Chapter 2Atomic Structure

Almost all the mass of an atom is concentrated in the nucleus because both the heavy particles, i.e. protons and neutrons, are found to be present here. A proton is 1836 times heavier than an electron, and the mass of a neutron is almost the same as that of a proton.

Although the nucleus is less than one hundred-thousandth (1/100 000) of the size of the atom, it contains more than 99.9 % of the mass of the atom. The electrons outside the nucleus are so light that their mass is not even included in the mass number.

Electrons, protons and neutrons are called the fundamental particles of all types of matter; the atoms of all the elements contain the same electrons, protons and neutrons. However, an atom of one element differs from an atom of another element because it contains a different number of the fundamental particles.

The number of protons (atomic number) is unique to each element, and the element is identified by this number. The difference in the properties of elements is due to the difference in the properties of their constituent atoms: sulphur looks very different from gold, which in turn is very different from bromine.

Each bismuth atom will have 83 protons and 83 electrons. The number of neutrons in bismuth will be calculated as follows:

So each ²¹⁰₈₃Bi atom will have 127 neutrons, 83 protons and 83 electrons.

Several isotopes of the same elements exist whose nuclei are unstable. They emit excess energy in the form of radiation; this is called radioactivity. Tritium (³₁H) is a radioactive isotope, while the other two isotopes of hydrogen are stable and do not emit any radiation.

A tritium nucleus has 1 proton and 2 neutrons. This proportion of neutrons to protons is too high for a stable nucleus, so it emits radiation (a beta particle) and turns into helium-3.

According to Bohr's model, when the electron is revolving in one of the orbits, its energy is fixed. The orbits that are further away from the nucleus possess successively greater energy, and the electron is not allowed to occupy a space in between the orbits.

The number of protons equals the atomic number Z, and in a neutral atom the number of electrons is the same as the number of protons. The number of neutrons is N = A − Z.

Since the chemical properties of the elements are determined by the number of electrons, all isotopes of an element show almost the same chemical behaviour, although their physical properties may be different.

Isotopes have the same number of protons and electrons but a different number of neutrons, so their masses differ: ²₁H has twice the mass of ¹₁H while ³₁H is thrice as heavy. Properties that depend on mass, such as density and rate of diffusion, are therefore different.

Several isotopes of the same elements exist whose nuclei are unstable. They emit excess energy in the form of radiation. This process of emission of radiation is called radioactivity, and the isotope which emits energy is called a radioactive isotope.

When a radioactive element emits radiation, it is transformed into another chemical element. This new element may be stable or may be radioactive so that it also emits radiation.

Chemists compare the masses of atoms on the relative atomic mass scale, on which the unit mass is 1/12 of the mass of an atom of carbon-12. On this scale Mg = 24.31 amu and Cl = 35.45 amu (C = 12.01).

So a chlorine atom is about 1.46 times as heavy as a magnesium atom; Mg is about 2 times and Cl about 3 times as heavy as a carbon atom.

The pictures show sulphur (yellow crystals), gold (shiny yellow metal) and bromine (a red-brown liquid in a flask).

Sulphur looks very different from gold which, in turn, is very different from bromine. Similarly iron is a heavy metal while aluminium and zinc are light metals. Metals are mostly lustrous while non-metals like sulphur and carbon appear dull. The difference in the properties of elements is due to the difference in the properties of their constituent atoms.

The idea of atom was first proposed in Greece when the philosopher Democritus declared that all matter is made of tiny particles. He named this particle atom, a particle that cannot be further subdivided.

Around 1800, the English chemist John Dalton did many experiments with compounds that provided the evidence for the existence of atoms.

In the last decade of the nineteenth century, scientists passing electricity through gases at reduced pressure (discharge tube experiments) discovered that atoms are no longer the smallest particles of matter. Atoms are composed of negatively charged particles called electrons and positively charged particles called protons. A proton is 1836 times heavier than an electron.

Electrons, protons and neutrons are called the fundamental particles. They are present in all atoms, except ordinary hydrogen which does not have a neutron.

The total number of protons and neutrons present in an atom almost accounts for the total mass of that atom. The mass of an electron being very small is not included in the mass number: a proton is 1836 times heavier than an electron.

In the nucleus, protons and neutrons are held together by a strong nuclear force. It is a very strong attractive force acting between proton and proton, neutron and neutron, and proton and neutron, over very short distances.

Although the nucleus is less than one hundred-thousandth (1/100 000) of the size of the atom, it contains more than 99.9 % of the mass of the atom.

The largest atom caesium is approximately nine times bigger than the smallest atom helium.

The size of an atom is so small that it is not possible to see it with the naked eye. However, a transmission electron microscope can be used to see atoms.

A discharge tube is a hard glass tube provided with two metallic electrodes and a vacuum pump to evacuate the gas present in it. When a very high voltage is applied to a gas at a very low pressure present in a glass tube, the glass surface behind the positive electrode started to glow, due to the rays emitted from the cathode. These rays were named cathode rays.

In 1897, British physicist Joseph John Thomson studied the properties of cathode rays by passing them through oppositely charged electric plates.

The presence of positively charged particles in an atom was first observed by E. Goldstein in 1886. He performed a series of experiments with a gas-discharge tube having a perforated cathode. A new type of rays were produced from the anode which moved towards the cathode. He called these new rays canal rays or anode rays.

The properties of canal rays vary depending on the gas used in the discharge tube; in fact what Goldstein discovered was gas ions, including hydrogen ions (H⁺). Cathode rays are electrons, and electrons are the same in all elements.

In 1917, Rutherford performed experiments which proved that the hydrogen nucleus is present in other nuclei. Rutherford thought that a hydrogen nucleus, or a proton, must be the fundamental building block of all nuclei and also possibly a new fundamental particle as well.

Later on, another particle, the neutron, was also discovered, which is known to carry no charge. The mass of a neutron is almost the same as that of a proton.

Lord Rutherford, in 1911, hit a stream of special type of particles (alpha particles) at a very thin gold foil. From this experiment he concluded that an atom has two portions: a tiny central portion which he called the nucleus, and a relatively large area surrounding it, which he called the extra-nuclear portion.

It was also discovered that almost all the mass of an atom is concentrated in the nucleus.

In 1913, Niels Bohr proposed a model for the hydrogen atom which is called Bohr's atomic model. According to this model the electron can revolve around the nucleus of the atom in specific paths called orbits or shells. When the electron is revolving in one of these orbits, its energy is fixed. When the electron is present in the orbit which is closest to the nucleus, its energy is minimum and it is called the ground state of the atom.

Since the electron present in each shell has a fixed energy, these shells are also named energy levels. The shell nearest to the nucleus is called the first shell or K shell; the second shell is at a greater distance from the nucleus and the electron in it possesses a greater value of energy.

The number of sub-shells in a shell is equal to the value of n: K has s; L has s, p; M has s, p, d; N has s, p, d, f.

To find out the number of electrons which can be accommodated in the extra-nuclear shells, scientists have devised a formula called the 2n² formula, where n = 1, 2, 3 … represents the number of the shell.

The K shell can, however, have less than two electrons or no electron at all.

According to the modern approach, electrons are like charged clouds whose location around the nucleus cannot be predicted with one hundred percent certainty. All we can say is that there is a certain probability of finding the electron at a certain probable distance from the nucleus.

The number of protons present in the atoms of an element is always fixed and it is called the atomic number of that element. It is represented by Z. Since an atom as a whole is electrically neutral, the number of electrons present in an atom will be the same as the number of protons.

Atomic number is unique to an element and the element is identified by this number. In the periodic table, elements are arranged in ascending order of their atomic numbers.

The total number of protons and neutrons present in an atom almost accounts for the total mass of that atom and hence it is called its nucleon number or mass number. It is represented by A.

Example: oxygen has 8 protons and 8 neutrons, so its atomic number is 8 and its mass number is 16: ¹⁶₈O.

The atomic number is written as a left subscript and the mass number as a left superscript. So an oxygen atom is symbolised as ¹⁶₈O, and carbon ¹²₆C has 6 protons and 6 neutrons.

Changing the number of electrons of an element forms ions, while changing the number of neutrons of an element forms isotopes.

Each barium atom will have 56 protons and 56 electrons. The number of neutrons will be calculated as follows:

So each ¹³⁷₅₆Ba atom has 81 neutrons, 56 protons and 56 electrons.

Technetium was the first element created by scientists in the laboratory (1937). Most elements beyond uranium are also synthetic.

All the atoms of an element must necessarily have the same atomic number, but their mass number may vary depending upon the number of neutrons present in the nucleus. Atoms of the same element having different number of neutrons in their nuclei are called isotopes.

Example: carbon has three isotopes, ¹²₆C, ¹³₆C and ¹⁴₆C, whose atoms have six, seven and eight neutrons.

Hydrogen exists as three isotopes, hydrogen (protium), deuterium and tritium, represented by ¹₁H, ²₁H and ³₁H. Hydrogen (¹₁H) is the only atom which does not have a neutron. ²₁H has twice the mass of ¹₁H while the mass of ³₁H is thrice the mass of ¹₁H.

Element carbon has three isotopes as its atoms have six, seven and eight neutrons in their nuclei. They are represented as ¹²₆C, ¹³₆C and ¹⁴₆C; each has 6 protons. Almost all natural carbon is carbon-12, and carbon-14 is radioactive.

The isotopes of the same element do not have the same physical properties. Several isotopes of the same elements exist whose nuclei are unstable. They emit excess energy in the form of radiation. This process of emission of radiation is called radioactivity, and the isotope which emits energy is called a radioactive isotope.

Example: tritium ³₁H; the other two isotopes of hydrogen are stable.

When a radioactive element emits radiation, it is transformed into another chemical element. This process is called radioactive decay. The new element may be stable or may be radioactive so that it also emits radiation.

Radiocarbon dating is a method for finding out the age of a historical object containing organic material with the help of the radioactive isotope of carbon, ¹⁴₆C. The method involves measuring the proportion of ¹⁴C in a sample from a dead plant or animal, like a piece of wood or a bone, which tells when the animal or plant died. The older the sample is, the less ¹⁴C is to be detected.

Doctors diagnose disease by injecting the patient with a small amount of radioactive fluid. Technetium-99m is used for diagnostic imaging across human organs like the brain and lungs. Doctors use a special camera to watch how the radioactive fluid moves.

Radioactive isotopes are used to test the strength of metals and concrete mixtures. They are used to generate cheap nuclear power and to find oil fields. In medicine they are used to diagnose and treat many diseases, including cancer and thyroid disorders.

Radiation emitted from a radioactive source causes atoms to ionise. For example, radiation emitted by radium-226 can remove electrons from an atom, if it has enough energy to remove the tightly bound electron. If an atom of sodium is hit by ionising radiation, it may lose an electron and become a positively charged ion (cation).

Gallium has many interesting properties. Its melting point is below body temperature, so it melts in the hand, and it does not evaporate easily.

Every year, our body replaces about 98 % of its atoms.

Yes. The same elements occur everywhere in the universe. Matter on Mars or on any other planet consists of the same elements found on Earth.

Without knowing the relative masses of atoms, we would not know in which ratio of masses we mix the reactants to carry out a chemical reaction. In 1961, chemists adopted a new scale for the relative masses of atoms based on carbon-12.

The relative atomic masses of elements are expressed in atomic mass unit (amu). It is defined as one-twelfth the mass of an atom of carbon-12. The mass of an atom is far too small to be used in grams conveniently, so a unit on this small scale is needed.

Example: one atom of hydrogen-1 is 1.007 amu, and one atom of sulphur-32 is 31.972 amu.

The relative atomic mass of an element is the mass of an atom of that element relative to the mass of the light isotope of carbon taken as 12. The unit mass on this scale is 1/12 of the mass of carbon-12; the mass of one carbon-12 atom is exactly 12.

An element usually consists of a few different isotopes with different mass numbers. These mass numbers are called relative isotopic masses. Each isotope also has its own naturally occurring abundance, which is called its isotopic abundance.

The relative atomic mass of an element can be calculated from the relative isotopic masses (m) and isotopic abundances (p) by the formula:

Carbon-14 is present only in traces, so it does not change the answer.

Abundance of Cl-37 = 24.23 %; abundance of the light isotope = 75.77 %.

The relative atomic mass of the light isotope of chlorine is 34.95.

According to Bohr's model, the orbits that are further away from the nucleus possess successively greater energy. The second shell is at a definite distance from the nucleus which is greater than that of the first shell, so the electron present in it possesses a greater value of energy.

The reason: the negative electron is attracted by the positive nucleus. To move it further away, to the second shell, work must be done against this attraction, and that work is stored as energy. So E₁ < E₂ < E₃ < E₄.

The discharge tube experiments were done by passing electricity through gases at reduced pressure. Cathode rays appear only when a very high voltage is applied to a gas at a very low pressure; the tube has a vacuum pump to evacuate the gas.

Classical concept

According to Bohr's model, the electron revolves around the nucleus of the atom in specific paths called orbits or shells, with a fixed energy in each orbit, much like a planet moving around the Sun. It was pictured as a tiny charged particle with a definite position and path.

Modern concept

According to the modern approach, electrons are like charged clouds whose location around the nucleus cannot be predicted with one hundred percent certainty. All we can say is that there is a certain probability of finding the electron at a certain probable distance from the nucleus.

Electrons are quantum particles with probabilistic paths; by the uncertainty principle their exact paths and locations cannot be mapped.

Several isotopes of the same elements exist whose nuclei are unstable; they emit their excess energy in the form of radiation.

In a nucleus the protons repel each other while the strong nuclear force holds all the nucleons together. When the numbers of neutrons and protons are not in a stable proportion (too many or too few neutrons), or the nucleus is very large (more than 83 protons) so that the repulsion becomes too great, the nucleus has excess energy. It becomes stable by giving out radiation and changing into another element (radioactive decay).

The experiments were repeated with different gases and different metals for the electrodes. Cathode rays always had the same properties and the same mass to charge ratio, so it was shown that electrons are the subatomic particles of all elements.

Canal rays changed with the gas, but they were always positive ions, and the lightest of them was H⁺. Rutherford later proved that the hydrogen nucleus (proton) is present in other nuclei. So electrons, protons and neutrons are shown to be present in all atoms, irrespective of the fact that these atoms behave very differently from one another; only their numbers differ.

The hydrogen atom is the simplest of all atoms. It has a tiny central nucleus with one proton and one electron revolving around it. Its atomic number is 1.

Nucleus

Rutherford showed that an atom has two portions: a tiny central portion called the nucleus and a relatively large area surrounding it called the extra-nuclear portion. Ordinary hydrogen ¹₁H is the only atom which does not have a neutron, so its nucleus is a single proton. Almost all its mass is in this proton, which is 1836 times heavier than the electron.

Electron: Bohr's model

In 1913, Niels Bohr proposed a model for the hydrogen atom. According to this model the electron can revolve around the nucleus in specific paths called orbits or shells. When the electron is revolving in one of these orbits, its energy is fixed. When it is present in the orbit closest to the nucleus (the K shell), its energy is minimum and it is called the ground state of the atom. The orbits further away possess successively greater energy, and the electron is not allowed to occupy a space in between the orbits.

Transition of the electron

When energy is given to the atom, the electron jumps to a higher shell (an excited state). When it returns to the ground state it gives out the energy as light; this is why hydrogen gives light of definite colours.

Modern view

According to the modern approach, the electron is like a charged cloud around the nucleus whose location cannot be predicted with one hundred percent certainty; there is only a certain probability of finding it at a certain distance from the nucleus.

Isotopes of hydrogen

Hydrogen exists as three isotopes, hydrogen (protium) ¹₁H, deuterium ²₁H and tritium ³₁H. All have one proton and one electron, but 0, 1 and 2 neutrons. ²₁H has twice and ³₁H thrice the mass of ¹₁H. Tritium is radioactive.

Theory of atomic structure

An atom has a nucleus containing protons and neutrons, surrounded by electrons in shells. The electrons in the outer shell are furthest from the nucleus and held least firmly.

Ionisation by a radioactive source

Radiation emitted from a radioactive source causes atoms to ionise. For example, radiation emitted by the radioactive element radium-226 can remove electron or electrons from the atom. However, this ionising radiation should have enough energy to remove the tightly bound electron from the orbit of an atom. An electron can be lost because the ionising radiation collides with the atom and forces the electron away from the atom.

If an atom of sodium is hit by ionising radiation, it may lose an electron. This process converts the atom into a positively charged ion (cation):

Formation of an anion

The electron set free may be captured by another atom, which then becomes a negatively charged ion (anion):

Radioactivity

The isotopes of the same element do not have the same physical properties. Several isotopes of the same elements exist whose nuclei are unstable. They emit excess energy in the form of radiation. This process of emission of radiation is called radioactivity and the isotope which emits energy is called a radioactive isotope. Tritium ³₁H is a radioactive isotope, while the other two isotopes of hydrogen are stable and do not emit any radiation.

Radioactive decay

When a radioactive element emits radiation, it is transformed into another chemical element. This process is called radioactive decay. The new element may be stable or may be radioactive so that it also emits radiation.

1. Medical imaging and treatment

Radioactive isotopes are useful in medical imaging. Doctors use them to diagnose disease by injecting the patient with a small amount of radioactive fluid. Technetium-99m is used for diagnostic imaging across human organs like the brain and lungs. Doctors use a special camera to watch how the radioactive fluid moves. In medicine they are also used to treat many diseases, including cancer and thyroid disorders.

2. Radiocarbon dating (archaeology)

Radiocarbon dating is a method for finding out the age of a historical object containing organic material with the help of the radioactive isotope of carbon, ¹⁴₆C. The method involves measuring the proportion of ¹⁴C in a sample from a dead plant or animal, like a piece of wood or a bone, which provides information that can be used to calculate when the animal or plant died. The older the sample is, the less ¹⁴C is to be detected.

3. Industry and power generation

Radioactive isotopes are used to test the strength of metals and concrete mixtures. They are used to generate cheap nuclear power and to find oil fields.

The relative atomic mass of an element can be calculated from the relative isotopic masses (m) and isotopic abundances (p):

Using the book's formula: 19 938.70 ÷ 100 = 199.39.

The abundances given add up to only 99.37 %, so dividing by their total gives the accurate value: 19 938.70 ÷ 99.37 = 200.6 (the accepted value for mercury is 200.59).

An element is fixed by its number of protons, so a new element is made by changing the nucleus. Ordinary chemical reactions only move electrons, so this needs nuclear reactions, carried out in particle accelerators and nuclear reactors. Technetium was the first element created by scientists in the laboratory.

1. Particle bombardment

Light particles (neutrons, protons, deuterons, alpha particles or carbon ions) are given very high energy in a particle accelerator and fired at a target of heavy atoms. When a particle enters a target nucleus, the number of protons changes and a new element forms.

2. Nuclear fusion

Two nuclei are made to join into one heavier nucleus, releasing energy. Superheavy elements (Z = 104 to 118) are made by fusing a beam of medium nuclei, such as calcium-48, with a heavy target such as californium.

3. Neutron capture and decay (in a nuclear reactor)

In a reactor, heavy nuclei absorb neutrons and then give out beta radiation, which turns a neutron into a proton. Uranium-238 becomes neptunium (Z = 93) and then plutonium (Z = 94) this way.

4. Nuclear fission

A heavy nucleus such as uranium-235 is split by a neutron into two lighter nuclei plus more neutrons and a great deal of energy. Fission produces lighter elements already known, and its neutrons drive the reactions above; it is the process used in nuclear power stations.

The idea that an atom is like a mini solar system is helpful to explain atomic structure. Rutherford found a tiny central nucleus with the electrons in the large extra-nuclear portion, and Bohr's model has the electron revolving around the nucleus in specific paths called orbits or shells, just as planets revolve around the Sun.

How far it is true

Where it is wrong

Experiment

A discharge tube is a hard glass tube provided with two metallic electrodes and a vacuum pump to evacuate the gas present in it. When a very high voltage is applied to a gas at a very low pressure present in a glass tube, the glass surface behind the positive electrode started to glow, due to the rays emitted from the cathode. These rays were named cathode rays.

Characteristics of cathode rays

In 1897, British physicist Joseph John Thomson studied the properties of cathode rays by passing them through oppositely charged electric plates.

Discovery

The presence of positively charged particles in an atom had been first observed by E. Goldstein in 1886. It was based on the concept that atoms are electrically neutral, having the same number of positive and negative charges.

Experiment

He performed a series of experiments with a gas-discharge tube having a perforated cathode. A new type of rays were produced from the anode which moved towards the cathode. He called these new rays canal rays or anode rays.

Properties

Discovery of the proton

In 1917, Rutherford performed experiments which proved that the hydrogen nucleus is present in other nuclei. Rutherford thought that a hydrogen nucleus or a proton must be the fundamental building block of all nuclei and also possibly a new fundamental particle as well.

Discovery of the neutron

Later on, another particle, the neutron, was also discovered, which is known to carry no charge. The mass of a neutron is almost the same as that of a proton.

These three particles, i.e. electron, proton and neutron, were given the name fundamental particles and are shown to be present in all atoms (except ordinary hydrogen, which does not have a neutron), irrespective of the fact that these atoms behave very differently from one another. However, the number of these particles is different in different atoms.

Discovery of the nucleus

After the discovery of these particles, a very important question arose: how are these particles arranged in a tiny place called the atom? Lord Rutherford, in 1911, provided the answer. He carried out a remarkable experiment in which he hit a stream of special type of particles (alpha particles) at a very thin gold foil.

Structure of the atom

From this experiment he concluded that an atom has two portions: a tiny central portion which he called the nucleus, and a relatively large area surrounding it, which he called the extra-nuclear portion. Almost all the mass of an atom is concentrated in the nucleus because both the heavy particles, protons and neutrons, are present here. In the nucleus these two particles are held together by a strong nuclear force. The electrons are in the extra-nuclear portion.

Bohr's atomic model

In 1913, Niels Bohr proposed a model for the hydrogen atom which is called Bohr's atomic model.

Energy levels

Since the electron present in each shell has a fixed energy, these shells are also named energy levels. The shell nearest to the nucleus is called the first shell or K shell. The second shell is at a greater distance from the nucleus and the electron present in it possesses a greater value of energy. Similarly, electrons may also be present in the third or higher shells.

Distribution of electrons in shells: 2n² formula

To find out the number of electrons which can be accommodated in the shells, scientists devised the 2n² formula, where n = 1, 2, 3 … is the number of the shell.

Sub-shells

Each shell is further sub-divided into sub-shells or orbitals. The number of sub-shells in a shell is equal to the value of n: the first shell has only the s sub-shell; the second has s and p; the third has s, p and d; the fourth has s, p, d and f. The maximum numbers of electrons in s, p, d and f sub-shells are 2, 6, 10 and 14. In the second shell, the 8 electrons are divided as 2 in s and 6 in p.

Modern approach

According to the modern approach, electrons are like charged clouds whose location around the nucleus cannot be predicted with one hundred percent certainty; there is only a certain probability of finding the electron at a certain distance from the nucleus.

Definition

All the atoms of an element must necessarily have the same atomic number, but their mass number may vary depending upon the number of neutrons present in the nucleus. Atoms of the same element having different number of neutrons in their nuclei are called isotopes.

Properties of isotopes

Since the chemical properties of the elements are determined by the number of electrons, all isotopes show almost the same chemical behaviour, although their physical properties may be different.

Isotopes of hydrogen

Hydrogen exists as three isotopes, hydrogen, deuterium and tritium, represented by ¹₁H, ²₁H and ³₁H. Hydrogen (¹₁H) is the only atom which does not have a neutron. ²₁H has twice the mass of ¹₁H while the mass of ³₁H is thrice the mass of ¹₁H.

Isotopes of carbon

Element carbon has three isotopes as its atoms have six, seven and eight neutrons in their nuclei. These isotopes are represented as ¹²₆C, ¹³₆C and ¹⁴₆C. Similarly, the masses of the three isotopes of carbon are different.

Need for relative atomic masses

Ever since the existence of the atom was recognised, chemists were trying to find a method to compare the masses of different atoms. Without knowing the relative masses of atoms, we would not know in which ratio of masses we mix the reactants to carry out a chemical reaction.

The carbon-12 scale

In 1961, chemists adopted a new scale. The unit mass on this scale is 1/12 of the mass of the light isotope of carbon taken as 12. The mass of one atom of carbon on this scale is exactly 12 and the masses of the other atoms are measured relative to this unit.

Calculation from isotopic abundance

An element usually consists of a few different isotopes with different mass numbers, called relative isotopic masses. Each isotope has its own naturally occurring abundance, called isotopic abundance. The relative atomic mass is calculated from the relative isotopic masses (m) and isotopic abundances (p):

Example 1: krypton

Example 2: light isotope of chlorine

Example 3: lead