SAT Subject Test Chemistry




Atomic Structure and the Periodic Table of the Elements


While the early separation experiments were in progress, an understanding was slowly being gained of the nature of the spontaneous emission from the various radioactive elements. Becquerel thought at first that there were simply X-rays, but THREE different kinds of radioactive emission, now called alpha particles, beta particles, and gamma rays, were soon found. We now know that alpha particles are positively charged particles of helium nuclei, beta particles are streams of high-speed electrons, and gamma rays are high-energy radiations similar to X-rays. The emission of these three types of radiation is depicted below.

The important characteristics of each type of radiation can be summarized as follows:

Alpha Particle     ((helium nucleus ) Positively charged, 2+

1. Ejection reduces the atomic number by 2, the atomic weight by 4 amu.

2. High energy, relative velocity.

3. Range: about 5 cm in air.

4. Shielding needed: stopped by the thickness of a sheet of paper, skin.

5. Interactions: produces about 100,000 ionizations per centimeter; repelled by the positively charged nucleus; attracts electrons, but does not capture them until its speed is much reduced.

6. An example: Thorium-230 has an unstable nucleus and undergoes radioactive decay through alpha emission. The nuclear equation that describes this reaction is:

     In a decay reaction like this, the initial element (thorium-230) is called the parent nuclide and the resulting element (radium-226) is called the daughter nuclide.

Beta Particle     (fast electron) Negatively charged, 1−

1. Ejected when a neutron decays into a proton and an electron.

2. High velocity, low energy.

3. Range: about 12 m.

4. Shielding needed: stopped by 1 cm of aluminum or thickness of average book.

5. Interactions: weak because of high velocity, but produces about 100 ionizations per centimeter.

6. An example: Protactinium-234 is a radioactive nuclide that undergoes beta emission. The nuclear equation is:

Gamma Radiation     (electromagnetic radiation identical with light; high energy) No charge

1. Beta particles and gamma rays are usually emitted together; after a beta is emitted, a gamma ray follows.

2. Arrangement in nucleus is unknown. Same velocity as visible light.

3. Range: no specific range.

4. Shielding needed: about 13 cm of lead.

5. Interactions: weak of itself; gives energy to electrons, which then perform the ionization.