91Ó°ÊÓ

The atomic mass of an element is a critical value in understanding isotopes. It is represented by the upper number in isotope notation and is the sum of the protons and neutrons in the nucleus. For each isotope symbol, it shows the total number of these subatomic particles.
For example, in the isotope \(rac{197}{79}\ \text{Au}\), the atomic mass (A) is 197. This includes both the protons and neutrons within the nucleus of this particular isotope of gold.

• Gold: 197 (from \(\frac{197}{79}\ \text{Au}\))
• Sodium: 23 (from \( \frac{23}{11}\ \text{Na} \))
• Plutonium: 239 (from \( \frac{239}{94}\ \text{Pu} \))

Understanding atomic mass is essential because the isotopes of an element will have the same atomic number but different atomic masses due to varying numbers of neutrons.

The atomic number (Z) is vital for identifying elements in the periodic table, as it defines the element's identity by indicating the number of protons in its nucleus.
It is represented by the lower number in isotope notation. For example, in \(\frac{197}{79}\ \text{Au}\), 79 represents the atomic number of gold.

• The atomic number of Gold (Au) is 79.
• The atomic number of Sodium (Na) is 11.
• The atomic number of Plutonium (Pu) is 94.

These atomic numbers offer a unique identity to elements, distinguishing one from another. For atoms that are neutral, the number of electrons equals the atomic number, ensuring electrical neutrality by balancing the positive charge of protons.

Understanding the simple calculations of subatomic particles—protons, neutrons, and electrons—is crucial for any student studying chemistry. Each plays a specific role in defining the characteristics of an element.

Protons

These positively charged particles are found in the nucleus and define the element. The atomic number already tells us how many protons are there.

Neutrons

The number of neutrons affects the isotope of the element and can be calculated by subtracting the atomic number from the atomic mass. For example:

• In gold (\( \frac{197}{79}\ \text{Au} \)): 118 neutrons (197 - 79).
• In sodium (\( \frac{23}{11}\ \text{Na} \)): 12 neutrons (23 - 11).
• In plutonium (\(\frac{239}{94}\ \text{Pu}\)): 145 neutrons (239 - 94).

Electrons

In a neutral atom, the number of electrons is equal to the number of protons. This makes the atom electrically neutral. Thus:

• Gold has 79 electrons.
• Sodium has 11 electrons.
• Plutonium has 94 electrons.

These calculations help reveal the structure of an atom, showcasing its balance of charges and its specific isotope characteristics.