Problem 207 - Chemistry Matter and Change

Question

Contrast atomic number and mass number. Compare these numbers for isotopes of an element. (Chapter 4\()\)

Step-by-Step Solution

Answer

The atomic number indicates protons and remains constant for isotopes; mass number is the sum of protons and neutrons and varies among isotopes.

Step 1: Define Atomic Number

The atomic number of an element, denoted as \( Z \), represents the number of protons found in the nucleus of an atom of that element. It also determines the chemical properties of the element and its position in the periodic table.

Step 2: Define Mass Number

The mass number, denoted as \( A \), is the total number of protons and neutrons present in the nucleus of an atom. It is calculated as \( A = Z + N \), where \( N \) is the number of neutrons.

Step 3: Understand Isotopes

Isotopes are variants of a particular chemical element that have the same number of protons (atomic number \( Z \)) but different numbers of neutrons, leading to different mass numbers \( A \).

Step 4: Compare Atomic Number and Mass Number for Isotopes

For isotopes of an element, the atomic number \( Z \) remains constant as they belong to the same element. However, the mass number \( A \) varies among isotopes due to the different number of neutrons. For example, carbon-12 and carbon-14 are isotopes of carbon with the atomic number 6 but with mass numbers 12 and 14, respectively.

Key Concepts

Atomic Number

The atomic number, symbolized as \( Z \), is an essential identity marker for each element on the periodic table. It indicates the number of protons present in the nucleus of an atom, and this number is crucial for defining the element's chemical behavior.

• The atomic number is unique to each element, meaning no two different elements can have the same atomic number.
• For example, hydrogen has an atomic number of 1, while helium has an atomic number of 2. This means hydrogen atoms have one proton and helium atoms have two protons.

Because the atomic number is fundamental in understanding an element’s identity, it also determines the position of the element in the periodic table. All atoms of a single element share the same number of protons, which ensures consistency in their chemical properties, even if other subatomic particles vary.

Mass Number

Mass number, noted by the symbol \( A \), is the sum of the protons and neutrons in the atom's nucleus. While the atomic number focuses solely on protons, the mass number gives a fuller picture of an atom's composition.

• This number is not typically found on the periodic table, as it can vary for atoms of the same element due to different numbers of neutrons.
• The formula to determine the mass number is \( A = Z + N \), where \( Z \) is the atomic number and \( N \) represents the number of neutrons.

The mass number is especially important in identifying isotopes, which are atoms of the same element that have different numbers of neutrons and, consequently, different mass numbers. For instance, the isotope carbon-14 has a mass number of 14 because it contains 6 protons and 8 neutrons.

Neutrons

Neutrons are neutrally charged particles located within an atom's nucleus alongside protons. While they do not influence the chemical properties directly like protons do, neutrons are crucial in stabilizing the nucleus, preventing the repulsive forces between positively charged protons from tearing it apart.

• The number of neutrons can vary in the atoms of an element, leading to the formation of isotopes.
• For example, while every atom of carbon has 6 protons (atomic number 6), the number of neutrons can be different, resulting in isotopes like carbon-12 (6 neutrons) and carbon-14 (8 neutrons).

Through this variability, neutrons contribute to the different mass numbers of isotopes and can impact an atom’s stability. In nature, elements exist as a mixture of isotopes, each with its unique mass number due to differing neutron numbers.