Urea and fertilizers containing urea are excellent sources of nitrogen for crop production.

Placing ¹⁄₂ in front of O₂ balances oxygen (making it 1 as it is at the right side of the equation).

Copper and sulphur are elements. Diamond is made up of only carbon elements so can't be classified as a compound because compounds are made up of two or more different elements.

Sucrose (C₁₂H₂₂O₁₁) is simply the chemical name for sugar and is a compound that consists of various elements i.e. carbon, hydrogen, and oxygen that combine chemically in a definite ratio.

Placing ¹⁄₂ in front of O₂ balances oxygen (making it 1 as it is at the right side of the equation).

Copper and sulphur are elements. Diamond is made up of only carbon elements so can't be classified as a compound because compounds are made up of two or more different elements.

Sucrose (C₁₂H₂₂O₁₁) is simply the chemical name for sugar and is a compound that consists of various elements i.e. carbon, hydrogen, and oxygen that combine chemically in a definite ratio.

A compound consists of two or more different elements chemically combined in a fixed ratio. Oxygen, magnesium and sodium are all elements but water is a compound made up of hydrogen and oxygen (H₂O)

CO (carbon mono oxide) systematic name is carbon (II) oxide

Number of Na at left = 2 (2 x 1) and at the right = 2 (2 x 1)
Number of Cl at left = 2 and at the right = 2 (2 x 1)

Since the number of elements at each side of the equation is the same, the equation is balanced

CaO + H₂O → Ca(OH)₂

CO (carbon mono oxide) systematic name is carbon (II) oxide

Compounds are made up of two or more different elements. All the options except water are elements/atoms.

The chemical formula of water is H₂O which is made up of hydrogen and oxygen atoms (Two different elements).

Ideally sulphur shares 4 of it's electron with oxygen in a covalent bond so there won't be any charge in this case. But in the sense that sulphur donates 4 of it's electron to oxygen so that each of the two oxygens' configuration can be stable (2,8), then it can be said to be +4.

Note: The atomic number of Oxygen is 8, hence its electronic configuration for a neutral Oxygen will be 2,6 (missing 2 electrons to be stable, thus to complete the 8 electrons which is suppose to be the total number of electrons in the second, third, fourth etc. shells).

Hydrogen (H), Sulphur (S) and Oxygen (O)

The valency of aluminium is 3 and that of oxygen is 2. During the chemical combination, there is exchange of valency. Aluminium will have the valency of oxygen and oxygen in turn will have the valency of aluminium.

The elements are Calcium (Ca), Oxygen (O) and Hydrogen (H)

When writing the chemical formula, the element(s) or compound(s) exchange their valencies.The sulphur has that of the aluminium element's valency (3) and so the value of y will also be the valency for sulphur

The atomic number for sulphur is 16 so on the electronic configuration, sulphur will have 2,8 and 6 on the last shell.

In order for sulphur to attain stability, it has to gain 2 electrons to make it 8 in the last shell, hence the valency of sulphur is 2 and so the value of y will be 2 (the valency of sulphur)

NOTE: The atomic number for aluminium is also 13, hence the electronic configuration will be 2,8 and 3. Aluminium could lose its 3 electrons in order to be stable, hence the valency of aluminium is also 3

Hence the chemical formula for aluminium sulphide is Al₂S₃