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.
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Chapter 4: Structure of the Atom
Charged Particles in MatterCan we deduce from these actions that rubbing two items together causes them to become electrically charged? The presence of charged particles in an atom was discovered thanks to the efforts of many scientists. By 1900, it was established that the atom was an indivisible particle that contained at l
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What is Atom?Atoms are tiny particles that comprise all the things in the known universe. Atoms of an element are responsible for all chemical reactions occurring in nature. We know that atoms are made up of three fundamental particles namely, ElectronsProtonsNeutronsThese particles are also called subatomic par
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Thomson's Atomic ModelThomson's Atomic Model is one of the fundamental models of the atom that tries to explain the working and structure of the atom. this model was proposed by famous Scientist JJ Thomson in 1904. Thomson during his cathode ray experiment proved the existence of a negatively charged particle called elec
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Rutherford Atomic ModelRutherford Atomic Model was proposed by Ernest Rutherford in 1911. It is also called the Planetary Model of the Atom. It introduced the concept of a dense, positively charged nucleus at the center of an atom, with electrons orbiting around it, forming the basis for modern atomic theory. In this arti
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Drawbacks of Rutherford's Atomic ModelIn Rutherford's Model of the Atom, Rutherford performed an experiment based on that experiment some of the hypotheses are given. At that time those hypotheses are considered revolutionary as there was an experiment to back that hypothesis. But as the experiment performed by Rutherford is rudimentary
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Bohr's Model of an AtomNeil Bohr proposed the Bohr model and was based on the modification of Rutherford’s model of an atom. Rutherford's model introduced a nuclear model of an atom in which it was explained that the nucleus which is positively charged is surrounded by negatively charged particles called electrons. Bohr m
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NeutronsNeutrons are fundamental subatomic particles of the atom. An atom is made up of electrons, protons and neutrons. James Chadwick, an English physicist, discovered the neutron in 1932. Neutrons are particles with no charge and higher mass. They are represented by n. They reside inside the nucleus of t
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ValencyValency of an element is a measure of an atom's ability to combine with other atoms to create molecules or chemical compounds. The characteristics of an element that indicate how many more atoms can join one of its atoms in a covalent bond are known as valence, or valency, in chemistry. The term, wh
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Valence ElectronsAny of the fundamental negatively charged particles in the outermost area of atoms that participate in the creation of chemical bonds are referred to as valence electrons. Changes in the atomic structure are confined to the outermost, or valence, electrons regardless of the kind of chemical connecti
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Mass NumberMass Number of an atom is the total number of protons and neutrons present in an atom. We know that an atom consists of electrons, protons, and neutrons but the mass of the atom is contributed by protons and neutrons as the mass of electrons is very low hence it doesn't contribute to the mass of an
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Relation Between Mass Number and Atomic NumberAtoms are the fundamental building blocks of matter, and they may combine in a variety of ways to produce various substances. You must be aware that all atoms are made up of electrons, protons, and neutrons. But did you know that the presence of these subatomic particles may explain an element's pro
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IsobarsIsobars are a group of elements that have the same mass number but different atomic numbers. In an isobar, we have different numbers of protons but the same number of nucleons, i.e. the sum of the number of protons and neutrons in the nucleus of isobars of the elements are the same. An example of is
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Why do all the Isotopes of an Element have similar Chemical Properties?The fundamental building units of matter are atoms and molecules. The existence of many types of matter is due to the various atoms that contain them. The atom was no longer seen as a simple, indivisible unit by 1900. It did, however, contain at least one subatomic particle, the electron. J.J. Thoms
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Why Isotopes have different Physical Properties?The atom is divided into two parts: the atomic structure and the extra nucleus. Positively charged protons and neutral neutrons make up the atomic structure. Electrons in the extra nucleus are negatively charged. Atoms, like all elements and compounds, have mass. Because of protons, the mass of an a
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What is Fractional Atomic Mass?The smallest possible amount of matter which still retains its identity as a chemical element, consisting of a nucleus surrounded by electrons is called an atom. These are the atoms that make up an element. An element is a material with the same number of protons in all of its atoms. Protons, electr
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Radioactive IsotopesRadioactive Isotopes or Radioisotopes are the isotopes of the elements that are not stable, i.e. they emit some radiation and change the composition of their nucleus. Radioactive isotopes are defined as the isotopes of any chemical element that have different masses and unstable nuclei and release e
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Discovery of ElectronsThe basic idea of the discovery of the elementary particles was generated by Dalton's Atomic Theory. John Dalton in 1808 gave the first scientific theory about atoms, in which, he stated that atoms are the smallest particle of any matter. They are indivisible and indestructible. According to Dalton
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What is a Proton?Protons are the fundamental particles that reside inside the nucleus of any atom. They are the positive charge particle and are responsible for balancing the negative charge of the electron to make the atom electrically neutral. Proton was discovered by the famous scientist Ernest Rutherford. Atoms
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Rutherford's Alpha Scattering ExperimentRutherford's Alpha Scattering Experiment is the fundamental experiment done by Earnest Rutherford's Alpha Scattering Experiment that gives the fundamental about the structure of the atom. Rutherford in his experiment directed high-energy streams of α-particles from a radioactive source at a thin she
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Atomic NucleusThe 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 nucl
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How did Neil Bohr explained the Stability of Atom?Bohr model of the atom was explained by Neil Bohr in 1915. It came into existence with the modification of Rutherford’s model of the atom. Rutherford’s model introduces the nuclear model of the atom, in which he explained that a nucleus (positively charged) is surrounded by negatively charged electr
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Electron ConfigurationElectron Configuration of an element tells us how electrons are filled inside various orbitals of the atom. The distribution of electrons inside various orbital of atoms is very useful in explaining various properties of the atoms and their combination with other atoms. The electron configuration of
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Potassium and Calcium - Atomic Structure, Chemical Properties, UsesAn atom is the smallest part of an element or compound that takes part in a chemical reaction. They are made of tiny particles known as protons, neutrons, and electrons. The Greek philosopher Democritus was the first person to use the term atom. The structure of the atom was discovered by John Dalto
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Noble GasInert gases are chemically unreactive. Noble gases, also termed inert gases are group 0 elements. There are six types of noble gases, helium (He), neon (Ne), argon (Ar), krypton (Kr), xenon (Xe), and radioactive radon (Rn). The noble gases are the type of elements belonging to group 18. Under standa
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What is meant by Chemical Combination?The composition of an element's nucleus and the arrangement of electrons around it are referred to as its atomic structure. Protons, electrons, and neutrons are the building blocks of matter's atomic structure. The nucleus of the atom is made up of protons and neutrons, which is surrounded by the at
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Difference between Electrovalency and CovalencyWe're all familiar with how electrons in an atom are organised into orbitals or shells. Valence electrons are the electrons that exist in the atom's outermost orbit. According to the Bohr-bury theory, the outermost shell may hold a maximum of 8 electrons. When the outermost shell is entirely filled,
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