Atomic Nucleus

The nucleus of an atom is a small, dense, round region located in the centre of an atom. It has two subatomic particles, protons and neutrons. Protons are electrically positively charged ions having a mass of approximately one atomic mass unit (amu). Neutrons are electrically neutral.

The atomic nucleus is bonded together by the strong force (also known as nuclear force), the most powerful known fundamental force in nature. The atomic nuclei have more than 99.9% of the atomic mass and are 10,000 times smaller. In 1911, Ernest Rutherford discovered the nucleus of an atom.

Before learning about the Nucleus we first need to learn about the Structure of atoms in brief so let’s start with that.

Structure of Atom

The structure of atom is a complex arrangement of negatively charged electrons organized in defined shells around a positively charged nucleus. The image of the electrons is shown below showing positively charged nuclei and negatively charged electrons revolving about the nucleus in their fixed orbit.

This nucleus, which is made up of protons and neutrons, contains the majority of the atom’s mass (except for common hydrogen which has only one proton). Every atom is roughly the same size. The Angstrom (Å), defined as 1 x 10^-10 m, is a useful unit of length for measuring atomic sizes. An atom has a diameter of about 2-3 Å.

Protons

A proton is a stable subatomic particle with a positive electric charge. Protons reside in the nucleus of every atom, determining its atomic number. This number also dictates the count of atomic electrons and the element’s chemical properties.

Comparatively, protons are slightly less massive than neutrons but 1,836 times more massive than electrons. Protons and neutrons together form nucleons.

The term “proton” originates from the Greek word meaning “first”. Ernest Rutherford coined the term “proton” for the hydrogen nucleus in 1920.

Positively charged, protons are particles of matter used in radiation treatment. Specially designed equipment can generate streams of protons for this purpose.

Neutrons

Neutrons are subatomic particles present in the nucleus of every atom, except hydrogen. Electrically neutral, they carry no charge and have a mass slightly greater than a proton.

When bound in an atomic nucleus, neutrons remain stable. However, as free particles, they have an average lifetime of approximately 1,000 seconds.

Neutrons, alongside protons and electrons, constitute the trio of subatomic particles within an atom. While protons bear a positive charge and electrons a negative one, neutrons maintain neutrality.

Isotopes refer to atoms of the same chemical element differing only in neutron number. For instance, carbon-12 contains 6 neutrons, whereas carbon-13 contains 7.

James Chadwick discovered neutrons in 1932.

Electrons

Electrons are negatively charged subatomic particles and belong to the first generation of the lepton particle family. They are considered elementary particles, lacking any known substructure or components.

Electrons can exist either bound to an atom or in a free state. When bound, they constitute one of the three primary particle types within the atom, alongside protons and neutrons, forming the atom’s nucleus.

Compared to other components of an atom, electrons are extremely small. While the charge of an electron equals that of a proton in magnitude, it bears the opposite sign. Therefore, electrically neutral atoms and molecules possess an equal count of electrons and protons.

Due to opposite charges, electrons are attracted to protons. Collectively, the electrons in an atom create a negative charge that offsets the positive charge of the protons in the atomic nucleus.

Joseph John Thomson discovered the electron in 1897.

Nucleus Discovery

The development of atomic theory underwent significant advancements through the pioneering experiments of scientists such as Thomson and Rutherford, leading to groundbreaking insights into the structure of the atom.

Thomson’s Experiments with Cathode Ray Tubes

• Thomson began experimenting with cathode ray tubes, which are vacuum-sealed glass tubes with most of the air removed.
• A high voltage was applied across two electrodes at one end of the tube, causing a beam of particles to flow from the negatively charged cathode to the positively charged anode.
• Thomson used oppositely charged electric plates to surround the cathode ray, redirecting it from the negatively charged to the positively charged electric plate.
• This led to the conclusion that cathode rays are composed of negatively charged particles, later named electrons.
• Scientists gradually accepted Thomson’s discoveries about cathode rays.
• This gave us Thomson’s Atomic Model.

Rutherford’s Gold Foil Experiment and Atomic Model

• Rutherford’s Atomic Model demonstrated that the atom is mostly empty space with a tiny, densely packed, positively charged nucleus.
• Rutherford conducted the Gold Foil Experiment, where he fired an alpha beam at a thin sheet of gold foil.
• The majority of the alpha particles passed through the foil without being deflected significantly.
• However, a small number of particles were slightly deflected, and an even smaller fraction was deflected more than 90 degrees from their original path.
• Based on these observations, Rutherford proposed the nuclear model of the atom, suggesting that atoms consist of a small, positively charged nucleus surrounded by negatively charged electrons.
• His experiment led to the calculation that the nucleus occupies only a tiny fraction of the volume of the atom.

Composition of a Nucleus

An atom’s nucleus is made up of a densely packed arrangement of nucleons namely protons and neutrons. Because these are the two heavy particles in an atom, 99.9% of the mass is concentrated in the nucleus. Because protons have a net positive charge, the nucleus of an atom is positively charged overall, while negatively charged electrons revolve around the central nucleus. Because the mass concentration at an atom’s nucleus is enormous, the nuclear forces that hold protons and neutrons together are also enormous. 

Since, the protons are so close to each other inside the tiny nucleus, electrostatic forces of repulsion also act inside the nucleus but it is very weak as compared to nuclear force of attraction that we neglect the same. Because the total number of protons in a nucleus equals the total number of electrons revolving around the nucleus, the atom as a whole is electrically neutral.

Characteristics of Nucleus

Various characteristics of the Nucleus are:

• Nucleus is located in the centre of the atom and contains protons and neutrons.
• Because of the presence of positively charged protons, the nucleus determines the overall charge on the atom.
• It is also in charge of the chemical properties of the element, such as the valency of the atom, the element’s reactivity to other elements, and the atomic number and mass number of the atom.
• While the neutron is electrically neutral, it contributes to the atom’s mass.
• The numeric sum of protons and neutrons is the atomic mass. The mass of an atom determines physical properties such as melting point, boiling point, density, and so on.
• The nucleus remains stationary at the centre of the atom, while the electron revolves around it due to the attraction between the positively charged nucleus and the negatively charged electron.
• The electron spins on its axis as it moves around the nucleus. An electron’s mass is considered negligible in comparison to the nucleus, but it is responsible for the valency of the atom and determines the atom’s ability to participate in any chemical reaction.
• Protons and Neutrons are combinedly called the Nucleons as they reside inside the Neutrons.

Mass of a Nucleus

Nucleus contains almost all of an atom’s mass, with only a minor contribution from the electron cloud. The mass of an atom is related with its atomic mass number, which is the total number of protons and neutrons in its nucleus. An isotopes of an element have same atomic number but distinct mass number. The mass number is written after the element name or as a superscript to the left of an element’s symbol. Carbon-12, or ¹²C, is the most common carbon isotope.

Atoms are so small in size and mass that using standard measuring units, while possible, is frequently inconvenient. On atomic scale, units of mass and energy have been defined to make measurements easier to express. The atomic mass unit is the unit of mass measurement (amu).

1 amu = 1.66 x 10^-24 Grams

Size of Nucleus

The nucleus of an atom is significantly smaller than the atom itself. Its radius spans from 1^–10 × 10^–15 meters, while the atom’s radius is typically 10^–10 meters, making the nucleus occupy a tiny volume within the atom.

Comprised of nucleons—protons and neutrons—the nucleus holds most of the atom’s mass since these particles are much heavier than electrons. However, electrons occupy almost the entire volume of the atom.

The diameter of the nucleus varies from 1.70 fm (1.70×10^–15 m) for hydrogen to about 11.7 FM (femtometre) for uranium.

Atomic Number

The atomic number of an element is the number of protons present in the nucleus of the atom of that element. It’s also known as the nuclear charge number or proton number. Atomic number is represented by the letter Z.

The atomic number equals the number of electrons in orbit around the nucleus in a nonionized atom. Therefore, atoms with the same atomic number belong to the same element. For instance, an element with 6 protons is carbon.

Atomic Mass

Atomic mass refers to the combined mass of all particles within an atom, including protons, neutrons, and electrons. However, electrons contribute so little mass that they’re usually disregarded in calculations. Atomic Mass is also known as mass number, atomic mass unit (AMU) or dalton (Da).

Atomic Mass is represented by the letter A.

Expressed in daltons, a non-SI unit, atomic mass derives its standard from carbon-12, where one dalton equals 1/12 of its mass at rest in ground state. It represents the average mass of all an element’s atoms, considering all its isotopes.cClosely linked to an atom’s mass number, atomic mass denotes the total mass of a single atom, typically measured in atomic mass units (amu). For instance, carbon-12, with six neutrons, has an atomic mass of 12 amu.

Practice Problems on Nucleus of Atom

Q1. Describe the composition of an atomic nucleus and explain its significance in determining the properties of an atom.

Q2. Illustrate why the nucleus is considered the central core of an atom and discuss the relative masses of its constituent particles, protons, and neutrons.

Q3. Explain the difference between the radius of an atom and the radius of its nucleus. Provide numerical values for each.

Q4. Why does the nucleus contain most of the atom’s mass, even though electrons take up a significant volume within the atom?

Q5. Define the term “nucleon” and identify the two types of nucleons found in an atomic nucleus.

FAQs on Nucleus of Atom

What is an Atom?

An atom is the smallest particle with which all the matter in the universe is made up of. It is believed that an atom can not be further broken down but recent studies have shown that Atoms can be further divided into smaller parts.

What is Atomic Nucleus?

A Nucleus is the part of the atom that contains all its mass it is at the centre of the atom and occupies very very tiny space of atom but contain 99.99% mass of atom. It contains all the protons and neutrons of the atom and electrons revolve around it in an fixed orbit.

Whom Discovered the Atomic Nucleus?

Atomic Nucleus was discovered by Rutherford.

What does the Atomic Nucleus Contain?

The atomic nucleus contains protons and neutrons

What is the Density of Atomic Nucleus?

The average density of Atomic Nucleus is 2.5 × 10¹⁷ kg/m³

What is the Radius of Atomic Nucleus?

The radius of atomic nucleus is in the order of 10^-13

What are Subatomic Particles?

Subatomic particles are the particles that make up an atom. As the name suggest they are smaller than Atom. The three subatomic particle are,

• Electrons
• Protons
• Neutrons

What are Quarks?

The subatomic particles of Protons and Neutrons are further made up of even smaller particles called the Quarks. There are six different kinds of quarks.

What are Isotopes?

Atoms having same atomic number but different atomic mass are called Isotopes. Example C-12 and C-14 are isotopes both have atomic number of 6 but mass number or atomic mass of C-12 is 12 and C-14 is 14.

What are Isobars?

Atoms having same atomic mass but different atomic numbers are called Isobars. Example ₁₈Ar⁴⁰ and ₂₀Ca⁴⁰ both are isobars both have same atomic mass of 40 but different atomic number. Atomic number of Ar is 18 and of Ca is 20.