Chemistry eText

Standard 1f Preknowledge

1f) Students know the three most common forms of radioactive decay (alpha, beta, and gamma) and know how the nucleus changes in each type of decay.

CALIFORNIA FRAMEWORKS SUMMARY: Radioactive isotopes transform to more stable isotopes, emitting particles from the nucleus. These particles are helium-4 nuclei (alpha radiation), electrons or positrons (beta radiation), or high-energy electromagnetic rays (gamma radiation). Isotopes of elements that undergo alpha decay produce other isotopes with two less protons and two less neutrons than the original isotope. Uranium-238, for instance, emits an alpha particle and becomes thorium-234.

Isotopes of elements that undergo beta decay produce elements with the same number of nucleons but with one more proton or one less proton. For example, thorium-234 beta decays to protactinium-234, which then beta decays to uranium- 234. Alpha and beta decay are ionizing radiations with the potential to damage surrounding materials. After alpha and beta decay, the resulting nuclei often emit high-energy photons called gamma rays. This process does not change the number of nucleons in the nucleus of the isotope but brings about a lower energy state in the nucleus.

Nuclear Decay Organizer

Alpha Particle Emission

Beta Particle Emission

Gamma Ray Emission

Symbol

⁴₂He²⁺ or ⁴₂a ²⁺

⁰_-1e or ⁰_-1b

Mass

Heavy

Light

No Mass

How it changes the nucleus

Decreases the mass number by 4
Decreases the atomic number by 2

Converts a neutron into a proton
Increases atomic number by 1

No change to the nucleus

Balancing Nuclear Equations

While the state standards do not explicitly say that students must be able to balance nuclear equations, these equations have appeared in released test questions. In addition, it is beneficial to students to learn how to balance these equations, because doing so drives home the manner in which the nucleus is altered by the various forms of decay.

The Basics:

Example:

⁹₄Be + ¹₁H → ⁶₃Li + ⁴₂He

A balanced nuclear equation is simply a mathematic expression of equivalence. In fact, it may be beneficial to think of the "yield" sign (→) as an equal (=) sign.

⁹₄Be + ¹₁H = ⁶₃Li + ⁴₂He

The sum of the atomic numbers (nuclear charge) on the left of the yield sign must be equal to the sum of the atomic numbers (nuclear charge) on the right:

4 + 1 = 3 + 2

The sum of the mass numbers on the left of the yield sign must be equal to the sum of the mass numbers on the right:

9 + 1 = 6 + 4

Working with Coefficients:

Coefficients are dealt with algebraically as well, multiplying both the atomic number and mass number of the particle to which it is attached.

Example:

²³⁵₉₂U + ¹₀n → ¹⁴²₅₆Ba + ⁹¹₃₆Kr + 3¹₀n

Atomic numbers (nuclear charges):

92 + 0 = 56 + 36 + 3(0)

Mass numbers:

235 + 1 = 142 + 91 + 3(1)

Now, try this review activity to test your understanding of these concepts.

ScienceGeek.net

California Standards Chemistry

Standard 1f Preknowledge

Nuclear Decay Organizer

Balancing Nuclear Equations