[Chemistry Class Notes] on Difference Between Atom and Ion Pdf for Exam

Before explaining the difference between atoms and ions, let us know what are atoms and ions in detail.

Introduction to Atoms

The word ‘atom’ was coined from the Greek word ‘atomos’; the meaning of ‘a’ in the word is ‘not’ and ‘tomos’ means ‘cut’. Together it became impossible to cut. Investigations on atoms have been started over a long period of time and still, it is continuing. The basic definition of atoms is “Atoms are small indivisible particles and every matter that we see are made up of atoms”

The first explanation of the atom was given by an English chemist, Dalton in 1803. It was known as Dalton’s atomic theory. 

According to this theory, the atoms are the building blocks of matter. But this clarification of atoms didn’t exist for a long time and in 1815 another scientist named Prout suggested that elements are made up of hydrogen atoms. Since the atomic weight of some elements could not relate with that of hydrogen, this theory was not accepted. In 1889, J.J Thompson conducted many experiments and based on some observations, he put forward the ‘plum pudding model’ of an atom. According to this, an atom is a positively charged sphere and negatively charged electrons are embedded in it like a plum in the pudding. The positive charge of the sphere is neutralised by the negative charge of the electron. So an atom is electrically neutral. Discovery of subatomic particles like protons and neutrons made this explanation invalid. 

In 1909, Rutherford presented the basic structure of an atom through his famous alpha particle scattering experiment. Rutherford atomic model states that an atom has a central part known as the nucleus and electrons are revolving around it. The positive charge of the nucleus is neutralised by the negative charge of the electrons. But his theories couldn’t give the details regarding the stability of an atom. Further studies on the Rutherford atom model were conducted by the scientist Niles Bohr and in 1913; he put forward the atom model which explains the stability of an atom and his observations were quite successful. Bohr’s atomic model could not explain some concepts like the fine structure of spectral lines of hydrogen, distribution of electrons etc. Somerfield modified Bohr’s atomic model and he explained the path of the electron and gave more explanations regarding the structure of spectral lines of hydrogen.

Features of Atom on the Basis of Modern Atomic Theory

1. The Modern atomic theory states that though atoms are small, they are divisible.

2. Nucleus of an atom consists of subatomic particles, protons and neutrons which are collectively known as nucleons. Electrons are revolving around the nucleus in specific orbits. Since the number of protons and number of electrons are equal, atoms are always neutral.

3. Molecules are formed by the combination of atoms.

4. The arrangement of the atoms in the periodic table is in such a way that down the group, the radius of the atom increases and across the period radius of the atom decreases.

5. On the basis of nuclei, atoms can be classified as isotopes, isobars and isotones.

• Isotopes– If the atoms of the same element have the same atomic number, (represented as ‘Z’) and different mass number (represented as A). They are known as isotopes. Protium (₁H¹), deuterium (₁H²), tritium (₁H³) etc are isotopes of hydrogen. In this, the number of protons =1 and the number of neutrons are different.

• Isobars– If the atoms of different elements have the same mass number (A) and different atomic number (Z), they are known as isobars. ₈O¹⁶ and ₇N¹⁶ are examples of isobars.

Here, the number of neutrons = 16 and the number of protons are different

Number of neutrons in the carbon is A–Z = 14–6 = 8

Number of neutrons in oxygen is 16–8 = 8

Ions

Unlike atoms, ions are charged particles. But they also consist of electrons, protons and neutrons. When an atom loses or gains an electron, it becomes an ion. Depending upon the gain or loss, atoms can be classified as positive ions or cations and negative ions or anions. 

Positive ions or cations are formed when an atom loses an electron. This concept can be cleared with the help of an example. A sodium atom consists of 11 protons and 11 electrons. It is electrically neutral when it loses an electron it becomes Na⁺ ion. The charge of this ion is +1. 

Negative ions or anions are formed when an atom gains an electron. For example, a chlorine atom consists of 17 electrons and 17 protons. When it gains an electron, it becomes Cl⁻ ion. The charge of this ion is −1.

This gain and loss of an electron can change the chemical and physical properties of an atom.

Combination of ions gives compounds. For example, sodium ions and chlorine ions combine to form sodium chloride, salt that we use for cooking.

Neutral sodium atom and chlorine atom are highly reactive than the Na⁺ and Cl⁻ ions. The sodium and chlorine in common salt are not reactive and it is safe to consume. 

Ions can be classified as monatomic and polyatomic ions.

Monatomic Ions- Ions which contain only one atom are called monatomic ions. Examples: Na⁺ and K⁺ 

Polyatomic Ions- Ions which contain more than one atom are called polyatomic ions. Examples: NO₃^– and CO₃^2- 

Difference between Atoms and Ions

1. Atoms are electrically neutral, which means the number of protons and electrons will be the same. While ions are charged particles, positive ions (cations) or negative ions (anions).

2. Combination of atoms results in the formation of molecules and the combination of ions results in the formation of compounds.

3. Independent existence of atoms is not possible. But, ions can exist independently.

4. Ions are stable whereas atoms are unstable.

5. Depending upon the valency, atoms can form a covalent bond or ionic bond. But ions can form only an ionic bond.

6. Atoms can take part in chemical reactions but ions cannot take part in chemical reactions. Ions can only be a part of ionic reactions.

7. Ions can have a complete octet. Hence they are stable. Atoms do not have a complete octet. Hence, they are unstable.