Introduction
It is very essential to know the composition of matter to determine both of its physical and chemical properties. Atomic theory is a theory of the nature of matter. The composition of matter is discrete units called atoms, as opposed to the notion that matter could be broken into any arbitrarily small quantity. It began in ancient Greece and India as Philosophical science and the field of chemistry showed that matter indeed behaves as if it is made up of particles.
John Dalton, in 1808 proposed a theory in which he stated that matter consists of very small indivisible particles called atoms. The word "atom" (Greek adjective atomos = uncut, 'indivisible’) was applied to the basic particles. The atomic structure itself was imagined to be the indivisible particle. However, around the 20th century, through various experiments with electromagnetism and radioactivity, physicists discovered that the atomic structure is actually an aggregate of various subatomic particles. Since atoms were found to be divisible, physicists introduced the term "elementary particles" to describe indivisible particles. This field of science was believed to be the basis to discover the true fundamental nature of matter.
The ratio of charge (e) of the electron to its mass (m) was found to be 1.76x 1011 coulomb per kilogram. Since an atomic structure is neutral, it should contain as much positive charge as negative charges carried by all electrons.
The lightest atom known is the hydrogen atom. It contains an electron and a positively charged particle. The positively charged particle obtained by removing the electron from a hydrogen atom was called a proton. It is represented as 1p1.
Mass of a proton is found to be 1.672 x 10-27 kg.
Its charge is +1.602 x 10-19C.
In 1932, James Chadwick discovered a new particle called a ‘neutron’, when he bombarded a thin beryllium foil with alpha (α) particles. The electrons, protons and neutrons are the fundamental particles present in atomic structure.
The number of protons present in the nucleus of an atom is known as atomic number (Z) of the element. However, as the atomic structure is neutral in nature, it should contain an equal number of positive charges and negative charges. Hence, atomic number is also equal to the number of electrons present in the atom of the element.
The sum of the number of protons and neutrons present in the nucleus of the atom is called mass number (A). The protons and neutrons are called ‘nucleons’.
Thus, Z = atomic number = no. of protons.
A = mass number = no. of protons + no. of neutrons
Therefore, A-Z = no. of neutrons.
It is very essential to know the composition of matter to determine both of its physical and chemical properties. Atomic theory is a theory of the nature of matter. The composition of matter is discrete units called atoms, as opposed to the notion that matter could be broken into any arbitrarily small quantity. It began in ancient Greece and India as Philosophical science and the field of chemistry showed that matter indeed behaves as if it is made up of particles.
John Dalton, in 1808 proposed a theory in which he stated that matter consists of very small indivisible particles called atoms. The word "atom" (Greek adjective atomos = uncut, 'indivisible’) was applied to the basic particles. The atomic structure itself was imagined to be the indivisible particle. However, around the 20th century, through various experiments with electromagnetism and radioactivity, physicists discovered that the atomic structure is actually an aggregate of various subatomic particles. Since atoms were found to be divisible, physicists introduced the term "elementary particles" to describe indivisible particles. This field of science was believed to be the basis to discover the true fundamental nature of matter.
Fundamental particles – Constituents of atoms
J J Thomson studied the conduction of electricity by gases at low pressure. A kind of negatively charged particles were found to be emitted by the cathode. These were called ‘cathode rays’. The properties of these particles were identical for all gases. This indicated that these particles existed in all substances. These particles were called ‘electrons’ and are represented as e—.The ratio of charge (e) of the electron to its mass (m) was found to be 1.76x 1011 coulomb per kilogram. Since an atomic structure is neutral, it should contain as much positive charge as negative charges carried by all electrons.
The lightest atom known is the hydrogen atom. It contains an electron and a positively charged particle. The positively charged particle obtained by removing the electron from a hydrogen atom was called a proton. It is represented as 1p1.
Mass of a proton is found to be 1.672 x 10-27 kg.
Its charge is +1.602 x 10-19C.
In 1932, James Chadwick discovered a new particle called a ‘neutron’, when he bombarded a thin beryllium foil with alpha (α) particles. The electrons, protons and neutrons are the fundamental particles present in atomic structure.
| Particle | Mass | Charge Unit = 1.602x10-19C | |
| kg | a.m.u | ||
| Electron | 9.109x10-31 | 0.0005486 | -1 |
| Proton | 1.672x10-27 | 1.007277 | +1 |
| Neutron | 1.675x10-27 | 1.008665 | 0 |
Atomic Number vs Atomic Mass
After the discovery of the neutrons, it has been established that the nucleus contains two types of particles namely protons and neutrons. The protons are responsible for the positive charge of the nucleus.The number of protons present in the nucleus of an atom is known as atomic number (Z) of the element. However, as the atomic structure is neutral in nature, it should contain an equal number of positive charges and negative charges. Hence, atomic number is also equal to the number of electrons present in the atom of the element.
The sum of the number of protons and neutrons present in the nucleus of the atom is called mass number (A). The protons and neutrons are called ‘nucleons’.
Thus, Z = atomic number = no. of protons.
A = mass number = no. of protons + no. of neutrons
Therefore, A-Z = no. of neutrons.
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