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Bonding
The atomic mass (or mass number A) is the number of protons plus the number of neutrons in the nucleus and is designated the symbol A.
The atomic number (or proton number Z) is the number of protons in the nucleus and is equal to the number of electrons in an atom which is always electrically neutral.
These numbers are sometimes given with the symbol for the element as shown below:
The sodium atom can therefore be represented as
Stability in atoms
When atoms have a full outer shell they achieve a stable state. Evidence for this are the group 0 elements or inert gases/noble gases that are very stable and do not react with other substances except for a few special reactions. The atoms of the group 0 elements all have a full outer shell.
When atoms of elements react with other atoms they try to achieve a stable state (full outer shell). This is important in understanding how atoms can bond together and to be able to predict the molecules that are likely to form. The two important types of bonding are ionic and covalent bonding.
Ionic bonding
Ionic bonding involves the transfer of an electron or electrons from one atom to another so that the balance of positive and negative charges in the atom is disrupted to form charged particles called ions. Atoms will normally bond when a full outer shell can be achieved, as in the case of NaCl (sodium chloride). The following diagram shows how the ions are formed and the giant lattice structure of NaCl.
[2, 8]+ [2, 8, 8]-
Both the dots and crosses represent electrons where dots have been used for the chlorine atom and crosses for the sodium atom. As can be seen the sodium atom has lost it’s outer electron and has formed a positive sodium ion whereas chlorine has gained an electron from sodium to form a negative chloride ion. Positive ions are smaller than their atoms and negative ions are bigger than their atoms. Since the ions formed above have opposite charges there is electrostatic attraction between them that holds them together.
Ionic substances such as NaCl are solids at room temperature have high melting points and boiling points and form a giant ionic lattice structure. Each sodium ion is bonded to six chloride ions and vice versa forming a cubic structure part of which is shown below.
This giant lattice structure shown has a very large number of bonds so that a lot of energy is required to break these bonds resulting in the high melting and boiling points observed.
Ionic bonding usually occurs between metals and non-metals as seen in the above example, sodium and chlorine.
Exercise 1
1.
a. Complete the table shown below:
a. What change takes place when a sodium ion is formed from a sodium atom?
______________________________________________________________________________
b. What change takes place when a chloride ion is formed from a chlorine atom?
_______________________________________________________________________________
c. State two changes that take place in a crystal of sodium chloride when melting occurs.
_______________________________________________________________________________
_______________________________________________________________________________
2. Provide the chemical symbol and charge for ions that are likely to form from the atoms of the following elements (use the periodic table to help you:
a. Li
b. Be
c. B
d. O
e. F
3. In the space provided below show how atoms of magnesium and chlorine will react to form the ionic substance magnesium chloride by using a dot and cross diagram. Show the charges on the ions using square brackets.
4. Do the same as in question 3 to show how atoms of aluminium and chlorine will react to form the ionic substance aluminium chloride.
Covalent bonding
Covalent bonding is another way that atoms can chemically combine to achieve a full outer shell. In covalent bonding there is no transfer of electrons as in ionic bonding but instead there is sharing of electrons. Electrons are shared by overlap of the outer shell of the atoms that are involved in bonding. Non-metal and non-metal atoms bond in this way and again as before from knowledge of the number of electrons in the outer shell of atoms it is possible to predict how they may share electrons to achieve a stable state.
Using a very simple atom (hydrogen atom) it is easy to observe how two atoms may react covalently to produce a hydrogen molecule, as shown below.
Each hydrogen atom shares one electron in its outer shell with another atom by overlap of the shell thus both have effectively two electrons in their outer shell to produce a stable molecule of hydrogen. The hydrogen atoms, as shown in the dot and cross diagram, are held strongly to each other by the covalent bond but the forces holding the molecules together are negligible so that hydrogen is a gas. The line between the atoms of hydrogen (where hydrogen atoms are shown using their symbol) represents a shared pair of electrons and is equivalent to a covalent bond. This is the displayed formula of hydrogen and shows the relative positions of the atoms and the number of bonds between them.
Hydrogen is a diatomic gas as it is made of two atoms and there are other examples of diatomic molecules that are part of the mixture of gases called air.
Below is another covalent molecule HO more commonly known as water. Two hydrogen atoms are bonded to one oxygen atom by overlap of shells. As can be seen both the oxygen atom and the two hydrogen atoms obtain a full shell by covalent bonding.
Carbon is an atom that can produce many covalent compounds where many are organic in nature. It also has the ability to bond with itself to produce different materials that depends on the way it has bonded. Carbon has 4 electrons in its outer shell and therefore needs 4 more electrons to achieve a full outer shell. It can do this by bonding with 4 more carbon atoms where each carbon atom shares one of its outer electrons with another carbon atom to produce a giant molecular structure as shown below.
Giant covalent molecules such as these are very hard, have high melting points and are a good demonstration of the great strength of a material bonded in this manner with a vast number of covalent bonds. Other examples of giant molecular structure are silicon dioxide, SiO2 (most commonly found in nature as quartz in sand and rocks).
Some useful points to remember for both ionic and covalent bonding
As a general rule for ions remember
- Group 1 elements form ions with one positive charge (they lose one electron)
- Group 2 elements form ions with two positive charges (they lose two electrons)
- Group3 elements form ions with three positive charges (they lose three electrons)
- Group 6 elements form ions with two negative charges (they gain two electrons)
- Group 7 elements form ions with one negative charge (they gain one electron)
Also
- Ions are formed when atoms gain or lose electrons
- Metals form positive ions by losing electrons
- Non-metals form negative ions by gaining electrons
- Covalent bonds are formed when non-metal atoms share pairs of electrons in their outer shells
Exercise 2
1. State which of the following compounds is likely to be covalent and why?
a. HI
b. CH 4
c. NaOH
d. NH 3
e. SO 2
2. In covalent bonding atoms share electrons, explain how electrons are shared.
3. If one pair of shared electrons is equivalent to one covalent bond then state how many covalent bonds two pairs of shared electrons are equal to?
4. The chemical formula for a number of covalent compounds is given. For each of these draw a dot and cross diagram to show how the atoms are bonded (you need only show the outer shell):
a. Cl 2
b. HCl
c. NH 3
d. CH 4
e. C 2H 6