By: Ben Thompson and Kevin Easterbrook
Potassium Bromide (KBr)
The Ionic bond formed between Potassium and Bromine is created through the transfer of electrons from Potassium (metal) to Bromine (nonmetal). Potassium is located in group 1 which means that potassium has one valence electron; Bromine is located in group 17 which means that it has seven valence electrons. In order for each atom to be satisfied, the octets cannot be partially filled. Due to this, Bromine needs one more electrons to fill its octet or, have a full valance/ closed shell. In order to achieve this full octet, potassium transfers its one valence electron causing bromine to have 8 valence electrons or, a full octet/closed shell. This transfer of electrons is caused by the high electronegativity of Bromine (3.0 E.N.) and lower ionization energy of Potassium. The 3.0 electronegativity of Bromine attracts the valence electrons in potassium (1 valence) to bromines protons (35 protons) located in the nuclei. This also occurs due to potassium having a relatively low ionization energy. This means that it easily loses an electron; potassium easily loses an electron because it only has nineteen protons in the nucleus and has four shells surrounding it. In order to hold all the electrons, more shells or, orbits, are required which in turn, increases the distance from the positively charged nucleus to the negatively charged valence; the force of attraction between the nucleus and valence is not strong enough to hold onto those electrons. In addition to this, Bromine is located in a high period number and a relatively low group number which means it does not have a massive amount of shielding; due to this, the protons (35) have a stronger pull because there is less distance from the radius to the valence and a stronger positive charge in the nucleus. Consequently, Bromine has lower ionization energy than potassium because although it has the same amount of shells, it has 26 more protons in the nucleus causing a stronger force of attraction between nuclei and valence. This leads to Bromine (nonmetal) receiving an electron from Potassium (metal).
Bromine has a high electronegativity because it has a high amount of protons with a low amount of shielding. Due to a low amount of shielding, the positively charged protons attract the negatively charged electrons in potassium's valence. This causes the transfer of electrons from Potassium to Bromine.This transfer of two valence electrons gives the formula for Potassium Bromide as KBr(s) ⟬K(s)+ Br(l)→ KBr(s)⟭. In addition to this, the loss of an electron in potassium and, the gaining of electrons in Bromine causes the formations of ions. Potassium now has one less electron giving it a 1+ ion while Bromine now has one more electrons giving it a 1- ion. As a result of these oppositely charged ions, a force of electrostatic attraction is created between the cation (Potassium) and the anion (Oxygen). This attraction between opposite charges causes a strong ionic bond. Now, although these ions are oppositely charged and, have a strong bond between them, it is one of the weaker ionic bonds. In order for a stronger bond to occur there has to be greater charged ions (3-, 2-, etc). Since there is only a 1± ion created, the bond is not a very strong in correlation to other ionic compounds, such as, Aluminum Phosphide which results in 3± ions.
The image above shows the postivie and negative charges of the atoms subsequently to the transfer of electrons in an ionic bond
As a result of the relatively strong bond created by the strong electrostatic attraction of opposite ions, potassium bromide forms a close packing cubic lattice which is a classification of a crystalline structure and a subclassification of cubic lattices. As stated previously, Potassium Bromide forms a close-packing cubic lattice. This cubic lattice is only one mere crystalline structure of seven. A cubic lattice is one of the most common lattice structures, and arranges itself in a cube-hence the name “cubic lattice.” A cubic lattice forms when the atoms create layers upon each other (3 layers in physical diagram. See Potassium Bromide model for visual.) in order to allow for maximum contact of each atom; this is important because the atoms are always experiencing constant forces of attraction to one another. Consequently, potassium bromide is not very reactive because the molecules are closely packed in this cubic lattice with many points of contact. This type of crystalline structure is only one of seven crystalline structures.
A crystalline structure is formed through the repeating of unit cells (atoms) which arrange themselves in a repeating geometric structure containing ions. This is an ionic lattice. The ionic lattice is based on the principle that ions are hard spheres which allows them to form these lattices. In addition to this, the anions in the ionic lattice structure are generally much larger which in turn, form the lattice structure; since the anions form this structure, the cations reside in between the larger anions. Conclusively, in order to form an ionic lattice, oppositely charged ions must be present in a repeating geometric array. Some of these other ionic lattices include Triclinic, Monoclinic, Orthorhombic, Rhombohedral, Tetragonal, and Hexagonal.
Triclinic lattices are formed by vectors of unequal length and have inclined angles which are non-perpendicular to each other. As a direct result of this, triclinic systems are the least symmetrical lattice structures of the 7; this gives certain properties such as rigidity as well as low melting points. Secondly, Monoclinic lattices are lattices which are formed also by unequal vector lengths but however have two vectors which are perpendicular to each other; this causes a rectangular base to form at the base of the lattice. In addition, monoclinic lattices do not have inclined angles. This gives monoclinic structures to be quite dense because the rectangle formed allows for the molecules to be compact. In addition to this compactness, monoclinic systems have high melting and boiling points Thirdly, Orthorhombic lattices are essentially cubic lattices which have been stretched by factors resulting in a rectangular shape similar but not the same as monoclinic structures. These Orthorhombic systems are very dense. A rhombohedral lattice is a three dimensional crystalline structure like a cube, except that its faces are not squares, but instead are rhombi. Tetragonal lattices are the result of stretching a cubic lattice along one of its vectors, so that the cube changes into a rectangular prism shape with a square base. A hexagonal lattice is a crystalline structure in which three axes of equal length intersect at 60 degree angles. A fourth axis of a different length to the other three intersects with them perpendicularly.
The presence of these oppositely charged ions allow for Potassium Bromide to have certain properties. Also, because of this crystalline structure (ionic lattice), and that potassium bromide appears crystalline in solid form, Potassium Bromide is rigid in nature; due to this rigidity, Potassium Bromide in solid form (exists as a salt) resists changes in shape and therefore, makes it not flexible or pliable. Since the ions are so strongly attracted to one another it requires a lot of energy to break this attractive force between atoms. Due to this, Potassium Bromide has a high melting point of 7340 Celsius. It requires this much energy to melt Potassium Bromide or, separate its ionic bonds. The electrostatic attraction between the two opposite ions (1-, 1+) that are created during the transfer of electrons results in high melting points because of attraction. Similarly to this, Potassium Bromide has a boiling point of 1,4350 Celsius. The bonds require this much heat energy in order to be broken (liquid to vapour). In addition to this, potassium Bromide is soluble in only polar solvents because of these charged ions. As stated previously, in order for an ionic lattice to form, there must be ions present; it is because of these ions that potassium bromide is soluble in only polar solvents. The polar-covalent solvents contain dipoles (positive and negative end) which attracts the charged ions of the ionic compound (potassium=cation and Bromine=anion). Due to this potassium bromide is able to be dissolved in polar solvents. When Potassium Bromide is dissolved in water (polar solvent), it forms a neutral solution. KBr+HOH→ HBr(aq) +KOH (aq) . In turn, potassium bromide is neutral in nature when it is dissolved in water (ph close to 7) because the hydrobromic acid formed by the hydrogen ions (HBr(aq)) is neutralized by the potassium hydroxide ions created (KOH(aq)).
