Animated Solution for Physics - Atoms and Nuclei: Which of the following is a correct statement ?

When early physicists began exploring the mysterious world of radioactivity and atomic structure, they encountered various types of "rays" and particles. To understand the universe at a fundamental level, they had to decode exactly what these emissions were made of. Let's take a thrilling journey through the options provided in this problem and uncover the true identities of these subatomic phenomena.

Cathode Rays

Let's start with the first statement. What exactly are beta rays? When certain unstable atomic nuclei undergo radioactive decay, they eject high-speed particles to reach a more stable state. These ejected particles are called beta rays, and physicists soon realized they were simply fast-moving electrons ($e^{-}$).

Now, what about cathode rays? In the late 19th century, J.J. Thomson experimented with vacuum tubes and observed a glowing beam traveling from the cathode to the anode. He discovered that these "cathode rays" were also streams of negatively charged particles—electrons!

Since both beta rays and cathode rays are fundamentally just streams of electrons, they are essentially the same thing, differing only in their origin. Therefore, the statement "Beta rays are same as cathode rays" is absolutely correct.

The second statement claims that gamma rays are high-energy neutrons. This is a classic misconception! Gamma rays ($\gamma$) are not particles with rest mass like neutrons or protons. Instead, they are highly energetic, high-frequency electromagnetic waves. They belong to the same family as visible light, radio waves, and X-rays, but pack a much more powerful punch. Because they are photons, they have zero rest mass and no electrical charge. Thus, they are definitely not neutrons.

Next, let's examine the claim that alpha particles are singly ionized helium atoms. To break this down, we need to look at the structure of a helium atom. A neutral helium atom consists of a nucleus (containing two protons and two neutrons) surrounded by two electrons.

If you strip away one electron, you get a singly ionized helium atom ($H{e}^{+}$), which still retains one electron in its orbit. However, an alpha particle ($\alpha$) is purely the helium nucleus itself. It has been stripped of both of its electrons, making it a doubly ionized helium atom ($H{e}^{2+}$). Because an alpha particle has no electrons at all, the statement is incorrect.

Finally, do protons and neutrons have exactly the same mass? While they are incredibly close, nature loves subtle differences. A free neutron is slightly heavier than a proton.

Mathematically, the mass of a neutron is $m_n\approx 1.008665\text{ u}$, whereas the mass of a proton is $m_p\approx 1.007276\text{ u}$. This tiny mass difference is profoundly important in nuclear physics; it is the very reason why a free neutron is unstable and can decay into a proton, an electron, and an antineutrino. Therefore, they do not have exactly the same mass.

After rigorously analyzing each option, the physics is clear. The only scientifically accurate statement is that beta rays and cathode rays are the same, as both are composed of electrons.