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Meet a Scientist Homi This does not mean that any one atom of chlorine has Jehangir a fractional mass of 35.5 u. It means that if you take, say Bhabha was 35 1 million chlor37e atoms, they will contain 7.5 lakh 17Cl an Indian and 2.5 lakh 17Cl atoms with a weighted average atomic physicist. mass of 35.5 u. He is known as the This comparison shows that the simple average ignores father of the Indian nuclear abundance, whereas the weighted average accurately reflects the mass of the element as it occurs in nature. programme. He made a pioneering contribution to the development of Pause and Ponder atomic energy in India. He established key institutions 17. Two different atoms have 11 protons each, but one has 12 neutrons, and the other has 13 neutrons. How do their like the Tata Institute of Fundamental Research (TIFR) atomic numbers and mass numbers compare? Are they and the Bhabha Atomic the same element or different elements? Research Centre (BARC), 18. If a bromine atom is available in the form of, say two for peaceful uses of atomic 79 81 isotopes,35Br (49.7%) and 35r (50.3%), calculate the energy to generate electricity, average atomic mass of the bromine atom. support agriculture, and advanced medical treatments. You have learnt that the two atoms that have the same atomic number but different mass numbers are called Ready to Go Beyond isotopes. What if they have the same mass number but Electron microscopes, different atomic numbers? What are such atoms called? Let such as Scanning us find out! Tunnelling Microscopes 8.9.2 Isobars (STMs) and Transmission Electron Microscopes Letusconsiderthreeelements — calcium(atomicnumber 20), (TEMs) can produce potassium(atomicnumber19),andargon(atomicnumber 18). images of materials with These elements have different numbers of protons, yet each atomic-level details. STMs has a mass number of 40. This shows that the total number mainly study surfaces of nucleons in their atoms is the same, even though they are (Fig. 8.15), while TEMs different elements. When atoms of different elements have reveal how atoms are arranged inside very thin the same mass number, but different atomic numbers, they are called isobars. samples. As we end our journey into the structure of the atom, it is important to know that the story does not end here. The exploration of the structure of the atom is still being discovered (Fig. 8.16). Later, scientists discovered that even Bohr’s model was not entirely correct. Electrons do not follow well-defined paths like the fixed Bohr orbits. Today, we understand that they exist as ʻelectron cloudsʼ around Fig. 8.15: STM image showing the nucleus. We can predict regions where they are most individual atoms on a surface Next likely to be, not exactly where they are. You will learn Level all these details in higher grades. The journey of Up exploring the mysteries of the atom is far from over, and exciting discoveries still lie ahead! 156 Exploration|Grade 9