Atoms and Nuclei

Everything that has mass and occupies space is matter. Atoms and Nuclei are the fundamental building blocks of matter. Learn in detail about Atoms and Nuclei

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Atoms and nuclei are the fundamental building blocks of matter. Atoms are defined as matter composed of subatomic particles such as protons, neutrons, and electrons. Whereas, Nuclei are the central region of an atom that contains positively-charged protons and neutral neutrons which occupy most of the atom’s volume, except for a small space between the nucleus and an electron cloud called the orbital zone or nucleus. In this ‘Atoms and nuclei study material’, we will be covering everything that you need to know about atoms and nuclei in detail, from their structure to the composition.

The Structure of an Atom

The structure of an atom refers to the constitution of its nucleus and the way electrons are arranged around the nucleus. The nucleus of the atom is made up of protons and neutrons, which are surrounded by the atom’s electrons. The total number of protons in an element’s nucleus is described by its atomic number.

Subatomic particles:

Protons
Neutrons
Electrons

Different elements have different numbers of protons and electrons. As a result, their atomic structures differ. This is why various elements have distinct qualities.

Atomic Models

Many scientists attempted to describe the structure of the atom using atomic models in the 18th and 19th centuries. Each of these models had its own set of advantages and disadvantages, and they were all important in the creation of the present atomic model. Scientists including John Dalton, J.J. Thomson, Ernest Rutherford, and Niels Bohr made significant contributions to the discipline. These atoms and nuclei class 12 notes delve into their theories about the structure of the atom.

Model	Postulates	Limitations
Dalton’s Atomic Model	All matter is composed of atoms that are indivisible. Particular elements have only particular types of atoms in them Each atom bears a constant mass that differs from element to element Atoms rearrange themselves during a chemical reaction Atoms can neither be created nor be destroyed	This theory fails to explain the existence of isotopes The structure of the atom was not appropriately explained The claim that atoms are indivisible was later scrapped when subatomic particles were discovered.
Thomson’s Atomic Model	He described an atom as a positively shaped sphere with negatively charged electrons embedded into it. Commonly referred to as the ‘plum pudding model’ Described atoms as electrically neutral	This model fails to explain the stability of an atom. Thomson’s atomic model couldn’t support the further discoveries of subatomic particles
Rutherford’s Atomic Model	Most of the charge and mass are concentrated at the centre of the atom, which is called the nucleus The structure of an atom is spherical Electrons tend to revolve around the nucleus in a circular orbit	Stability of atom is not explained Electrons’ continuous revolution around the nucleus should result in a continuous spectrum, instead, we see a linear spectrum
Bohr’s Atomic Model	Electrons are placed in discrete orbits called ‘stationary orbits’ The energy levels of these shells are called ‘quantum numbers’ Electrons have a tendency to move to a higher or lower level by either absorbing or emitting energy When an electron is at rest in its own stationary orbit, there is no absorption or emission of energy	This model works only for single-electron elements A combination of smaller discrete lines were observed when the emission spectrum of hydrogen was seen under a more accurate spectrometer. Doesn’t explain Stark or Zeeman effects

Structure of Nuclei

The nucleus of an atom is a tightly packed structure of protons and neutrons. Protons and neutrons present in the nucleus are the heaviest particles in an atom and, hence, the nucleus makes up for 99.9% of the mass of the atom.

The Radius of a Nucleus

‘R’ represents the radius of the nucleus.

R=R0A1/3

Where,

Ro is the proportionality constant
A is the mass number of the element

Total Number of Protons and Neutrons in a Nucleus

The mass number (A), also known as the nucleon number, is the total number of neutrons and protons in a nucleus.

A = Z + N

Where,

N is the neutron number
A is the mass number
Z is the proton number

Nature of Nuclear Force

Just like gravitational force binds masses together, the nuclear forces bind the charges together. It is the force present between the protons and neutrons of atoms. The nuclear force is stronger in comparison to the Coulomb force. The nature of nuclear forces in atoms and nuclei are given as:

Nuclear forces are attractive
Independent of charges
They have a short-range
Lesser the distance between two nucleons, weaker the nuclear force
Dependent on the spin

Radioactivity

Radioactivity is a process that occurs as a result of the decay of the nucleus. In this process, the nucleus of the atom loses energy due to emission radiation. It is derived from the law of conservation of charge. The rate of decay is directly proportional to the number of atoms present at that instant.

Types of Radioactive Decay:

Alpha α
Beta β
Gamma γ

Conclusion

Atoms and nuclei are the fundamental building blocks of matter. Atoms are defined as matter composed of subatomic particles such as protons, neutrons, and electrons. Many scientists including John Dalton, J.J. Thomson, Ernest Rutherford, and Niels Bohr attempted to describe the structure of the atom using atomic models. The nucleus of an atom is a tightly packed structure of protons and neutrons. Protons and neutrons present in the nucleus