The simplest way to understand the attraction or bonding of atoms is to examine the electronic structure of their outer electron shells. Elements with complete outer shells, such as helium and neon (colored blue above), are extremely stable and chemically inert.
On the other hand, elements with a half-filled outer shell, such as sodium and chlorine (colored red above), are very reactive and form ions (positively charged atoms) when they lose one or two electrons from their innermost valence shell.
Electronegativity is a term that describes the degree to which an element attracts electrons in a chemical bond. It is a very important concept in chemistry, as it helps us to explain why certain compounds have different properties.
The atomic number, which is the number of protons in the nucleus, and the valence electrons, which are screened from the nucleus by electron shells, influence an element’s ability to attract shared electrons. A higher number of protons means a more positive charge and greater attraction for shared electrons, but the shielding effects of the electron shells counteract this effect.
This attraction can lead to the formation of ions. For example, when sodium and chlorine form a covalent bond, they are attracted to each other more strongly than they are to hydrogen. This causes them to form an ionic bond, in which the two atoms exchange their valence electrons and form sodium cations (Na+) and chloride ions (Cl-).
It is important to remember that electronegativity increases from left to right across a period of the periodic table, except for the noble gases. For instance, in period 3 on the periodic table, Sodium (Na) has an electronegativity of 0.93, while Chlorine (Cl), which is in the same group but in a different period, has an electronegativity of 3.16.
Another factor affecting an atom’s ability to attract electrons is its size. A smaller atom will have a stronger attraction to bonding electrons than a larger one, because the smaller atom will be closer to the electrons that it is interacting with.
For this reason, fluorine and bromine have very different atomic sizes. Fluorine has nine electrons arranged between two electron shells, whereas bromine has 35 electrons that occupy four electron shells. The shorter atomic radius of fluorine means that it is closer to the bonding electrons in the bonds that it forms, making it more attractive than bromine.
In 1908, chemist Linus Pauling proposed the concept of electronegativity. He based it on a series of measurements that demonstrated how atoms would fight for electrons in chemical bonds, and he created a scale to represent the results. This is the scale that is still used today, and it is often referred to when calculating reactivity and polarity in chemistry.
A covalent bond is formed when two atoms share electron pairs to achieve a stable configuration, called a valence shell octet. In organic chemistry, this type of bonding occurs more often than ionic bonds. The reason is that a covalent bonding pair can fill the outermost energy level, or valence shell, of an atom without needing to donate or gain electrons.
A molecule is formed from a group of covalently bonded atoms. It is represented by a molecular formula that gives the number of atoms of each type in the compound.
The covalent bond is created when the atoms of one element share electrons with an atom of another element. Generally, the electrons are shared equally between the atoms of each element. However, some covalent bonds are polar covalent bonds, which result when the shared electrons between the atoms are unequally distributed.
In a polar covalent bond, the atom with a higher electronegativity will attract the shared electrons more than the atom with a lower electronegativity. This imbalance in the distribution of electrons causes an imbalance in the physical properties of the molecule.
For example, a water molecule (H2O) forms when two hydrogen atoms share a pair of electrons with an oxygen atom. The Lewis structure of the molecule shows the pair of electrons that form the covalent bond as a dash, and the nonbonding electrons are shown as a pair of dots around the symbols for the atoms.
Students can learn more about covalent bonds by drawing a model of the covalent bond between the hydrogen atoms in H2 (hydrogen), H2O (water), O2 (oxygen), CH4 (methane), and CO2 (carbon dioxide). This lesson may take more than one class period to complete.
To explain a covalent bond, students must know that there are attractive and repulsive forces between the nuclei of the bonded atoms and the electrons that they share. The attraction between the nuclei results in a distance between the nuclei, and the repulsive force between the electrons and the nuclei determines the length of the bond.
Ionic bond is formed by the transfer of valence electrons from a metal to a non-metal during chemical reactions. During this process, the non-metal gains electrons to become a negative ion (called an anion) and the metal loses electrons to become a positive ion (called a cation).
Ionic bonds can be found in both crystalline solids and liquids. Aqueous solutions of ionic substances are strong conductors of electricity. Unlike covalent substances that can dissolve in organic solvents, most ionic substances do not dissolve in water.
Electronegativity is a measurement that tells us how strongly an element will fight with other elements for electrons in chemical bonds. This property can also help determine which type of bond will form, and which molecule will have which properties.
Atoms of elements with great differences in electronegativity tend to form ionic bonds, while those with intermediate differences in electronegativity prefer to form covalent bonds. Pure covalent bonds are formed when the electronegativity difference between the atoms is zero, and all covalently bonded molecules have some degree of ionic character.
An example of an ionic bond is the sodium-chlorine bond, in which a sodium atom loses a valence electron to become a chlorine ion. The resulting sodium ion has the same electron configuration as neon (1s2 2s22p6), while the chlorine ion has the same electron configuration as helium (3s23p5).
When two ions form an ionic bond, it is called coordination, and compounds that contain these are referred to as ionic solids. Ionic compounds are found in the majority of crystalline solids, but they can be difficult to dissolve in water.
One important thing to note about coordination is that it can lead to polar covalent bonds, in which the ionic bond becomes partially ionic and partially covalent. For example, if the electronegativity of the ionic atoms is very different, then a single pair of ion electrons will move entirely to the iodine end of the bond.
The other important aspect of coordination is that it can lead to ionic and covalent bonds that have different properties. For example, an ionic bond between two cations may have a higher melting point than a covalent bond between the same ions.
In chemistry, the degree to which an element attracts electrons is called the polarity of an element. Whether an element is polar or nonpolar depends on the difference in electronegativity between the atoms in a bond.
The higher the difference in electronegativity between atoms, the more polar a bond will be. For example, a bond between hydrogen and fluorine is a polar bond because the electrons in the bond spend more time around the fluorine atom than they do the hydrogen atom.
Another reason why a bond is polar is because one atom of an element will attract the electrons in the bond more strongly than the other atom. This imbalance in electron distribution will lead to the buildup of a partial negative charge on one side of the bond and a partial positive charge on the other.
These polar covalent bonds are found in many chemical species, including some organic molecules. They are also a key feature of some biomolecules, such as proteins.
Electronegativity can be seen as a measure of how greedy an atom is for electrons. A highly electronegative element will want to grab as many of them as possible, while a less electronegative element will be happy to let them go.
A molecule that has no differences in electronegativity is called nonpolar, and is often referred to as a pure covalent molecule. A molecule that has a large difference in electronegativity is called polar, and is usually considered to be an ionic molecule.
The most polar bonds are those between carbon and oxygen or nitrogen. Because oxygen is more electronegative than carbon, it will attract the electrons in the bond more highly than carbon does. The resulting imbalance in electron distribution will result in the formation of a partial negative charge on the oxygen end of the bond and a partial positive charge in the carbon end of the bond.
An ionic bond is different from a polar bond in that the electron density of the molecule is evenly distributed, as shown in Figure 4.4.1. This is different from the distribution of electrons in a covalent bond, where there will always be an atom that attracts the bonding electrons more strongly than the other atom does.
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