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Difference Between Atom and Ion for JEE Main 2024

Last updated date: 19th Jun 2024
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Features of Atom and Ion

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

  • Isotones- If atoms of different elements have the same number of neutrons, they are known as isotones. 6C14 and 8O16 are examples of isotones.

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

Number of neutrons in oxygen is 16–8 = 8


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 (Cations)

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 (Anions)

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 CO32- 

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.

FAQs on Difference Between Atom and Ion for JEE Main 2024

1. What is the empirical formula of compound?

The empirical formula is the simplest formula for a compound. The empirical formula is also known as the simplest formula of a given compound. An empirical formula for a given compound is the formula of a substance noted with the smallest possible integer subscript. The empirical formula provides us with information about the ratio of the numbers of atoms in the given compound. The percentage composition of a given compound directly relates to its empirical formula. Students can learn more about the empirical formula and other topics from the Chapter at Vedantu's official website and Youtube Channel.

2. What are some properties of an ionic bond between two molecules?

There exists a strong force of attraction between the cations and anions in molecules bonded by ionic bonds. Some of the properties they show are-

  • These bonds are the strongest of all bonds. 

  • These bonds have a charge separation, which makes them the most reactive in their medium.

  • The ionic bonded molecules generally have high melting and boiling points. 

  • They are also good conductors of electricity because they contain ions that act as charge carriers. This has led to their widespread use in a wide range of industries, including medicine, electronics and construction.

3. What is a molecule?

A molecule of a given substance is the smallest unit of a compound. Molecules are made up of several groups of atoms. Atoms consist of smaller units: electrons, protons, and neutrons. Protons and neutrons are contained in the nucleus of an atom, and electrons revolve around it. Protons are positively charged subatomic particles while electrons are negatively charged subatomic particles. Neutrons do not carry any charge. The nucleus of a given atom is positively charged because of the presence of protons. Atoms are mostly vacant. Every element has a specific atomic number. The atomic number of an element is the total number of protons present in its nucleus. Atomic numbers are positive integers. They are denoted by Z. When discussing the mass of an atom, the mass of its particles is taken into account. Electrons have negligible mass, so only the proton and neutron masses are added together to calculate the total atomic mass. The number of protons in a neutral atom’s nucleus is called its “atomic number” and is denoted by Z. A molecule is the smallest unit of a given compound that has the same characteristics. For example, all water molecules are made up of two atoms of hydrogen (H) and one atom of oxygen (O), but they can be broken down into atoms or parts of atoms.

4. How were neutrons discovered?

Neutrons are neutral subatomic particles. In 1932, James Chadwick, a scientist, fired alpha radiation at a beryllium sheet from some polonium source and this experiment led to the production of uncharged and penetrating radiations. This radiation was created by paraffin wax which is a hydrocarbon having a relatively higher hydrogen content. In 1927, Carl D. Anderson discovered the positron through experiments on cosmic rays. In his experiment, he fired positive protons at a target of paraffin wax, which emitted both protons and electrons when hit. He then used an ionization chamber to observe the protons that were not deflected by the electric field. Radiation and the interaction of radiation and the atoms of several gases were studied carefully by Chadwick. In 1934, James Chadwick proved that the unusual penetrating radiation that was emitted from radioactive elements consisted of uncharged particles with approximately the same mass as a proton. These were later called “neutrons''. This is how neutrons were discovered.

5. How were protons discovered?

Protons are positively charged subatomic particles. Ernest Rutherford, a scientist,  observed that his scintillation detectors detected hydrogen nuclei when a beam of alpha particles was shot into the air. This showed that most of the mass of an atom is in its nucleus, with electrons being very light. Rutherford investigated further and discovered that the hydrogen nuclei were produced from nitrogen atoms present in the atmosphere. Deuterium was discovered in the 1930s by Harold Urey at the University of California, Berkeley and Samuel Ruben from Columbia University. They observed that deuterium is present in natural water but not free gas or free air.  It was concluded that the nuclei of Hydrogen originated from the nitrogen atom, proving the fact that the hydrogen nucleus was a part of all other atoms. Neils Bohr discovered that the hydrogen nuclei originate from the nitrogen atom, proving that all atoms have a nucleus. In the 1930s, two German chemists named Otto Hahn and Fritz Strassman experimented to study how neutrons are formed. They found that bombarding a sample of nitrogen with protons created oxygen and a new element, which they initially called “barium”. This particular experiment was the 1st to report a nuclear reaction. It was given by the equation: 14N + α → 17O + p. The hydrogen nucleus was then named the proton and recognized as one of the building blocks of an atomic nucleus. This is how protons were discovered.