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The atom’s mass is densely concentrated at the centre; this forms the nucleus of an atom. By scattering α particles at the centre of an atom, the radius of a nucleus is determined. It is concluded that the radius of the nucleus was 104 times smaller than the radius of the atom itself. A particular nucleus in an atom defined by its mass number and atomic number is known as a nuclide of the atom. In the structure of an atom, it is the nucleus that contains almost all the atom’s mass.
Nucleus and nuclide
In the structure of an atom, the nucleus is the dense and tiny region that consists of positively charged particles known as protons. The nucleus of an atom also contains neutrally charged particles known as neutrons. And outside the nucleus, the structure of an atom is largely hollow, with only revolving negatively charged electrons.
But, what is Nuclide? The nuclide definition is given as that particular nucleus of an atom associated with a particular atomic number, and a particular atomic mass and energy of the atom.
Composition of nucleus
The nucleus and nuclide of an atom are made of small, densely concentrated particles; the main particles found in the nucleus of all atoms are protons and neutrons. The protons are positively charged in the nucleus, and the neutrons are neutral. The mass of a nucleus is the sum of the mass of protons and neutrons in the nucleus.
The electrons are particles in the atoms; unlike protons and neutrons, electrons are not situated in the centre of an atom; instead, the electrons revolve around the nucleus, the electron particles also carry a negative electric charge.
The mass of a positively charged single proton is 1.6726 X 10-24 gm, the mass of a negatively charged single electron is 9.108 X 10-28 gm, and a single neutral neutron has a mass of 1.675 X 10-24 gm.
Atomic mass unit (u)
An atom has long been considered the smallest unit of matter. Because of this, it is difficult to measure its mass in standard measuring units, like kilograms or grams; for instance, the mass of a Carbon atom (12C) in kilograms is around 1.99265 X 10-26 kg. The mass of a single carbon atom is so small that studying it in standard measurement units makes little sense; therefore, a new unit is developed to express such values.
To conveniently express the atomic mass, a new unit was created. It is called the atomic mass unit and is represented by (u). The atomic mass unit is 1/12th of the carbon twelve (12C) atom.
Atomic number
An atomic number of an element is given by the total number of protons present in the atom’s nucleus; it is represented by Z in the calculations. The atomic number of an element is written in the subscripts of the atomic symbol—for example, 6C, 8O, and 1H.
Atomic mass
The atomic masses of various elements are expressed in the atomic mass unit (u). The atomic mass of the elements is mostly close to the multiples of the mass of a single hydrogen atom, i.e., 1H. However, there are some exceptions to this statement—for example, Chlorine.
Isotopes and isobars
There are several elements whose atoms, while sharing the same chemical properties in the same physical conditions, possess a different atomic mass; these are called isotopes.
In simpler words, certain atoms in an element exhibit the same chemical properties in normal conditions but do not share the same mass with other atoms; these atoms are called isotopes of the elements.
Many atoms have isotopes; for example, Carbon has at least three isotopes, namely, Carbon-12, Carbon-13, and Carbon-14. Similarly, Uranium-234, Uranium-235, and Uranium-238 have known isotopes of Uranium.
On the other hand, atoms of the same elements also express the same atomic mass but a different atomic number; these are called isobars. The isobars are Carbon-14 and Nitrogen, as they both share the same atomic mass, i.e., 14 but different atomic numbers, i.e., 6 and 7, respectively.
Size of the nucleus
Scattering of α particles at the nucleus of different elements has been one of the most successful experiments in discovering the properties and compositions of the atoms. Similarly, scattering fast-moving electrons at the nucleus of different elements provides accurate measurements for the size and shape of the nucleus and nuclide.
The expression to determine the radius of a nuclide with mass number A and radius R is given by,
R = R0A1/3
Where, R0 = 1.2 X 10-15 m, and A = mass number.
Conclusion
The nucleus of an atom is a tiny, positively charged area that consists of almost all the atom’s mass. The nuclide definition is given as that particular nucleus of an atom associated with a particular atomic number, and a particular atomic mass and energy of the atom.
The total number of protons in the nuclide defines the atom’s atomic number, and the total number of protons and neutrons defines the atom’s mass number.
