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Elements exist in a single atom state, sometimes in two atoms states such as oxygen, chlorine, and a few more. However, there are so many chemicals that aren't elements but different forms of elements. Let's understand how this is possible.
What is a Compound?
Take an example of water. The chemical formula of water is H2O. If you look closer, there are two elements, hydrogen and oxygen. Elements are always denoted with a capital letter. Furthermore, there is a numeric value too between both elements. The question is what kind of molecule is water? The answer is compound.
A compound is a molecule that is made up of two or more elements that are chemically combined with each other.
The molecule of water is made up of hydrogen and oxygen which chemically reacted with each other. Let's do one more example for better understanding. Carbon dioxide (CO2) is a gas that we breathe out. Yes, our breath is also a chemical. The molecule of carbon dioxide is made up of carbon and oxygen which chemically reacted with each other.
The list of compounds is uncountable. There is a numeric value in the compounds. The value indicates the number of atoms of the specific element. In water, two hydrogens are reacting with one oxygen to form water.
How Compounds are Formed? And Their Properties
Compounds are formed when two or more elements chemically react. Even two or more compounds can react with each other to form a new compound. The key point to note is that chemical reaction is a must thing. If elements or compounds don't undergo a chemical reaction then the final product will not be a compound.
Physical Properties
A compound will have different physical and chemical properties than its reactants. For example water. Two hydrogens react with one oxygen to form water. Hydrogen and oxygen are both gases yet the product formed is a liquid. Do you see a physical change? The reactants were gas while the product is a liquid. There are more physical differences between water and its reactants such as colour, solubility, and many more.
Chemical Properties
Furthermore, when compounds are formed, not only physical properties change but also chemical properties. For example, water isn't as reactive as hydrogen. The boiling point of pure water is at 100°C, however, the boiling points of hydrogen and oxygen are -253°C and -183°C. That is quite a big change, isn't it? Let's do one more, hydrogen is a very flammable gas. As a matter of fact, it is used in a fuel cell. On the other hand, water is not flammable. In fact, water is used to extinguish the fire. If we compare the reactant and product then it is observable that the reactant is causing fire and the product is opposite of it, i.e. extinguishing fire.
Separation Property
There are a few more properties that make the compound special. Compounds are not only different in chemical and physical properties but in other properties as well. When two or more elements or compounds chemically react with each other, they start to make bonds. It means that elements or compounds create bonds with each other and these bonds are very strong because of high intermolecular forces of attraction(you will learn more about bonding in upcoming resources). Although the force of attraction is very high but they are still separable. Compounds can be separated using thermal decomposition and electrolysis.
Composition
The word composition means the ratio of all substances that are making up a unit. In the case of compounds, you cannot vary the composition. In order to initiate a chemical reaction, there are some requirements and one of them is the composition. For example, 2 molecules of hydrogen and 1 atom of oxygen are required to make water. If you provide only 1 atom of hydrogen to the 1 atom of oxygen then no chemical reaction will occur. Therefore, the composition of compounds are fixed, you have no control over it.
Names of Compounds
Since compounds are newly formed chemicals that are different from their reactants, they will have names of their own. Although they don't have unique names like elements have, however, their names are derived from their reactants. For example, the chemical name for table salt is sodium chloride. The chemical equation of sodium chloride is below:
Sodium + chlorine → sodium chloride
The compound has the name of its both reactants. All compounds got their name from their reactants and their name indicates the elements that reacted with each other to form this compound. For example, carbon dioxide, the name is telling that the compound, carbon dioxide, is made up of carbon and oxygen (oxide is the name of oxygen ion) but here, the word "di" might be new for you. Many compounds have these letters such as nitrogen dioxide, sulfur dioxide, and many more. The word "di" represents the number two. It means that the compound contains two atoms of that specific element. In carbon dioxide, one carbon atom is attached with two atoms of oxygen to form a compound. These are Greek prefixes that declare the number. Below are the Greek numerical prefixes:
Di → 2
Tri → 3
Tetra → 4
These Greek prefixes are used to denote the number of atoms of a specific element. For example, trimethylamine, the word tri is used before methyl means the above compound contains three methyl atoms bonded with the amine.
Below is the example table of names of compounds that were made by their reactants.
Chemical Formula
In chemistry, taking names of chemicals isn't a common practice. Chemicals have a specific formula that defines them. Compounds are usually represented by a chemical formula. These chemical formulas are made by writing the chemical symbols of elements and compounds that reacted to create a new compound.
The chemical formula of a compound also provides us with information about the compound. This information is not available in the names of compounds and that makes the chemical formula more important. There are two things that one can learn from the chemical formula.
- Elements that were used to make the compound.
- The ratio of different elements that made the compound.
Take a look at the water. The chemical formula is H2O. We have already mentioned how to read chemical formulas, why don't you give it a try and evaluate yourself, the answer is also written in one of the above paragraphs.
We have another compound that is a bit different and the name of that compound is lead (II) nitrate. The chemical formula for this compound is Pb(NO3)2. This is something that we didn't mention in the above headings, so let's understand the meaning of brackets.
Lead (II) nitrate is a compound and as the name tells it contains lead atoms, nitrogen atoms, and oxygen atoms (nitrate is a combination of nitrogen and oxygen and its chemical formula is NO3).
To learn the ratio of atoms, we need to take a good look into its chemical formula.
Pb(NO3)2
This compound contains one atom of lead, 2 atoms of nitrogen, and six atoms of oxygen. Some of you might be confused about how nitrogen has two atoms? How does oxygen has six atoms when the subscript is saying three atoms. Take a look at the brackets and the subscript at the end of the bracket. Nitrate is already a compound and lead is an element. The nitrate compound reacted with the lead element to form lead (II) nitrate. However, to carry out this reaction, the requirement is that lead requires two molecules of nitrate and that is why we trapped the nitrate chemical formula in brackets and gave a subscript. That brackets are like a trap, it means that the subscript will be applied to whatever is inside those brackets. In the case of lead (II) nitrate, nitrate is within brackets and that means the subscript on the bracket will be applied on the nitrate only, not the lead. We will teach you how to make chemical formulas in detail in the periodic table chapter.
Since you understood the purpose of brackets, now calculate the atoms of lead (II) nitrate.
Lead = 1 atom
Nitrogen = 1 atom x 2 = 2 atoms
Oxygen = 3 atoms x 2 = 6 atoms
Decomposition of Compounds
Usually, two or more elements or compounds react with each other to form a new compound but that isn't the case for all compounds. Some compounds decompose into other compounds or elements. The decomposition process is called thermal decomposition which means breaking down a compound by means of heat energy. Take an example of sugar. The chemical formula of sugar is C12H22O11. When you heat sugar, it decomposes into carbon dioxide and water, below is its chemical reaction:
C12H22O11 + 12 O2 → 12 CO2 + 11 H2O
Conclusion
There are many other properties of compounds but for GCSE level, this is enough. The best way to understand compounds is to read the theory thoroughly and then solve questions that appeared in past exams of GCSE chemistry. This way you will have a clear idea of how to answer questions and what the examiner is expecting from you.
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Lithium is used in place of Fluorine
Hi Madan. You’re right to point that out – thanks very much for your comment!
amazing side
It’s very educative,I learnt alot thanks ❤️
Was very educative and help. Gave me a broader understanding of the topic