Journey Inside the Atom

Are atoms the smallest indivisible particles? Why do electrons not fall into the nucleus even though they are attracted to protons in it? Why did scientists keep modifying atomic models?

Suppose you made up your own 'atom', as Thomson described, using clay for the positive charge and small beads for the electrons spread through it. What will happen if: (i) the positive charge on the clay is lesser than the total negative charge of the beads? (ii) by mistake, the clay itself carries a bit of negative charge? Would your model still represent a neutral atom?

Could an orange or a lemon, which also contain seeds inside soft pulp, be a good comparison? In what ways does it match Thomson's idea and where does it fall short?

Why did Thomson conclude that electrons are present in all atoms?

What do you think would happen if α-particles were replaced with negatively charged particles in Rutherford's gold foil experiment?

Rutherford found that a few α-particles bounced back sharply. How does this single surprising result completely rule out Thomson's 'plum pudding model' of the atom?

If you could ask Rutherford one question about his work, what would it be?

Assertion (A): Rutherford concluded that most of the mass of an atom is concentrated in a small region at the centre called the nucleus. Reason (R): According to Thomson's model, electrons are embedded in a uniformly distributed positive charge sphere. Choose the correct option: (i) Both A and R are true, and R is the correct explanation of A. (ii) Both A and R are true, but R is not the correct explanation of A. (iii) A is true, but R is false. (iv) A is false, but R is true.

Imagine you are a scientist who has discovered a new element. Name this element after yourself and justify that the symbol you have chosen follows the IUPAC rules.

What problems could arise if every scientist used different symbols for the same element?

An atom with an atomic number of 26 has 56 nucleons. Find out its number of electrons, protons and neutrons.

The nucleus of an atom contains 20 protons. If its mass number is 41, find the number of neutrons in it.

An atom has 18 neutrons and an atomic number of 17. What is its mass number?

An atom ²³A has 11 electrons. Find the number of neutrons in it.

Identify the number of electrons in the outermost shell of the following elements: (i) ¹²₆C (ii) ¹⁹₉F (iii) ²⁸₁₄Si

Write the electronic configuration of the elements having atomic numbers 12, 16 and 18.

Solve this riddle: I am an atom with a mass number of 23 and 11 protons. I am a soft metal and react vigorously with water. Who am I and how many neutrons do I have? You can also create one such riddle.

Two different atoms have 11 protons each, but one has 12 neutrons, and the other has 13 neutrons. How do their atomic numbers and mass numbers compare? Are they the same element or different elements?

If a bromine atom is available in the form of, say two isotopes, ⁷⁸₃₅Br (49.7%) and ⁸¹₃₅Br (50.3%), calculate the average atomic mass of the bromine atom.

Can you calculate how many atoms would be needed to make a sheet of paper that is 0.1 mm thick, like the one in your textbook? If the diameter of one atom is about 10⁻¹⁰ m and the sheet is 0.1 mm (10⁻⁴ m) thick, number of atoms ≈ (10⁻⁴ m) ÷ (10⁻¹⁰ m) = 10⁶. That is about one million atoms need to be stacked together! When these atoms come together, they form the solid materials we see and touch. What feels like a simple sheet is actually a vast assembly of tiny building blocks, all neatly arranged. It's amazing how the unseen atomic world shapes everything we see and use every day!

The discovery of the neutron opened a new era in atomic physics. Neutrons, being uncharged, can easily penetrate nuclei, leading to breakthroughs, such as the creation of artificial radioactive elements and the splitting of uranium atoms. This gave birth to the 'atomic age', allowing the development of both nuclear power and nuclear weapons.

Atoms are too tiny to be weighed in kilograms or grams like everyday objects. Just as it is easier to weigh a grain of wheat in milligrams rather than kilograms, scientists use a special unit called the unified atomic mass unit (u) to measure the mass of atoms. This is because a kilogram is too large for such small particles. Earlier, atomic mass was expressed in atomic mass unit (abbreviated as 'amu').

Electron microscopes, such as Scanning Tunnelling Microscopes (STMs) and Transmission Electron Microscopes (TEMs) can produce images of materials with atomic-level details. STMs mainly study surfaces (Fig.8.15), while TEMs reveal how atoms are arranged inside very thin samples.

Why are Bohr's shells called K, L, M, N... and not A, B, C, D? The naming came from early X-ray experiments by the physicist, Charles Barkla, who called the first observed X-ray line K. He didn't start naming from A to leave room for possible discovery of a series earlier than the K series, although none were ever found. Bohr adopted the same notation for atomic shells.

Lighter atoms often have an equal number of protons and neutrons (like carbon with six each or oxygen with eight each). However, as atoms get heavier, their nuclei have many more neutrons than protons. Iron has 26 protons and 30 neutrons, and by the time we reach uranium, the nucleus has 92 protons and 146 neutrons. You may wonder that since all the protons with like charges are squished together in a nucleus why do they not push each other away? Think of it this way—every proton inside the nucleus repels every other proton because they all carry positive charge. Neutrons being neutral help reduce this repulsion by intervening and increasing the distance between protons, and also by strengthening the force, called the nuclear force, that binds all particles together. So, heavier atoms need many more neutrons to hold everything in the nucleus tightly bound.

Some isotopes have special properties that are useful in various fields. Some of the applications of isotopes are: ²³⁵U is used as fuel in nuclear reactors. ⁶⁰Co is used in radiation treatment for cancer. ¹³¹I is used to treat goitre and thyroid cancer. ¹⁴C is used in archaeology to determine the age of ancient fossils and artefacts.

J. J. Thomson's most significant discovery was that of the electron, the first subatomic particle to be identified and a part of every atom. He received the Nobel Prize in Physics in 1906 for his studies of the electrical conductivity of gases. This research led him to discover electrons. As the head of the famous Cavendish Laboratory in Cambridge, he guided and inspired many scientists, including Ernest Rutherford.

Ernest Rutherford worked with J. J. Thomson and later became known as the Father of Nuclear Physics. He discovered the atomic nucleus and explained how some elements naturally break down, for which he won the 1908 Nobel Prize in Chemistry. In 1911, he proposed the nuclear model of the atom. His portrait now appears on New Zealand's $100 banknote.

Niels Bohr was a professor of physics at Copenhagen University, Denmark. He was curious about how atoms exist because the old models could not explain why electrons stay around the nucleus without collapsing. His explanation of the atomic structure provided more clarity. Niels Bohr received the Nobel Prize in 1922, for his work on the structure of the atom.

James Chadwick, working at the Cavendish Laboratory at the University of Cambridge, solved a key puzzle in 1932, when he discovered the neutron. This breakthrough explained atomic mass and earned him the Nobel Prize in Physics in 1935. The neutron immediately transformed research, enabling scientists to probe nuclear secrets, and sparked a chain of discoveries in understanding and harnessing atomic energy.

Homi Jehangir Bhabha was an Indian physicist. He is known as the father of the Indian nuclear programme. He made a pioneering contribution to the development of atomic energy in India. He established key institutions like the Tata Institute of Fundamental Research (TIFR) and the Bhabha Atomic Research Centre (BARC), for peaceful uses of atomic energy to generate electricity, support agriculture, and advanced medical treatments.

The Bhabha Atomic Research Centre (BARC, Fig. 8.8), Mumbai, leads advanced neutron-scattering experiments using reactors, such as Dhruva. This has revealed key insights into materials like superconductors, battery electrodes, and drug molecules, helping develop better medicines, energy storage, and industrial alloys right here in India. Research and explore more about Dhruva!

Observe Fig. 8.4 of the gold foil experiment. Predict the observations you would expect if the gold foil in the experiment were made thicker. Also, draw a simple diagram to show the observations you expect. Hint: Compare thin foil vs thick foil. How does the thickness affect the chances of hitting a nucleus?

an atom had no empty space? How would this have affected the size of various objects?