shell.
What's a valence electron?
A simple way to remember all the exceptions is as follows:
• Incomplete octet : These elements are stable with fewer than 8 electrons in their valence shell and include hydrogen (stable with 2 electrons), helium (2), lithium Li + (2), beryllium (4), and boron (6).
• Expanded octet : Any element in period 3 and greater can hold more than 8 electrons, including phosphorus (10), sulfur (12), chlorine (14), and many others.
• Odd numbers of electrons : Any molecule with an odd number of valence electrons cannot distribute those electrons to give eight to each atom; for example, nitric oxide (NO) has eleven valence electrons.
Expanded octets result in substances with more than an octet of valence electrons resulting in non-traditional geometries of molecular compounds. Which of the following elements cannot form an expanded octet as the central atom?
Ions
When an element has more or fewer electrons than protons, it becomes an ion. Positive ions are called cations ; negative ions, anions .
The representative elements make ions by forming the closest noble gas electron con figuration . Metal is form cations; nonmetals form anions. When the transition metals form ions, they lose electrons from their s sub-shell first and then from their d sub-shell. (You should take a look at periodic table section)
Cations are significantly smaller than their neutral a tom counterparts. For instance, sodium's outermost electron is located by itself on an outer shell. When this outer electron is removed, the sodium cation is significantly smaller because the remaining electrons are located in inner shells . The reverse is true for anions. The additional electrons are added to an outer shell making the anion much larger than its neutral atom counter part. Isoelectronic ions (ions with the same number of electrons) tend to get smaller with increasing atomic number because more protons pull inward on the same number of electrons. The sizes of the oxygen, fluorine, sodium and magnesium ions reflect this trend.
Coulomb's law: F = kq 1 q 2 /r 2 , describes the electrostatic forces holding an electron to its nucleus.
The distance between the electron and the nucleus is r . For q 1 we might plug in the positive charge of the nucleus, Z, and for q 2 the charge on the given electron. This would work fine for hydrogen , where the lone electron feels 100% of the positive charge on the nucleus.
However, in helium the first electron shields some of the nuclear charge from the second electron, so that the second electron does not feel the entire nuclear charge, Z. The amount of charge felt by the second electron is called the effective nuclear charge (Z eff ) .Z eff for the outermost electron of helium is not 2, even though there are two protons in the helium nucleus.The Z eff is the nuclear charge Z minus the average number of electrons between the nucleus and the electron in question. The Z eff ,and not Z , should be plugged in for q in Coulomb's law to find the force on the outermost electron. Note that the force is a function of both q (Z eff ) and r (the distance from the nucleus).