Question

Give the atomic number \((Z)\) and mass number \((A)\) for each of the following: a. an oxygen atom with 8 neutrons b. a chlorine atom with 18 neutrons c. a sodium atom with 12 neutrons d. a uranium atom with 143 neutrons

Short answer

a. Oxygen: Z = 8, A = 16. b. Chlorine: Z = 17, A = 35. c. Sodium: Z = 11, A = 23. d. Uranium: Z = 92, A = 235.

Step-by-step solution

Understand Atomic Number and Mass Number

The atomic number (Z) of an element corresponds to the number of protons in the nucleus of an atom, which is unique for each element. The mass number (A) is the sum of the number of protons and neutrons in the nucleus of an atom.

Determine Oxygen Atomic Number and Mass Number

Oxygen has an atomic number (Z) of 8, which means it has 8 protons. To find its mass number (A), add the number of protons to the number of neutrons: A = 8 protons + 8 neutrons = 16.

Determine Chlorine Atomic Number and Mass Number

Chlorine has an atomic number (Z) of 17, which means it has 17 protons. For its mass number (A), add the number of protons to the number of neutrons: A = 17 protons + 18 neutrons = 35.

Determine Sodium Atomic Number and Mass Number

Sodium has an atomic number (Z) of 11, indicating it has 11 protons. To find its mass number (A), calculate the sum of protons and neutrons: A = 11 protons + 12 neutrons = 23.

Determine Uranium Atomic Number and Mass Number

Uranium has an atomic number (Z) of 92, which tells us there are 92 protons. Find its mass number (A) by adding the number of protons to the number of neutrons: A = 92 protons + 143 neutrons = 235.

Key concepts

Atomic Number

The atomic number, denoted as \( Z \), is a fundamental characteristic of an element that defines its identity. It represents the number of protons present in the nucleus of an atom. Since each element has a unique number of protons, the atomic number effectively serves as an element's fingerprint.

For instance, oxygen always has 8 protons, so its atomic number is 8. This number can be found on the periodic table and is used to distinguish one element from another. Understanding the atomic number is crucial for grasping the basics of chemistry, as it sets the stage for chemical properties and behaviors in different substances.

Mass Number

Moving beyond the atomic number, the mass number, represented by \( A \), is the sum total of protons and neutrons residing in an atom's nucleus. Since protons and neutrons both carry a weight close to 1 atomic mass unit each, the mass number roughly indicates the atom's actual mass.

For a given oxygen atom with 8 neutrons, the mass number is \( 8 (protons) + 8 (neutrons) = 16 \). The mass number provides essential information, especially when discussing isotopes and understanding the stability of an atom. It's important to realize that while the atomic number is fixed for an element, the mass number can vary due to the presence of different numbers of neutrons.

Protons and Neutrons

Protons and neutrons are the subatomic particles that contribute to the majority of an atom's mass and are found in the nucleus. Protons are positively charged, and their number dictates the atomic number and, consequently, the chemical identity of an atom. Neutrons, on the other hand, have no charge but play a crucial role in adding mass and stability to the nucleus.

Understandably, the number of protons in an atom remains constant for a given element, while neutrons may vary, leading to the formation of isotopes. Protons and neutrons closely interact through the strong nuclear force, one of the four fundamental forces in physics, which holds the nucleus together despite the electrostatic repulsion between positively charged protons.

Isotopes

Isotopes are variant forms of a particular chemical element, sharing the same number of protons (and hence, atomic number) but differing in the number of neutrons, resulting in a variation of their mass number. This isotopic diversity allows for the existence of atoms of the same element that have different physical, and sometimes chemical, properties.

For example, carbon-12 and carbon-13 are both isotopes of carbon, possessing 6 protons each but with 6 and 7 neutrons respectively. These differences can greatly influence the stability and radioactive nature of the isotopes. Isotopes prove critical in various scientific disciplines, from tracking chemical processes in the environment to medical diagnostic imaging techniques.