Atomic Structure — Short Questions

Almost all the mass of an atom is concentrated in its nucleus because the nucleus contains protons and neutrons, which have much greater mass as compared to the electrons that orbit around the nucleus. Protons and neutrons are approximately 1836 times more massive than electrons.

Elements are identified by their atomic numbers. Elements are different from one another because each element has a unique atomic number and different properties. The number of electrons, protons and neutrons in an atom determine its chemical behavior which in turn distinguishes one element from another.

Each bismuth atom will have 83 protons and 83 electrons. The number of neutrons in bismuth will be calculated as follows:
n = A − Z
n = 210 − 83
= 127
So, each bismuth atom will have 127 neutrons in its nucleus.

Tritium is a radioactive isotope of hydrogen because it has unstable nucleus. Tritium undergoes radioactive decay, emitting low energy beta radiations. Tritium has two neutrons, one proton and one electron that making it heavier than the other two isotopes of hydrogen (Protium and Deuterium).

An atom can evolve or absorb energy through a process called electron transition. When an electron in an atom jumps from low energy orbit to high energy orbit, it absorbs energy and when it jumps back from high energy orbit to low energy orbit, it evolves energy. This energy is evolved or absorbed in the form of electromagnetic radiation, such as light or heat.

i. ¹⁹⁵₇₈Pt
Solution
Each Pt atom will have 78 protons & 78 electrons. The no. of neutrons in Pt will be calculated as follow:
n = A-Z
n = 195-78 ⇒ 117
So, each Pt atom will have 117 neutrons, 78 protons &78 electrons.

ii. ⁵⁵₂₅Mn
Solution
Each Mn atom will have 25 protons & 25 electrons. The no. of neutrons in Mn will be calculated as follows:
n = A − Z
n = 55 - 25
n = 30
So, each Mn atom will have 30 neutrons, 25 electrons & 25 protons.

iii. ¹²⁷₅₃I
Each ¹²⁷₅₃I atom will have 53 protons & 53 electrons. The no. of neutrons in (₅₃I¹²⁷) will be calculated as follows:
n = A − Z
n = 127 − 53
n = 74
So, each (I) atom will have 74 neutrons, 53 protons & 53 electrons.

i. Isotopes have same chemical properties because they have same electronic configuration and have same atomic number.
ii. Isotopes have different physical properties because they have different mass number.

Several isotopes of the same elements exist whose nuclei are unstable. They are high in energy. They emit excess energy in the form of radiation. This process of emission is called radioactivity and the isotope which emits energy is called radioactive isotope. Every element has one or more radioactive isotope.
Example:
²¹⁰₈₃Bi → ²⁰⁶₈₁Tl + Energy

The atomic mass of carbon is 12.01 amu
The atomic mass of magnesium is 24.30 amu
The atomic mass of chlorine is 35.45 amu.
To compare their masses, we divide the masses as:
mass of Mg / mass of C = 24.30 / 12.01 = 2.02
Mg atoms is 2.02 times havier than C atom.
mass of Cl / mass of C = 35.45 / 12.01 = 2.95
Cl atom is 2.95 times havier than C atom.
mass of Cl / mass of Mg = 35.45 / 24.30 = 1.45
Cl atom is 1.45 times havier than Mg atom.
Carbon is lightest atom among these three atoms.

Relative atomic mass of Lead (Pb)
= 2 × 204 + 24 × 206 + 22 × 207 + 52 × 208
100
= 207.22

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 strong attractive force that binds protons and neutrons together. This force is stronger than electrostatic or magnetic forces. This force exists between neutrons and neutrons, and between protons and neutrons and protons and protons.

Protons and neutrons have roughly the same mass, around 1 amu. This mass contributes significantly to the total mass of the atom. Electrons have much less mass, the mass of electron is about 1836 times less than the mass of proton and neutron so their contribution to the total mass of an atom is usually negligible.

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

Cathode rays are so named because they are emitted by the cathode in a discharge tube. A very high electrical potential of thousands of volts was applied in the discharge tube which ionized the residual gas atoms present in the tube. The positive ions thus produced travelled towards the cathode as anode or canal rays. When they collided with the cathode they knocked electrons out of its surface. This stream of electrons was called cathode rays.

Lord Rutherford, in 1911. He carried out an experiment in which he hit a stream of alpha particles to a very thin gold foil. From this experiment he concluded that an atom has two portions. A tiny central portion which he called as nucleus and a relatively large area surrounding this, which he called extra nuclear portion. The electrons are present in this extra nuclear portion in the form of cloud around the nucleus.

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 as atom, a particle that cannot be further subdivided.

i) Goldstein discovered positively charged particles called protons in 1886.
ii) Thomson found in an atom, the negatively charged particles known as electron in 1897.
iii) Chadwick discovered neutron in 1932.

Electron It is the negatively charged particle. It revolves around the nucleus. Mass of electron is 5.486 × 10⁻⁴ amu.

Neutron It is the neutral particle. It is present in the nucleus. Mass of neutron is 1.0087 amu.

Shell Each principle energy level in which an electron revolves around nucleus is called shell. These are represented by K, L, M, N etc.

Sub Shell Each shell further contains one or more sub energy levels called sub shells. These are represented by s, p, d, f.

Sub-atomic particles are the fundamental particles that make up atoms. The three main sub – atomic particles are:

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.

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 as cathode rays.

In 1897, British physicist Joseph John Thomson studied the properties of cathode rays by passing them through the oppositely charged electric plates.
i. Cathode rays bent towards the positively charged plate showing that they carry negative charge.
ii. Thomson used the findings of his experiments to calculate the mass to charge ratio of cathode rays which finally proved that cathode rays are in fact, negatively charged material particles.

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

Solution Each barium atom will have 56protons and 56 electrons. The number of neutrons in barium will be calculated as follows:
n= A-Z
n= 137 −56 = 81
So each ¹³⁷₅₆Ba atom will have 81 neutrons, 56 protons and 56 electrons.

Proton number refers to the number of protons in the nucleus of an atom. It is also known as the atomic number and is indicated by the symbol "Z".
For example: There is only one proton in the nucleus of H atom; therefore its atomic number is 1.

The sum of total number of protons and neutrons in an atom is known as its mass number or nucleon number. It is represented by A.
For Example: Mass of 1st isotope of hydrogen is 1.
A = Z + n or n = A − Z
Formula:
Mass number = atomic number + No. of neutron.

Copernicium (Cn) is a synthetic element and it was discovered in 1996. This metal turns into a gas at room temperature.

Isotopes are atoms of an element have the same atomic number but different mass number. This is because atoms of an element can differ in the number of neutrons.
Example: ¹H , ²H , ³H are isotopes of hydrogen.

Hydrogen has three isotopes. Hydrogen – 1 (Protium) has no neutron. Its symbols ¹H . Hydrogen – 2 (deuterium) has one neutron and hydrogen – 3 (Tritium) has two neutrons. Their symbols are ²H and ³H respectively.

Isotopes of Carbon
Carbon has three isotopes. Carbon-12, carbon-13 and carbon-14. Almost all the carbon is carbon-12. Its symbol is ¹²C . It has six neutrons and six protons. Carbon-13 has symbol ¹³C, it has seven neutrons and six protons. Carbon-14 has eight neutrons and six protons. Its symbol is ¹⁴C .

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 is called radioactivity and the isotope which emits energy is called radioactive isotope.
For example: ²³⁸₉₂U is a radioactive isotope of uranium.

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

The unit for relative atomic mass is called atomic mass unit. Its symbol is "amu". One atomic mass unit is ¹/₁₂th the mass of one atom of carbon-12 isotope.
Need: The mass of an atom is too small to be determined practically. That's why to determine the atomic mass of various elements, atomic mass unit is needed.

An element usually consists of different isotopes. The relative atomic mass of an element can be calculated from the relative isotopic masses(m) and isotopic abundances (p) by formula:
Relative atomic mass
= m₁p₁ + m₂p₂ + m₃p₃
100

The relative atomic mass is a weighed average of the all the naturally occurring isotopes of an element, taking into consideration of their natural abundance. Use general formula Relative atomic mass of C =
RA of C-12 × at.mass of C-12+RA of C-13 × at.mass of C-13+RA of C-14 × at.mass of C-14
100

Relative atomic mass of C = 98.8 × 12+1.1 × 13+0.009 × 14
100

Relative atomic mass of C = 1185.6+14.3+0.126
100

Relative atomic mass of C = 12.00026 amu

Relative atomic mass of krypton
= 80× 2.0 + 82×12.0 + 83×12.0 + 84×57.0 + 85×57.0
100
= 83.7

Relative isotopic abundance of light isotope of chlorine = 75.77%
Relative atomic mass of chlorine = 35.45

Relative atomic mass of chlorine = Cl × 75.77 + 37 × 24.23
100

35.45 = Cl × 75.77 + 37 × 24.23
100

3545 = Cl × 75.77 + 37 × 24.23
3545 − 896.51 = Cl × 75.77
2648.49 / 75.77 = Cl
34.95 = Cl

Relative atomic mass of light isotope of chlorine is 34.95

Radiocarbon dating is a method for finding out the age of an historical object containing organic material with the help of radioactive isotope of carbon ¹⁴C. 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 an animal or plant died. The older the sample is, the less ¹⁴C is to be detected.

Gallium has many interesting properties. Its melting point is below body temperature so it is liquid at room temperature. It has water like viscosity. It does not evaporate.

Lord Rutherford, in 1911, carried out a remarkable experiment in which he hit a stream of alpha particles to a very thin gold foil. From this experiment he concluded that an atom has two portions. A tiny central portion which he called as nucleus and a relatively large area surrounding this, which he called extra nuclear portion. It was also discovered that 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. In the nucleus these two particles are held together by a strong nuclear force.

In 1913, Niel 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.

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

To find out the number of electrons which can be accommodated in these-extra nuclear shells, the scientists have devised a formula called (2n²) formula where n can have values 1,2,3… and so on and they represent the number of shells. Shells have also been named as K, L,M,N … and so on.

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

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

The energy of an electron increases as we move from the first to the second shell because the second shell is farther away from the nucleus which means that electron in the second shell has high energy as compared to the electron in the first shell. This increase in distance from the nucleus results in a higher energy level for electron in our shell.

Lowering the pressure of the gas inside the discharge tube is necessary because it helps to create a vacuum. This vacuum provides minimum opposition to the electrons emitting from the electrode creating smoothly moving cathode/ canal rays.

The classical concept of an electron is that electron being the charged particles should release or emit energy continuously and they should ultimately fall into the nucleus. Electron revolve around the nucleus of an atom similar to planets revolve around the sun in the solar system.

However, as scientific knowledge advanced, the concept of electron evolved. Now, it is considered that electron behave like particle as well as wave. Electrons are now described by wave functions that represent the probability of finding an electron in a particular region around the nucleus rather than following a definite path like a planet in orbit. This change in concept has led to a more convinced understanding of the behavior of electrons in atoms and molecules.

The nuclei of radioactive elements are unstable because they contain an imbalance of protons and neutrons which leads to an excess of energy within the nucleus. This imbalance causes the nucleus to undergo radioactive decay in order to become more stable.

During radioactive decay, the nucleus emits energy in the form of radiation to achieve a more balanced and stable configuration.

During discharge tube experiments, scientists observed that the properties of the electrons and protons were the same regardless of the element being studied. They conclude that the same type of electrons and protons are present in all elements. From discharge tube experiments, scientists suggested that electrons are negatively charged particles and protons are positively charged particles. These fundamental particles are consistent across different elements.