Radioactivity and Nuclear Physics

Radioactive substances emit alpha particles (helium nuclei), beta particles (electrons), and gamma rays (electromagnetic radiation). They do NOT emit free neutrons - neutrons are only released during nuclear fission, not standard radioactive decay.

Irene Joliot-Curie and Frederic Joliot Curie jointly discovered artificial radioactivity. These isotopes rapidly became important tools in biomedical research and in the treatment of cancer and related diseases. 

An atomic pile (early nuclear reactor) produces radioactive substances through controlled nuclear reactions. The pile sustains a chain reaction that creates radioactive isotopes through neutron bombardment of stable nuclei. Wilson cloud chambers detect radiation tracks, electronic guns emit electrons in vacuum tubes, and transistors are semiconductor devices - none produce radioactivity.

Gamma rays have the highest penetrating power among all electromagnetic radiations due to their high energy and lack of charge, allowing them to pass through most materials. Alpha particles are least penetrating (stopped by paper), beta particles have moderate penetration (stopped by aluminum), and X-rays though penetrating are less so than gamma rays.

Uranium-238 undergoes a series of radioactive decays (the uranium decay series) through multiple intermediate elements including radium, thorium, and polonium, ultimately stabilizing as lead-206. This decay chain is used in uranium-lead dating of very old rocks and minerals.

Alpha particles have the least penetrating power because they are heavy (helium nuclei) and carry double positive charge, causing them to interact strongly with matter. They can be stopped by a sheet of paper or few centimeters of air. Beta particles (electrons) penetrate further, and gamma rays (high-energy photons) have the highest penetration, requiring dense materials like lead for shielding.

Protium (¹H), Deuterium (²H or D), and Tritium (³H or T) are the three isotopes of hydrogen, differing only in neutron count. Yttrium (Y) is a separate transition metal element (atomic number 39) in Group 3 of the periodic table, not related to hydrogen.

Curie is the traditional unit of radioactivity, defined as 3.7 × 10^10 disintegrations per second (originally based on the activity of 1 gram of radium-226). The SI unit is becquerel (Bq), where 1 curie = 3.7 × 10^10 Bq. Einstein and Planck are units used in photochemistry and quantum mechanics respectively, not radioactivity.

A physical change doesn't produce new substances and is often reversible. Magnetizing a needle only aligns magnetic domains - the material remains the same and can be demagnetized. Radioactive decay (A) changes elements, photosynthesis (B) produces glucose from CO₂ and water, and rusting (D) creates iron oxide - all are chemical changes.

1. Alpha partices lack penetration power but they have more energy than beta. Alpha particles cannot penetrate the skin to reach the internal organs whereas beta particles can damage internal organs.
   1. Radioactive iodine - 131 accumulates in thyroid gland and causes cancer. The cancer occurs from residual tissue radiation damage caused by the I¹³¹, and usually appear years after exposure, long after the I¹³¹ has decayed.

1. The nuclear fuels uranium-235 and plutonium-239 are the materials from which nuclear weapons are made.
   1. Radioactive radon gas is a colourless gas produced from the decay of uranium.

Radium and francium are radioactive elements.

The sentence asks what "radioactive" means. "Means" is the correct verb to explain the definition of a term. "Explains" would need the term as subject (radioactivity explains), "gives" doesn't fit semantically, and "answers" would require the term to answer something, which is the opposite of what's happening here.

The text describes how heavy atoms "break down and change into other kinds" and calls this process "radioactivity." This breakdown is scientifically called "decay" - radioactive decay. "Tear off" is too informal, "failure" doesn't describe the transformation process, and "build up" is the opposite of what happens.

Each radioactive element disintegrates (breaks down) by giving off rays. "Disintegrates" precisely captures the atomic breakdown process in radioactive decay. "Divides" implies splitting into equal parts, "multiplies" means increasing in number, and "freezes" has no relevance to radioactive processes.