Why do all atoms of an element have the same atomic number, although they may have different mass numbers? What do we call atoms of the same element with different mass numbers? Explain the meaning of each term in the symbol \({ }_{Z}^{A} \mathrm{X}\).

The atomic number is a fundamental characteristic of a chemical element. It is denoted by the symbol 'Z' and represents the number of protons in the nucleus of an atom. Since protons carry a positive charge, the atomic number also indicates the positive charge of the nucleus.

Every atom of a particular element has the same atomic number, making it the defining feature of that element. For instance, every atom of hydrogen has an atomic number of 1, signifying that each hydrogen atom contains exactly one proton. Importantly, the atomic number directly determines an element's position in the periodic table and its chemical properties.

• Hydrogen (H) - Atomic number 1
• Oxygen (O) - Atomic number 8
• Gold (Au) - Atomic number 79

In simple terms, if two atoms have the same atomic number, they are atoms of the same element.

Mass number, represented by 'A', is another crucial term in understanding atomic structure. Unlike the atomic number, which counts protons, the mass number is the sum of the protons and neutrons within an atom's nucleus.

Since neutrons are neutral particles, they do not affect the atomic number. However, they do contribute to the atom's mass which is why atoms with the same number of protons but different numbers of neutrons (and thus different mass numbers) are known as isotopes. Isotopes of an element share chemical properties due to the same number of protons but can have different physical properties due to variations in mass.

For example:

• Carbon-12 (6 protons, 6 neutrons) - Mass number 12
• Carbon-13 (6 protons, 7 neutrons) - Mass number 13
• Carbon-14 (6 protons, 8 neutrons) - Mass number 14

Carbon-12, Carbon-13, and Carbon-14 are all isotopes of the element carbon.

A chemical element is a substance that consists of only one type of atom, indicated by a unique atomic number. Elements are the building blocks of all matter in the universe. Each element has a designated symbol, usually derived from its English or Latin name, that is used in the chemical shorthand to represent it in chemical formulas and equations.

The periodic table organizes the chemical elements in a structured way. It's formatted such that elements with similar chemical behaviors fall into the same groups or columns. The atomic number increases from left to right in the table, making it a straightforward reference for identifying elements and predicting their behavior in chemical reactions.

Examples include:

• Helium (He) - A noble gas with atomic number 2
• Iron (Fe) - A transition metal with atomic number 26
• Uranium (U) - A radioactive element with atomic number 92

Chemistry is deeply rooted in understanding the properties and interactions of these elements.

Isotopes are variants of a chemical element that have the same number of protons but differ in the number of neutrons, leading to different mass numbers. Isotopes play a critical role not only in chemistry but also in various fields such as medicine, archaeology, and environmental science.

Some isotopes are stable, while others are radioactive, meaning they decay over time and emit radiation. This property of radioactive isotopes is harnessed in medical imaging and radiometric dating techniques.

In nature, elements typically occur as a mixture of isotopes. For example, natural chlorine is composed of two stable isotopes, Chlorine-35 and Chlorine-37, in known proportions. Understanding isotopes enriches the comprehension of atomic weight and the intricate balance within atomic nuclei.

Applications of Isotopes:

• Medical diagnostics using radioactive isotopes
• Determining the age of fossils through carbon dating
• Studying environmental changes via oxygen isotope ratios

Isotopes exemplify the diversity within the atomic world, showcasing the variability even among atoms of the same element.