ACIDS

A hydrogen-containing compound which gives hydrogen ions (H₃O⁺ or H⁺) when dissolved in water, is called an acid.

 

Acids can be classified as:

v Naturally-occurring acids

v Mineral acids

 

Mineral acids can be classified into two groups:

Hydracids – The solutions of compounds of hydrogen with highly electronegative (nonmetallic) elements other than oxygen, are called hydracids.

 

Oxoacids – The acids which contain an oxygen-containing anion (called oxoanion), are called oxoacids (earlier called oxyacids).

 

Thus, oxoacids are the compounds which contain hydrogen, oxygen and another nonmetallic element.

 

Strong and Weak Acids

Strong Acids – Acids which are completely ionised/dissociated when dissolved in water are called strong acids.

Example:

Sulphuric acid (H₂SO₄)

Hydrochloric acid (HCl)

Perchloric acid (HClO₄)

 

Weak  Acids  – Acids which are partially ionised when dissolved in water are called weak acids.

Example:

Ethanoic acid (CH₃COOH)

Carbonic acid (H₂CO₃)

Hydrocyanic acid (HCN)

The Charged Particles

IONS

The electrically charged particle that is formed whenever an atom loses or gains one or more electrons is called an ion.

When an atom of hydrogen loses one electron, it forms a hydrogen ion.

 

Cation

v The cation is a positively charged ion.

v It is formed whenever an atom loses one or more electrons.

When a sodium atom (Na) loses an electron, it gives sodium ion (Na⁺). Na⁺ is a cation.

     

 

v The charge on any ion is shown as a superscript to the symbol of the corresponding element.

  

Anion

v The anion is a negatively charged ion.

v It is formed whenever an atom gains one or more electrons.

When a chlorine atom (Cl) gains an electron, it gives chloride ion (Cl^–). Cl^– is an anion.

Atomic Symbols

J. J. Berzelius introduced the modern system of denoting elements by their symbols.

 

Symbol

The symbol of an element is an abbreviation (short name) for its full name.

OR

The symbol of an element is a shorthand notation for its full name.

 

Significance of a Symbol

The symbol of an element:

Ø  Stands for the name of the element.

Ø  Stands for one atom of the element.

Ø  Represents the quantity of the element equal in mass to its atomic mass.

 

For example; symbol C stands for:

Ø    Element Carbon

Ø    One atom of element carbon

Ø    12 g of element carbon

 

The symbol N stands for:

Ø   Element Nitrogen

Ø   One atom of element nitrogen

Ø   14 g of element nitrogen

 

Modern Atomic Symbols

According to the modern system of atomic symbols,

F  The  first  letter  of  the  Latin  or  Greek  name  of  an  element  is  taken  as  its symbol. Such a letter is written as a CAPITAL letter.

F If more than one element has the same first letter, their symbols consist of two letters – the first letter is followed by another letter which is not common.

F The first letter is written as a capital letter while the second letter as a small letter.

 

For example, the symbol of Hydrogen is H and that of Helium is He.

The symbol of Calcium is Ca and that of Copper is Cu.

 

The common names of some typical elements, their symbols and the source from where these symbols are derived, are given below:

 

 

Atomic Symbols for Some Typical Elements

Atomic Number and Atomic Mass

Atomic Number

Ø Atomic number of an element is the number of protons present inside the nucleus of that element.

Ø It is denoted by Z.

Ø Atomic number cannot have a fractional value.

 

Mass Number

Ø Sum of the number of protons and neutrons inside the nucleus of an atom is called its mass number.

Ø It is denoted by A.

 

Atomic Mass

Ø The average mass of an atom of an element in atomic mass units is called its atomic mass.

Ø Atomic mass is denoted by A.

Ø The atomic mass of hydrogen is 1.008 atomic mass unit (or 1.008 u).

Commonly, the term ‘relative atomic mass (Ar)’ is also used.

 

Relative Atomic Mass

Ø The relative atomic mass (A_r) of an element is defined as the average relative mass of its atom as compared with that of an atom of ¹²C (carbon-12) taken as 12 atomic mass units.

Ø Relative atomic mass has no units.

Ø It is a pure number.

Number of electrons, protons and neutrons in an atom of any element are related to its atomic number and mass number as:

Number of electrons (e) = Number of protons (P) = Atomic number (Z)

 

The atomic masses, number of electrons, protons and neutrons in the atoms of certain common elements is given below.

Mass number (A) = Number of protons (P) + Number of neutrons (N)

Number of Electrons, Protons and Neutrons in an Atom