5.2 Bonding and Lattices

Atoms seek to have a full outer shell. That means there will be two electrons for hydrogen and helium, and eight electrons for the other elements. This is accomplished by transferring or sharing electrons with other atoms in chemical bonds.  The type of chemical bond is important for the study of minerals because the type of bond will determine many of a mineral’s physical and chemical properties.

Ionic Bonds

Consider the example of halite again, which is made up of sodium (Na) and chlorine (Cl).  Na has 11 electrons: two in the first shell, eight in the second, and one in the third (Figure 5.2, top). Na readily gives up the third shell electron so it can have the second shell with 8 electrons as its outermost shell.  When it loses the electron, the total charge from the electrons is -10, but the total charge from the protons is +11, so it is left with a +1 charge over all.

Chlorine has 17 electrons: two in the first shell, eight in the second, and seven in the third. Cl readily accepts an eighth electron to fill its third shell, and therefore becomes negatively charged because it has a total charge of -18 from electrons, and a total charge of +17 from protons.

In changing their number of electrons, these atoms become ions — the sodium loses an electron to become a positive ion or cation,[1] and the chlorine gains an electron to become a negative ion or anion (Figure 5.2, bottom). Since negative and positive charges attract, sodium and chlorine ions stick together, creating an ionic bond. Electrons can be thought of as being transferred from one atom to another in an ionic bond.

Electron configuration of sodium and chlorine atoms (top). Sodium gives up an electron to become a cation (bottom left) and chlorine accepts an electron to become an anion (bottom right). [SE]

Figure 5.2 Electron configuration of sodium and chlorine atoms (top). Sodium gives up an electron to become a cation (bottom left) and chlorine accepts an electron to become an anion (bottom right). [Steven Earle CC-BY 4.0]

 

Exercise 5.1 Cations, Anions, and Ionic Bonding

A number of elements are listed below along with their atomic numbers. Assuming that the first electron shell can hold two electrons and subsequent electron shells can hold eight electrons, sketch in the electron configurations for these elements. Predict whether the element is likely to form a cation (+) or an anion (–), and what charge it would have (e.g., +1, +2, –1). The first one is done for you.

Fluorine (9)

Fluorine (9)

   anion (-1)  

Lithium (3)

atom

________

Magnesium (12)

atom

________

Argon (18)

atom

________

 Chlorine (17)

atom

________

Beryllium (3)

atom

________

Oxygen (8)

atom

________

 Sodium (11)

atom

________

 

 

Covalent Bonds

An element like chlorine can also form bonds without forming ions. For example, two chlorine atoms can each complete their outer shells by sharing electrons.  Chlorine gas (Cl2, Figure 5.3) is formed when two chlorine atoms form a covalent bond.

FIgure 5.3 A covalent bond between two chlorine atoms. The electrons are black in the left atom, and blue in the right atom. Two electrons are shared (one black and one blue) so that each atom appears to have a full outer shell. [Steven Earle CC-BY 4.0]

Carbon is another atom that participates in covalent bonding.  An uncharged carbon atom has six protons and six electrons. Two of the electrons are in the inner shell and four are in the outer shell (Figure 5.4, left). Carbon would need to gain or lose four electrons to have a filled outer shell, and this would create too great a charge imbalance. Instead, carbon atoms share electrons to create covalent bonds (Figure 5.4, right). In the mineral diamond (Figure 5.5, left), the carbon atoms are linked together in a three-dimensional framework, where one carbon atom is bonded to four other carbon atoms and every bond is a very strong covalent bond.
The electron configuration of carbon (left) and the sharing of electrons in covalent C bonding of diamond (right). The electrons shown in blue are shared between adjacent C atoms. Although shown here in only two dimensions, diamond has a three-dimensional structure as shown on Figure 5.5. [SE]

Figure 5.4 The electron configuration of carbon (left) and the sharing of electrons in covalent C bonding (right). The electrons shown in blue are shared between adjacent C atoms. [Steven Earle CC-BY 4.0]

 

Figure 5.6 Covalently bonded structures. Left: Diamond with three-dimensional structure of covalently bonded carbon. Right: Graphite with covalently bonded sheets of carbon. [Source: Materialscientist CC-BY-SA 3.0, http://bit.ly/1Pe6res]

Figure 5.5 Covalently bonded structures. Left: Diamond with three-dimensional structure of covalently bonded carbon. Right: Graphite with covalently bonded sheets of carbon. [Materialscientist CC-BY-SA 3.0, http://bit.ly/1Pe6res]

Other Types of Bonds

Most minerals are characterized by ionic bonds, covalent bonds, or a combination of the two, but there are other types of bonds that are important in minerals. Consider the mineral graphite (Figure 5.5, right): the carbon atoms are linked together in sheets or layers in which each carbon atom is covalently bonded to three others. Graphite-based compounds are strong because of the covalent bonding between carbon atoms within each layer, which is why they are used in high-end sports equipment such as ultralight racing bicycles. Graphite itself is soft, however, because the layers themselves are held together by relatively weak Van der Waals bonds.  Van der Waals bonds, like hydrogen bonds (Figure 5.6), work because molecules or atoms can be electrostatically neutral, but still have an end that is more positive and an end that is more negative. The positive and negative ends attract each other. 

Figure 5.5 Hydrogen bonding in water. Neutral water molecules cling together because the positive charge is on one end of the molecule with the hydrogen atoms, and the negative charge is on the other end with the oxygen atoms. [Source: Benja-bmm27, public domain, http://bit.ly/1lNjqaI]

Figure 5.6 Hydrogen bonding in water. Neutral water molecules cling together because the positive charge is on one end of the molecule with the hydrogen atoms, and the negative charge is on the other end with the oxygen atoms. [Benja-bmm27, public domain, http://bit.ly/1lNjqaI]

 

Metallic bonding occurs in metallic elements because they have outer electrons which are relatively loosely held. (The metals are highlighted on the periodic table in Appendix 1.) When bonds between such atoms are formed, these electrons can move freely from one atom to another. A metal can thus be thought of as an array of positively charged atomic nuclei immersed in a sea of mobile electrons. This feature accounts for two very important properties of metals: their electrical conductivity and their malleability (they can be deformed and shaped).


  1. You can remember that a cation is positive by remembering that a cat has paws (paws sounds like "pos" in "positive"). You could also think of the "t" in "cation" as a plus sign.

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