What is an Atom in Chemistry | Definition, Examples

Definition of Atom

An atom is defined as the smallest unit that retains the properties of an element. An atom is composed of atoms and these cannot be made or destroyed. All atoms of the same element are identical and different elements have different types of atoms. Chemical reactions occur when atoms are rearranged.
what is an atom
It is the smallest constituent unit of matter that possess the properties of the chemical element. Atoms don’t exist independently, instead. They form ions and molecules which further combine in large numbers to form matter that we see, feel and touch. Atoms are much too small to be seen; hence experiments to find out their structure and behavior have to be conducted with large numbers of them. From the results of these experiments we may attempt to construct a hypothetical model of an atom that behaves like the true atom.

The smallest particle of an element, which may or may not have an independent existence. But always takes place in a chemical reaction is called an atom.

Atoms consist of three fundamental types of particles, protons, electrons and neutrons. Neutrons and protons have approximately the same mass and in contrast to this the mass of an electron is negligible.

A proton carries a positive charge, a neutron has no charge and an electron is negatively charged. An atom contains equal numbers of protons and electrons and therefore overall an atom has no charge.

The nucleus of an atom contains protons and neutrons only, and therefore is positively charged. The electrons occupy the region of space around the nucleus. Therefore, most of the mass is concentrated within the nucleus.

The center of the atom is called the nucleus. The nucleus contains neutrons and protons that give an atom its weight and positive charges. A neutron carries no charge and has a mass of one unit.

A proton carries a single positive charge and also has a mass of one unit, The atomic number of an
element is equal to the number of protons or positive charges in the nucleus.

The atomic weight of an element is determined by combining the total number of protons and neutrons in the nucleus. An electron carries a single negative charge.

If an atom of an element is to have zero charge, it must have the same number of electrons as protons. These electrons are arranged in orbits around the nucleus of the atom like the layers of an anion.

Most of the atom is empty space. The rest consists of a positively charged nucleus of protons and neutrons surrounded by a cloud of negatively charged electrons.

The nucleus is small and dense compared with the electrons, which are the lightest charged particles in nature. Electrons are attracted to any positive charge by their electric force. In an atom, electric forces bind the electrons to the nucleus.

The size of an atom is extremely small, much smaller than our imagination. A layer of an atom as thick as a thin sheet of paper is formed when more than millions of atoms are stacked together. It is impossible to measure the size of an isolated atom because it’s difficult to locate the positions of electrons surrounding the nucleus. However, the size of an atom can be estimated by assuming that the distance between adjacent atoms is equal to half the radius of an atom. Atomic radius is generally measured in nano meters.

1 m = [latex]10^{9}nm[/latex]

An atom is composed of three particles, namely, neutrons, protons and electrons with hydrogen as an exception without neutrons.

  • Every atom has a nucleus that bounds one or more electrons around it.
  • The nucleus has typically a similar number of protons and neutrons which are together known as nucleons.
  • The protons are positively charged, electrons are negatively charged and neutrons are neutral.

Atomic Theory

The smallest piece of an element that maintains the identity of that element is called an atom. Individual atoms are extremely small. It would take about fifty million atoms in a row to make a line that is 1 cm long. The period at the end of a printed sentence has several million atoms in it. Atoms are so small that it is difficult to believe that all matter is made from atoms—but it is.

John Dalton’s Atomic Theory

john dalton atomic model

The English chemist John Dalton suggested that all matter is made up of atoms, which were indivisible and indestructible. He also stated that all the atoms of an element were exactly the same, but the atoms of different elements differ in size and mass.

Chemical reactions, according to Dalton’s atomic theory, involve a rearrangement of atoms to form products. According to the postulates proposed by Dalton. The atomic structure comprised atoms, the smallest particle responsible for the chemical reactions to occur.

The following are the postulates of his theory :

  • Every matter is made up of atoms.
  • Atoms are indivisible.
  • Specific elements have only one type of atoms in them.
  • Each atom has its own constant mass that varies from element to element.
  • Atoms undergo rearrangement during a chemical reaction.
  • Atoms can neither be created nor be destroyed but can be transformed from one form to another.

Dalton’s atomic theory successfully explained the Laws of chemical reactions. Namely, the Law of conservation of mass, Law of constant properties, Law of multiple proportions and Law of reciprocal proportions.

Demerits of Dalton’s Atomic Theory –

  • The theory was unable to explain the existence of isotopes.
  • Nothing about the structure of atom was appropriately explained.
  • Later, the scientists discovered particles inside the atom that proved, the atoms are divisible.

The discovery of particles inside atoms led to a better understanding of chemical species. These particles inside the atoms are called subatomic particles. The discovery of various subatomic particles is as follows :

Thomson Atomic Model

jj thomson atomic model

The English chemist Sir Joseph John Thomson put forth his model describing the atomic structure in the early 1900s. He was later awarded the Nobel prize for the discovery of “electrons”. His work is based on an experiment called cathode ray experiment.

The construction of working of the experiment is as follows :

Cathode Ray Experiment

It has a tube made of glass which has two openings, one for the vacuum pump and the other for the inlet through which a gas is pumped in. The role of the vacuum pump is to maintain “partial vacuum” inside the glass chamber. A high voltage power supply is connected using electrodes i.e. cathode and Anode is fitted inside the glass tube.

  • When a high voltage power supply is switched on, there were rays emerging from the cathode towards the anode. This was confir med by the ‘Fluorescent spots’ on the ZnS screen used. These rays were called “Cathode Rays”.
  • When an external electric field is applied, the cathode rays get deflected towards the positive electrode. But in the absence of electric field, they travel in a straight line.
  • When rotor Blades are placed in the path of the cathode rays, they seem to rotate. This proves that the cathode rays are made up of particles of a certain mass. So that they have some energy.
  • With all this evidence, Thompson concluded that cathode rays are made of negatively charged particles called “electrons”
  • On applying the electric and magnetic field upon the cathode rays (electrons). Thomson found the charge to mass ratio (e/m) o f electrons. (e/m) for electron: 17588 × 1011 e/bg.

From this ratio, the charge of the electron was found by Mullikan through oil drop experiment.

 [Charge of e = 1.6 × 10-16 C and Mass of e = 9.1093 × 10-31 kg].

Conclusions : Based on conclusions from his cathode ray experiment. Thomson described the atomic structure as a positively charged sphere into which negatively charged electrons were embedded.

It is commonly referred to as the “plum pudding model” because it can be visualized as a plum pudding dish. Where the pudding describes the positively charged atom and the plum pieces describe the electrons.
Thomson’s atomic structure described atoms as electrically neutral, i.e. the positive and the negative charges were of equal magnitude

Limitations of Thomson’s Atomic Structure:

Thomson’s atomic model does not clearly explain the stability of an atom. Also, further discoveries of other subatomic particles, couldn’t be placed inside his atomic model.

Rutherford Atomic Theory

rutherford model of atom

Rutherford, a student of J. J. Thomson modified the atomic structure with the discovery of another subatomic particle called “Nucleus”. His atomic model is based on the Alpha ray scattering experiment.

Alpha Ray Scattering Experiment

  • A very thin gold foil of 1000 atoms thick is taken.
  • Alpha rays (doubly charged Helium He2+) were made to bombard the gold foil.
  • Zn S screen is placed behind the gold foil.
  • Most of the rays just went through the gold foil making scintillations (bright spots) in the ZnS screen.
  • A few rays got reflected after hitting the gold foil.
  • One in 1000 rays got reflected by an angle of 180° (retraced path) after hitting the gold foil.
  • Since most rays passed through, Rutherford concluded that most of the space inside the atom is empty.
  • Few rays got reflected because of the repulsion of its positive with some other positive charge inside the atom.
  • 1/1000th of rays got strongly deflected because of a very strong positive charge in the center of the atom. He called this strong positive charge as “nucleus”.
  • He said most of the charge and mass of the atom resides in the Nucleus.

Rutherford’s Structure of Atom

Based on the above observations and conclusions, Rutherford proposed his own atomic structure which is as follows

  • The nucleus is at the center of an atom, where most of the charge and mass are concentrated.
  • Atomic structure is spherical.
  • Electrons revolve around the nucleus in a circular orbit, similar to the way planets orbit the sun

Limitations of Rutherford Atomic Model

  • If electrons have to revolve around the nucleus, they will spend energy and that too against the strong force of attraction from the nucleus. A lot of energy will be spent by the electrons and eventually. They will lose all their energy and will fall into the nucleus so the stability of atom is not explained.
  • If electrons continuously revolve around the ‘nucleus, the type of spectrum expected is a continuous spectrum. But in reality , what we see is a line spectrum

Subatomic Particles


  • Protons are positively charged subatomic particles. The charge of a proton is 1e, which corresponds to approximately 1.602 × 10-19
  • The mass of a proton is approximately 1.672 × 10-24. Protons are over 1800 times heavier than electrons.
  • The total number of protons in the atoms of an element is always equal to the atomic number of the element.


  • The mass of a neutron is almost the same as that of a proton i.e. 1.674×10-24 .
  • Neutrons are electrically neutral particles and carry no charge.
  • Different isotopes of an element have the same number of protons but vary in the number of neutrons present in their respective nuclei.


  • The charge of an electron is -1e, which approximates to -1.602 × 10-19.
  • The mass of an electron is approximately 9.1 × 10-31.
  • Due to the relatively negligible mass of electrons, they are ignored when calculating the mass of an atom.

Atomic Structure

An atom is the smallest unit of matter that retains all of the chemical properties of an element. Atoms combine to form molecules, which then interact to form solids, gases, or liquids. For example, water is composed of hydrogen and oxygen atoms that have combined to form water molecules.

Many biological processes are devoted to breaking down molecules into their component atoms. So they can be reassembled into a more useful molecule.

Atomic Particles

Atoms consist of three basic particles: protons, electrons, and neutrons. The nucleus (center) of the atom contains the protons (positively charged) and the neutrons (no charge). The outermost regions of the atom are called electron shells and contain the electrons (negatively charged). Atoms have different properties based on the arrangement and number of their basic particles.

The hydrogen atom (H) contains only one proton, one electron, and no neutrons. This can be determined using the atomic number and the mass number of the element.

Structure of an Atom

Elements, such as helium, depicted here, are made up of atoms. Atoms are made up of protons and neutrons located within the nucleus, with electrons in orbitals surrounding the nucleus.

Atomic Mass

Protons and neutrons have approximately the same mass, about 1.67 × 10-24 grams. Scientists define this amount of mass as one atomic mass unit (amu) or one Dalton. Although similar in mass, protons are positively charged, while neutrons have no charge. Therefore, the number of neutrons in an atom
contributes significantly to its mass, but not to its charge.

Electrons are much smaller in mass than protons, weighing only 9.11 × 10-28 grams, or about 1/1800 of an atomic mass unit. Therefore, they do not contribute much to an element’s overall atomic mass. When considering atomic mass, it is customary to ignore the mass of any electrons and calculate the atom’s mass based on the number of protons and neutrons alone.

Electrons contribute greatly to the atom’s charge, as each electron has a negative charge equal to the positive charge of a proton. Scientists define these charges as “+1” and “-1”. In an uncharged, neutral atom, the number of electrons orbiting the nucleus is equal to the number of protons inside the nucleus. In these atoms, the positive and negative charges cancel each other out, leading to an atom with no net charge.

Protons, neutrons, and electrons

Both protons and neutrons have a mass of 1 amu and are found in the nucleus. However, protons have a charge of +1, and neutrons are uncharged. Electrons have a mass of approximately 0 amu, orbit the nucleus, and have a charge of -1.

Exploring Electron Properties

Compare the behavior of electrons to that of other charged particles to discover properties of electrons such as charge and mass.

Volume of Atom

Accounting for the sizes of protons, neutrons, and electrons, most of the volume of an atom—greater than 99 percent—is, in fact, empty space. Despite all this empty space, solid objects do not just pass through one another. The electrons that surround all atoms are negatively charged and cause atoms to repel one another, preventing atoms from occupying the same space. These intermolecular forces prevent you from falling through an object like your chair.

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