05 Particles (2) Molecules And Ions 2009

THE PARTICLES OF MATTER (PART II): IONS & MOLECULES THE COMPLETE OUTER SHELL RULE Let us revise what we’ve studied in the previous unit Atoms are electrically balanced particles that consist in one positively charged centre called the nucleus surrounded by a “cloud” of negatively charged particles called the electrons, to exactly balance the nuclear charge. Molecules are electrically balanced particles with more than one positive centre (nucleus). Molecules are sets of bonded atoms that act as a unit. Ions are atoms (or groups of atoms) with unbalanced charges The question arises: why should atoms form molecules or ions? The answer to this question was proposed by Lewis with his “rule of eight” or “complete outer shell rule”. This rule has a solid theoretical background that goes far beyond the high school syllabi Nevertheless it is strongly suggested by the simple observation of the following facts. • • • • •

Elements belonging to Group I form mono-positive particles, atoms belonging to Group II di-positive particles. To do so, they must get rid of the outer shell’s electrons becoming isoelectronic with the closest (preceding) noble gas. Isoelectronic means, they have as many electrons as the noble gas). Elements of Groups VI and VII form anions with two and one negative charges respectively. For this purpose they gain electrons (two electrons and one electron respectively) and they also become isoelectronic with the nearest noble gas in the Periodic Table. This can be extended to other cases as the bonding of nitrogen to three hydrogen atoms or of carbon to four of them, etc.

This Lewis’ rule or complete outer shell rule, or “the rule of eight” gives a fairly good explanation to the problem of chemical bond formation and with some modifications can be generally applied. It states that Atoms bond to each other giving, taking or sharing electrons in order to complete their outer shells (to resemble the nearest noble gas’ structure). In this Unit we will describe the different ways in which atoms bond to each other METALS BONDED TO NON METALS: IONIC BONDS Most metals have no more than two electrons in their outer shells. On the other hand most non metals have five or more electrons in the highest level. To resemble the nearest noble gas, metals give their outer shell’s electrons and non metals take them

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Ionic bonding in sodium chloride Sodium (2,8,1) has 1 electron more than a stable noble gas structure (2,8). If it gave away that electron it would become more stable. Chlorine (2,8,7) has 1 electron short of a stable noble gas structure (2,8,8). If it could gain an electron from somewhere it too would become more stable. The result is obvious. If a sodium atom gives an electron and a chlorine atom takes it, both become more stable.

Sodium has lost an electron, so it no longer has equal numbers of electrons and protons. Because it has one more proton than electron, it has a charge of 1+. If electrons are lost from an atom, positive ions are formed. Positive ions are called cations.

Chlorine has gained an electron, so it now has one more electron than proton. It therefore has a charge of 1-. If electrons are gained by an atom, negative ions are formed. A negative ion is called an anion. The sodium ions and chloride ions are held together by the strong electrostatic attractions between the positive and negative charges. This is known as an ionic bond. You need one sodium atom to provide the extra electron for one chlorine atom, so they combine together 1to1. The formula is therefore NaCl. As you can see, the formula of an ionic compound tells you the different classes of atoms it is made of and the proportion in which they are combined. The formula NaCl states that you can find one sodium cation per chlorine in this substance. When ions are formed they cluster together in a pattern that alternates cations and anions. In these patterns every cation is surrounded by negative particles and every anion by positive particles. There is no particular cation associated to any particular anion. Ionic substances are solid at room temperature and usually require very high temperatures to melt (the forces among the particles are very strong).

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Magnesium, in group II, has to get rid of two electrons to become stable. As chlorine takes up just one, two chlorine atoms are needed for every magnesium atom. The formula in that case will be MgCl2. The pattern of the particles in the solid will be different but the essentials of the ionic bonding still hold. NON METALS BOND OTHER NON METALS: THE COVALENT BOND In this case, none of the atoms will give electrons to the other! Instead they share one, two or three pairs of electrons in order to achieve the complete shell. These “shared pairs” are no longer located around one of the nuclei but around both and mainly in the zone between both nuclei. A molecule is formed (see at the beginning of the chapter) Chlorine For example, two chlorine atoms could both achieve stable structures by sharing their single unpaired electron as in the diagram. The fact that one chlorine has been drawn with electrons marked as crosses and the other as dots is simply to show where all the electrons come from. There is no difference between them. The two chlorine atoms are said to be joined by a covalent bond. The reason that the two chlorine atoms stick together is that the shared pair of electrons is attracted to the nucleus of both chlorine atoms. The formula of the molecule is written Cl2. This formula shows not just the relative amount but the actual number of atoms of different elements in a molecule of the compound Hydrogen Hydrogen atoms only need two electrons in their outer level to reach the noble gas structure of helium. Once again, the covalent bond holds the two atoms together because the pair of electrons is attracted to both nuclei. The formula of hydrogen gas, formed by hydrogen molecules is H2 Hydrogen chloride Now the sharing is between two different non metals. Once again sharing takes place and a molecule is formed. The hydrogen has a helium structure, and the chlorine an argon structure. The formula for this substance is HCl. Covalent substances can be gaseous, liquid or relatively low-melting-point solids. In this case the forces holding the particles (molecules) together are far weaker than in the case of ionic compounds. These forces are of electric nature but they will be described in later courses.

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Water Molecules are not formed just by two joining two atoms. This is the case for water, in which oxygen requires a share of two electrons to complete its outer shell but hydrogen atoms can afford just one electron to the share. Water is a tri-atomic molecule in which oxygen forms two covalent bonds one with each of the hydrogen atoms

METALS BOND TO METALS In this case none of the atoms will keep its outer shell’s electrons. What happens then if the electrons cannot go away and “nobody needs them”? Atoms in this case pack tightly forming a compact structure: their outer shells can be thought as bursting and collapsing into a super-multi-atom outer shell, a sea of electrons where these particles move freely as no atom will make any effort to keep them. These free electrons are the 'electronic glue' holding the particles together. The electrons can move freely within this region, and so each electron becomes detached from its parent atom. The electrons are said to be delocalised. The metal is held together by the strong forces of attraction between the positive nuclei and the delocalised electrons.

This is sometimes described as "an array of positive ions in a sea of electrons". If you are going to use this view, beware! Is a metal made up of atoms or ions? It is made of atoms. Each positive centre in the diagram represents all the rest of the atom apart from the outer electron, but that electron hasn't been lost - it may no longer have an attachment to a particular atom, but it's still there in the structure. Sodium metal for instance, is written as Na and not as Na+.

QUESTIONS AND PROBLEMS 1- An atom is located in the 2nd period and the 5th group. a- Find its Z b- Predict what kind of bond will it form when bonding to an identical atom c- Will it bond ionically or covalently to chlorine? 4

2- Bronze is not an element but an alloy formed by copper and tin. (Both metals). How does tin bond to copper atoms in the alloy? 3- Show in diagrams how magnesium (group II) bonds to fluorine (group (VII) 4- Write down the cations formed by the following atoms: Sodium Calcium Magnesium Aluminium. (e.g.: Lithium = Li+) 5- Write down the simple anions formed by the following atoms: Chlorine Oxygen Fluorine Sulphur. (e.g.: Nitrogen = N3- or N---) 6- Write down the electronic structure (or distribution or configuration)of all ions in exercises 5 and 6 7- In what proportion will the following elements combine? a- F and Na b- Mg and S c- Na and O b- sodium (group I) bonds to oxygen (group VI)

d- Cl and Ca

8- Water molecules are made by atoms of oxygen and hydrogen. a- Draw the electron configurations of both elements (Z = 8 and Z = 1 respectively) b- Are hydrogen and oxygen metals or non metals? c- What kind of chemical bond will be formed between them? d- How many electrons does oxygen need to comply with the complete outer shell rule? e- How many electrons does hydrogen need in turn? f- How can both requirements be fulfilled at the same time? g- Using the previous conclusions draw a diagram showing the bonds in the water molecule. Use cross-and-dots models (Cross and dot : see the diagrams of Cl2, HCl and H2) 9- Ammonia is a very important compound from scientific, technical and commercial points of view. It is formed by nitrogen and hydrogen atoms. Draw the bonds in ammonia following the same steps as in exercise 8. 10- Our bodies are formed by atoms being carbon, hydrogen, oxygen, nitrogen, sulphur and phosphorus those that are found in higher proportions. Are we essentially ionic or covalent? Justify your choice. 11- Two pots contain white powdered samples. One of them is glucose (C6H12O6) and the other one mercury (I) chloride (Hg2Cl2) a powerful poison. Considering the bonds between its elements, plan a simple experiment to decide which is which. State: a- what you would do b- what you would see c- how you would decide which is which.

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