What is Nucleus?: Structure, Mass, Isotope, Isobar & Isotone

WHAT IS NUCLEUS?

Scientist Rutherford reached two important conclusions regarding charge and distribution of mass in atoms with his famous alpha particle scattering experiment—

The entire positive charge and almost the entire mass of the atom is concentrated in a very small area of the atom. This is called the nucleus of the atom. Its volume is only about 1/10¹² of the total volume of the atom.
Scattered throughout the rest of the atom are negatively charged electrons, whose mass is negligible compared to the total mass of the atom. Except for these electrons, the rest of that part is empty space.

STRUCTURE OF NUCLEUS

CONSTITUENTS OF THE NUCLEUS

(I) PROTON: Proton is an elementary particle. Its charge is positive; The value of that charge is equal to the negative charge of the electron. That is, proton charge, +e = +4.8 X 10^-10 esu = +1.6 X 10^-19 C. On the other hand, the mass of proton is about 1836 times the mass of electron, i.e. mass of proton,

m_p = 9.11 X 10^-31 X 1836 = 1.673 X 10^-27 kg & equivalent energy of m_p = 938.8 MeV.

(II) NEUTRON: Neutron is also an elementary particle. It has no charge. That is, it is a neutral particle. Then the mass of the electron is about 1839 times, that is, slightly more than the mass of the proton. i.e. mass of neutron,

m_n = 9.11 X 10^-31 X 1839 = 1.675 X 10^-27 kg

and equivalent energy of m_p = 939.6 MeV.

(III) NUCLEAR FORCE: Protons and neutrons in the nucleus are bound together by strong attractive forces. One type of interaction gives rise to this force acting within the nucleus. This interaction is called the strong interaction, the nuclear interaction, and the acting force is called the nuclear force. The nature or intensity of this force cannot be explained by Newton’s law of gravitation or Coulomb’s law of constant-current attraction-repulsion.

Element	Number of Proton	Number of Neutron	Number of electrons outside the nucleus
Hydrogen	1	0	1
Helium	2	2	2
Carbon	6	6	6
Nitrogen	7	7	7
Oxygen	8	8	8
Sodium	11	12	11
Lead	82	126	82
Uranium	92	146	92

ATOMIC MASS & NUCLEAR MASS

UNIFIED ATOMIC MASS UNIT

Atomic masses are so small that grams are not usually expressed in kilograms. That is why the special unit used is the unified atomic mass unit. u of this unit. This unit is sometimes referred to as the dalton (Da). Earlier this unit was known as atomic mass unit (amu) and was denoted based on the mass of hydrogen atom or oxygen atom. Nowadays this unit is indicated based on the mass of carbon atoms rather than hydrogen or oxygen atoms.

Definition: 1/12 of the mass of a carbon-12 atom is called 1 unified atomic mass unit.

We know, mass of 1 g atom of carbon = 12 g and number of atoms in 1 g atom = Avogadro’s number = 6.023 X 10²³.

So, Mass of 1 carbon atom = 12/6.023 X 10²³ g

By definition, 1 u = 1/12 X Mass of carbon atom = 1/12 X 12/6.023 X 10²³ g = 1.66 X 10^-24 g

ATOMIC MASS: RELATIVE ATOMIC MASS

The mass of one atom of an isotope of an element is called the atomic mass of that isotope.

Again, the average mass of an atom of an element is calculated by considering the ratio in which the isotopes of an element are found in the Earth’s interior and in the atmosphere. This average mass is called the relative atomic mass of the element. It is also sometimes called atomic weight, although this physical quantity is actually mass, not weight. Relative atomic mass is sometimes referred to as average atomic mass.

Atomic mass or relative atomic mass is usually expressed in unit atomic mass units (in u).

Today it is possible to measure atomic mass or relative atomic mass very precisely with the help of advanced spectroscopic instruments.

Element	Isotope	Relative atomic mass	Atomic mass
Hydrogen	H1	1.00794	1.007825
Carbon	C12	12.0107	12.000000
Oxygen	O16	15.9994	15.994915
Argon	Ar40	39.948	39.962383
Helium	He4	4.002602	4.002603
Nitrogen	N14	14.0067	14.003074
Aluminum	Al27	26.9815386	26.981539
Uranium	U238	238.02891	238.050788

NUCLEAR MASS

The mass of the nucleus of an atom is obtained by subtracting the mass of the electrons in that atom from its mass. During this subtraction the mass of an electron, me = 0.00055u is taken. However, atomic mass and nuclear mass are assumed to be the same unless very precise calculations are required.

ISOTOPE

If atoms of the same element have more than one mass number, they are called isotopes of each other. Two atoms being isotopes of each other means that they are atoms of the same element. Hence their chemical religion is identical. That is why they are placed in the same place in the periodic table of elements. This is where the word ‘homogeneous’ comes from. Different isotopes are formed due to differences in the number of neutrons in the nucleus of an atom. Obviously, if there are multiple isotopes of the same element, each has the same number of protons. Notably, there are other isotopes of carbon besides C¹². It is therefore very important to use the term ‘carbon-12 atom’ and not just ‘carbon atom’ in the definition of the unified atomic mass unit.

HEAVY WATER

The isotope of hydrogen with mass number 2 is called heavy hydrogen or deuterium. Its symbol is H², but it is sometimes expressed simply by the symbol D. Its nucleus consists of a proton and a neutron. Just as normal hydrogen chemically combines with oxygen to form water molecules (H₂O), heavy hydrogen can also form similar molecules (D₂O). This is called deuterium oxide or heavy water. Just as normal hydrogen and heavy hydrogen have the same chemical religion, so do ordinary water and heavy water have the same chemical religion. But in physical religion there are some important differences between them. The relative importance of normal water is 1, but the relative importance of heavy water is 1.1. The boiling point of normal water is 100° C; 101.42° C of heavy water, 5000 parts of normal water contain a part heavy water. Heavy water is used as a ‘moderator’ in nuclear reactors.

ISOBAR

Atoms having equal mass numbers are called isobars of each other. Although their mass number is the same, both the number of protons and the number of neutrons are different.

Example: C¹⁴ and N¹⁴ are isobars of each other. C¹⁴ has proton number 6 and Newton number 8; On the other hand, N¹⁴ has both proton number and neutron number of 7.

ISOTONE

Atoms having the same number of neutrons are called isotones. They have the same number of neutrons but different number of protons and mass numbers.

Example: C¹⁴ and O¹⁶ are isotones of each other. C¹⁴ has 8 neutrons, 6 protons and 14 mass; On the other hand, O¹⁶ has 8 neutrons, 8 protons and 16 mass.

ATOMIC NUMBER

It has been observed that an atom will behave like an element, determined by the number of protons present in the nucleus of that atom. For example, an atom with 6 protons is a carbon atom, and an atom with 8 protons is an oxygen atom. The number of neutrons in the nucleus or the number of electrons orbiting outside the nucleus never indicates the element. Changing the number of neutrons creates isotopes of the same element, while changing the number of electrons creates ions of the same element. On the other hand, changing the number of protons changes the element. For example, the number of protons in each of the oxygen atoms in nature must be 8, but there is no fixed number of electrons or neutrons.

As we know, the elements are arranged in the periodic table according to the change of chemical nature of the elements starting with hydrogen as the first element. The change in the number of protons in the atom corresponds completely with the change in the chemical nature of the element. From this the definition of the atomic number of the first element is indicated.

Definition: The periodic number of an element in the periodic table, arranged in order of chemical properties starting with hydrogen as the first element, is called the atomic number of that element.

The following definition of atomic number is widely accepted today.

Definition: The number of protons in the nucleus of an atom of an element is called the atomic number of that element.

Since the number of protons and electrons in neutral atoms are equal, the number of electrons in an atom can also be called the atomic number of that atom.

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