Understanding formation of compounds ( chemistry f.4)

(9Stability of Noble Gases

What is Chemical Compound

• A chemical compound is a substance that is formed by more than one elements that bond together chemically in a fixed proportions.
• In periodic table, there are only 118 elements, and about 1/3 of them are synthetic elements.
• Only a few substances exist as element (Not Compound) in nature.
• The table below shows some examples of substance exist as element in nature.

Element exist as monoatomic gas	Element exist as diatomic Molecule(gas)	Element exist as solid
Helium (He) Neon (Ne) Argon (Ar) Krypton (Kr) Xenon (Xe) Radon (Rn)	Oxygen (O2) Nitrogen (N2)	Carbon (Graphite & Diamond) Gold Silver Platinum

• In nature, we can find millions of substances, which means most of the chemical substances exist as compound in nature.
• In short, elements tend to form compound in nature.

Why Elements Tend to Form Compound?

• A compound is formed by 2 or more elements hold together by a force called chemical bond.
• Before studying why elements like to bond together, we need to know why certain elements such as Helium and Neon do not form any bonds with other elements.

Why Noble Gases Don't Form Compound

• In previous chapter, we have discussed that Group 18 elements (Noble Gases) exist as monoatom in nature.
• They are inert in nature and do not react with any other elements (or themselves) to form any chemical compounds.
• In other words, they are chemically very very stable (or chemically very very non-reactive).

Duplet and Octet Electron Arrangement

• The charge on the nucleus and the number of electrons in the valence shell determine the chemical properties of an atom.
• The stability of noble gas is due to their electrons arrangement.
• The diagram above shows the first four elements of Noble Gas.
• We can see that the outer most shell (valence shell) of Helium has 2 electrons. We call this duplet electron arrangement. We should take notes that the maximum number of electrons can be filled in the first shell is 2 electrons, which means 2 electrons in the first shell is considered FULL.
• The valence shell all other Group 18 elements (including Xenon and Radon which is not shown in the diagram) has 8 electrons, and we call this octet electron arrangement.
• When the electron arrangement of an atom is duplet or octet, the energy of the electrons is very low, and it is very difficult (even though it is not impossible) to add or remove electrons from the atom.
• This explain why noble gases are reluctant to react with all other elements.

The Octet Rule

• So far we have learnt that the electon arrangement of noble gases are octet duplet, and this is the most stable electron arrangement of an atom.
• Atoms of other main group elements which is not octet tend to react with other atoms in various ways to achieve the octet.
• The tendency of an atom to achieve an octet arrangement of electrons in the outermost shell is called the octet rule.
• If the outermost shell is the first shell, then the maximum number of electrons is two, and the most stable electron arrangement will be duplet.
• A configuration of two electrons in the first shell, with no other shells occupied by electrons, is as stable as the octet electron arrangement and therefore is also said to obey the octet rule.

Important Notes Most of the elements (except noble gases) are chemically not stable. It is the aim of every atom to achieve the duplet or octet electron arrangement. This makes them very stable. It is only the valence electrons in the outermost shell involved in bonding. The electrons in the inner shells are not involved. The maximum number of electrons in the first shell is two. This is called a duplet. The maximum number in the second shell is eight. This is called an octet.

How Atoms Achieve Duplet or Octet Electron Arrangement?

• Atoms can achieve duplet or octet electron arrangement in 3 ways:

1. throw away the excess electron(s)
2. receiving electron(s) form other atom if they are lack of electron(s)
3. sharing electron

• 2 types of chemical bonds are commonly formed between atoms, namely

1. Ionic Bond
2. Covalent Bond

The Ionic Bond

• By releasing or receiving electron(s), the atoms will become ions and consequently form ionic bond between the ions.
• Ionic bonds are always form between metal and non-metal. For example, sodium (metal) react with chlorine (non-metal) will form an ionic bond between sodium ion and chloride ion.
• The compounds formed is called the ionic compound.
• Some time, an ionic bond is also called electrovalent bond.

[edit] The Covalent Bond

• By sharing electron(s), the atoms will form covalent bond between the atom and the molecule formed is call the covalent molecule.
• Covalent bond is always formed between non-metal with another non-metal.