General Chemistry

1.0Atom

An atom is the smallest particle of an element that can take part in a chemical reaction.

Atoms are the building blocks of all the matter around us.

2.0Element

An element is a fundamental unit that cannot be easily broken into smaller pieces.

3.0Molecule 

A molecule is the smallest particle of an element or a compound that is capable of an  independent existence and shows all the properties of that substance. In general, a molecule is a group of two or more atoms that are chemically bonded together.

Atoms of the same element or different elements can join together to form molecules. Accordingly, there are two types of molecules:

1. Molecules of elements

The molecule of an element contains two or more similar atoms chemically combined together.

For example, hydrogen gas consists of H₂ molecules.

2. Molecules of compounds

The molecule of a compound contains two or more different types of atoms chemically combined together in a definite proportion by mass.

For example, hydrogen chloride molecule contains hydrogen and chlorine atoms.

Atomicity of A Molecule of An Element 

The number of atoms which constitutes one molecule of an element is called its atomicity.

Phosphorus: P₄ - 4 atomicity

Sulphur: S₈ - 8 atomicity

Depending upon the number of atoms in one molecule of the element, it can be classified as:

4.0Symbol

The symbol of an element is a short way of representing an element. The first scientist to use the symbols of element was Dalton. Modern symbols were proposed by J. Berzelius.

Symbols can be formed as follows:

1. By using the first letter of the English name of the element.

2. By using the first and second letters of the English name of the element.

3. By using the first letter and any other prominent sounding letter of the English name of the element.

4. By using the first letter of the Latin name of the element.

Example: Potassium - Kalium - K.

5. By using the first and the second letter of the Latin name of the element, Sodium- Natrium- Na.

6. By using the first and any other prominent sounding letter of the Latin name. 

Remember

If the symbol has only one letter of the alphabet like in H, O, K, etc., it must be a capital letter.

If it has 2 letters, the first is always a capital letter, while the second is always a small letter.

Examples: Na, Cr, Mn, etc.

What does a symbol represent?

🟂 A symbol represents one atom of an element.

Example : H stands for one atom of hydrogen. 

🟂 However, if a symbol has a subscript and a superscript, as given along side, then more information can be derived from it.

🟂 The subscript 11 shows that one atom of sodium has 11 protons and 11 electrons in it or 11 positively charged, and 11 negatively charged particles and also that the net charge on the atom is zero.  

🟂 The superscript 23, shows that the atom contains 23 dense particles in the nucleus of the atom. The superscript also allows us to calculate the number of neutrons in an atom; this is done by subtracting the subscript from the superscript i.e., 23 – 11=12.

5.0Valency 

Valency is the combining capacity of an atom. It is equal to the number of electrons the atom loses or gains or shares when it combines with one or more atoms.

Example : Valency of sodium is 1, because it can lose one electron. Valency of oxygen may be 2. In the case of carbon dioxide, carbon and oxygen atoms do not lose or gain electrons and their valencies are 4 and 2 respectively. Actually valency refers to the number, not the charge.

Certain atoms exhibit different kinds of valencies. This is because such elements can lose more than one electron from their outer most shell depending on the available conditions. When such atoms exhibit variable valency, the name of the atom with the lower valency ends as -ous, and the one with the higher valency, ends as -ic.

Remember

Atoms with variable valencies have their symbols same but are named differently. 

Examples

🟂 Copper (Cu) has a variable valency of 1 and 2. It is represented as Cu⁺ and Cu²⁺. Cu⁺ is read as Cuprous and Cu²⁺ is read as Cupric.

🟂 Ferrous (Fe²⁺) and Ferric (Fe³⁺).

🟂 Plumbous (Pb²⁺) and Plumbic (Pb⁴⁺).

🟂 Variable valencies are also represented, using the symbols along with the respective valency number in Roman numerals in bracket.

Stock system

Stock nomenclature for inorganic compounds is a widely used system of chemical nomenclature developed by the German chemist Alfred Stock and first published in 1919. In the "Stock system", the oxidation states of some or all of the elements in a compound are indicated in parentheses by Roman numerals.

6.0Ions

An ion is an electrically charged particle.

It is formed by the loss or gain of one or more valence electrons by an atom, so it contains an unequal number of electrons and protons.

Classification of Ions

On The Basis of Charge

1. Cations are positively charged ions. 

They are formed by the loss of one or more electrons by an atom. 

For example, Sodium atom loses one electron to form a sodium ion, Na⁺, which is a cation.

Na             ⎯⎯→            Na⁺       +       e^–

Sodium atom         Sodium ion Electron

Since a cation is formed by the removal of electrons from an atom, therefore, a cation contains less electrons than a normal atom. 

The atomic number of the atom does not change upon the formation of a cation. This is because the number of protons in a cation is the same as that in the parent atom. 

A cation is smaller in size than its parent atom, since there is loss of electrons. 

A cation is stable as compared to its parent atom.

The number of electrons in a cation is less than the number of protons. 

A cation is unreactive as it usually contains 8 electrons in its outermost shell.

The ions of all the metal elements are cations. For example, Na⁺, K⁺, Ca²⁺, Mg²⁺, Al³⁺, etc. H⁺ and NH₄⁺ are the only cations formed from non-metals.

2. Anions are negatively charged ions. They are formed by the gain of one or more electrons by an atom.

For example, a chlorine atom gains one electron to form chloride ion, Cl^-, which is an anion.

Cl         +        1e^–        →               Cl^–

Chlorine atom        Electron Chloride ion

Since an anion is formed by the gain of electrons by an atom, therefore, an anion contains more electrons than a normal atom. The atomic number of the atom does not change upon the formation of an anion. This is because the number of protons in an anion is the same as that in the parent atom.

An anion is bigger in size than its parent atom, since there is gain of electrons.

An anion is stable as compared to its parent atom.

The number of electrons in an anion is more than the number of protons.

An anion is unreactive as it usually contains 8 electrons in its outermost shell.

The ions of all non-metals are anions (except H⁺ and NH₄⁺), For example, Cl^–, O^2–, S^2–, N^3–, etc.

On The Basis of Number of Atoms

1.  Simple ions

Those ions which are formed from single atoms are called simple ions. 

For example, sodium ions – Na⁺, magnesium ions – Mg²⁺, oxide ions – O^2–, etc.

Simple ions are also called monoatomic ions.

2. Compound ions

Those ions which are formed from a group of atoms joined together are called compound ions. For example, ammonium ion – NH₄⁺, hydroxide ion – OH^–, carbonate ion – CO₃^2–, etc. 

Compound ions contain more than one atom and can be classified as diatomic ions, triatomic ions or polyatomic ions.

List of common Monovalent cations

7.0Chemical Formulae 

Every chemical substance is known by a specific name. But many times, these names are cumbersome, confusing, and do not provide information about its chemical composition. 

To overcome this, each chemical compound is represented by a chemical formula that gives its composition (constituent elements present) and the number of elements of each type present.