Chemical Bonding and Farmulas

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Chemical Bonding, Chemical Formulas

A compound is a substance that is made up of two or more different elements chemically combined.

A compound is a substance that is made up of two or more different elements chemically combined.

Compounds

Most substances are compounds e.g. Water (H2O) and Carbon Dioxide (CO2) The Octet Rule states that when bonding occurs, atoms tend to reach an electron arrangement with eight electrons in the outermost shell.

The Octet Rule states that when bonding occurs, atoms tend to reach an electron arrangement with eight electrons in the outermost shell.

This chapter looks at chemical bonds – the attractive forces that hold compounds together.

So almost all the time atoms either gain or lose electrons until they have eight in their outer shell. OCTET RULE EXCEPTIONS
* Transition elements do not always form bonds with 8 electrons in the outer shell * Some elements, for example Hydrogen and Helium form bonds where there are only 2 electrons in the outer shell.

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Chemical Bonding, Chemical Formulas

Ionic Bonding
An Ion is a charged atom or group of atoms.

An Ion is a charged atom or group of atoms.

Ionic bonds are formed when electrons are TRANSFERRED from one atom to another. This forms ions. An Ionic Bond is the force of attraction between oppositely charged ions in a compound.

An Ionic Bond is the force of attraction between oppositely charged ions in a compound.

The name’s Bond, Ionic Bond, taken not shared
The name’s Bond, Ionic Bond, taken not shared

Positive ions are formed when an atom loses electrons (It is taking away a negative charge – making it more positive) Positive ions = cations
Negative ions are formed when an atom gains electrons (Its adding on a minus charge – making it more negative) Negative ions = anions
You are sometimes asked to show the formation of ionic bonds in certain compounds. For this you use “dot and cross diagrams”. You know how to do these so I’ll just put in a few examples to remind you. In general, ionic bonds are formed when a metal and a non-metal react.

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Chemical Bonding, Chemical Formulas

EXAMPLE
Show the formation of ionic bonds in (i) Magnesium Fluoride
(ii) Potassium Bromide
(i) Magnesium Flouride contains magnesium and fluorine. Look in the periodic table and find what group each of these are in. Every element wants to either gain or lose electrons until it has 8 electrons in its outer shell. Magnesium is in group 2 so it wants to lose 2 electrons.

Fluorine is in group 7 so it wants to gain 1 electron.
Therefore, there must be two fluorine atoms for each magnesium atom.

(ii) Potassium is in group 1. It wants to lose 1 electron. Bromine is in group 7. It wants to gain 1 electron.
Therefore there is one bromine atom for each potassium atom.

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Chemical Bonding, Chemical Formulas

Writing Formulas
When working out bonds the same number of electrons has to be lost and gained. This gives you the formula for the compound. You need to know your group ions/complex ions, and their charges, when writing formulas. They come up a lot so LEARN THEM NAME| FORMULA| CHARGE|

Hydroxide ionNitrate HydrogencarbonatePermanganate| OH-NO3-HCO3-MnO4-| 1 negative charge (-)| CarbonateChromateDichromateSulfateSulfiteThiosulfate| CO32-CrO42-Cr2O72-SO42-SO32-S2O32-| 2 negative charges (2-)| Phosphate| PO43-| 3 negative charges (3-)|

Ammonium| NH4+| One positive charge (+)|
Here is a...
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