Question

State the number of neutrons in an atom of each of the following isotopes: (a) \({ }_{10}^{20} \mathrm{Ne}\) (b) \({ }_{18}^{40} \mathrm{Ar}\) (c) \({ }_{36}^{80} \mathrm{Kr}\) (d) \({ }_{54}^{131} \mathrm{Xe}\)

Short answer

Ne-20: 10 neutrons, Ar-40: 22 neutrons, Kr-80: 44 neutrons, Xe-131: 77 neutrons.

Step-by-step solution

Understanding Atomic Number and Mass Number

For each isotope symbol, the subscript represents the atomic number (protons), and the superscript represents the mass number (protons + neutrons). To find neutrons, subtract the atomic number from the mass number.

Calculating Neutrons in Neon-20

Neon's isotope symbol is \({ }_{10}^{20} \mathrm{Ne}\). The atomic number (10) indicates 10 protons. The mass number (20) indicates the total count of protons and neutrons.\[ 20 - 10 = 10 \] Therefore, Neon-20 has 10 neutrons.

Calculating Neutrons in Argon-40

Argon's isotope symbol is \({ }_{18}^{40} \mathrm{Ar}\). The atomic number (18) indicates 18 protons. The mass number (40) gives the sum of protons and neutrons.\[ 40 - 18 = 22 \] Thus, Argon-40 has 22 neutrons.

Calculating Neutrons in Krypton-80

Krypton's isotope symbol is \({ }_{36}^{80} \mathrm{Kr}\). The atomic number (36) refers to 36 protons. The mass number (80) includes both protons and neutrons.\[ 80 - 36 = 44 \] Hence, Krypton-80 has 44 neutrons.

Calculating Neutrons in Xenon-131

Xenon's isotope symbol is \({ }_{54}^{131} \mathrm{Xe}\). The atomic number (54) denotes 54 protons. The mass number (131) represents total protons and neutrons.\[ 131 - 54 = 77 \] Therefore, Xenon-131 contains 77 neutrons.

Key concepts

Atomic Number

The concept of atomic number is one of the fundamental building blocks of understanding chemistry and atomic structure. Every element on the periodic table is assigned a unique atomic number.

It's all about the number of protons in the nucleus.

• The atomic number, denoted as a subscript in the isotope symbol, is crucial because it defines the identity of the element.
• For instance, hydrogen has an atomic number of 1 because it has just one proton.
• Neon, with an atomic number of 10, has 10 protons in its nucleus.

Even though the atomic number is small, its significance is immense. It forms the basis for the Periodic Table organization, and it also tells us how the elements will combine and react chemically with others.

Mass Number

Mass number is another critical concept essential to understanding isotopes and atomic structures. In isotope notation, the mass number is the superscript.

It represents the sum of both protons and neutrons in the nucleus:

• The mass number is not found on the periodic table as it can vary for isotopes of the same element.
• For instance, Argon-40 has a mass number of 40, signifying protons plus neutrons together equal 40.
• Krypton-80, therefore, tells us that the total count of protons and neutrons is 80.

Expression of the mass number helps us precisely determine the isotopic identity of each atom and how heavy the atom is (leaving out electron mass, which is trivial compared to protons and neutrons).

Isotope Calculations

Isotope calculations are vital for digging deeper into the finer details of atomic structure, particularly when figuring out the number of neutrons present in an isotope.

Here's how we do it:

• The rule of thumb is simple: subtract the atomic number from the mass number to find the number of neutrons.
• This step is essential because each isotope of an element has the same number of protons but a different number of neutrons.
• For example, Xenon-131 has an atomic number of 54, meaning it has 54 protons.
  To find the number of neutrons:
  \[ 131 - 54 = 77 \] This tells us that there are 77 neutrons in this particular isotope of Xenon.

Practicing isotope calculations helps students understand not only nuclear compositions but also the concept of isotopic stability and abundance, which play crucial roles in different scientific fields like chemistry, physics, and even archaeology.