Question

How is atomic number different from mass number?

Short answer

The atomic number is the number of protons, while the mass number is the sum of protons and neutrons.

Step-by-step solution

Define Atomic Number

The atomic number is the number of protons found in the nucleus of an atom. It uniquely identifies a chemical element, as each element has a different number of protons.

Define Mass Number

The mass number, also known as atomic mass number, is the total number of protons and neutrons in an atom's nucleus. It is not unique to a chemical element because isotopes of an element can have different mass numbers.

Highlight Key Difference

The key difference between atomic number and mass number is that the atomic number is solely the count of protons, which defines the type of element, while the mass number is the sum of both protons and neutrons, indicating the total mass of the nucleus.

Key concepts

Atomic Number

Atomic number is a fundamental attribute of an element, setting it apart from all others. It represents the number of protons located in the nucleus of an atom. This proton count is crucial, as it determines not only the atomic number but also which element the atom constitutes. Each element on the periodic table has its own distinct atomic number. For instance, hydrogen has an atomic number of 1 because it has one proton. If an atom has an atomic number of 6, it must be carbon, as carbon atoms possess six protons. This distinctive feature makes the atomic number the defining characteristic of an element.

Mass Number

The mass number is the culmination of the total count of protons and neutrons within an atom's nucleus. Unlike the atomic number, it is not unique to any one element. Elements can exist in different forms known as isotopes, each with varying numbers of neutrons, thus differing mass numbers. For example, carbon-12 and carbon-14 are isotopes of carbon, where the numbers represent their respective mass numbers. Both isotopes contain six protons, but while carbon-12 has six neutrons, carbon-14 has eight. This additional neutron content in isotopes like carbon-14 leads to a higher mass number, showcasing the mass number as a summation of both protons and neutrons.

Protons

Protons are subatomic particles that reside within the nucleus of an atom. Each proton carries a positive electrical charge, which is balanced by negatively charged electrons orbiting the nucleus. The number of protons is critical as it is synonymous with the atomic number and directly corresponds to an element's identity.

• Atoms of the same element always have the same number of protons.
• A change in this number transforms the element into another, entirely different, element.

Therefore, understanding protons is essential, as they are a pivotal part of an atom’s nucleus and play a significant role in the chemical behavior of elements.

Neutrons

Neutrons are neutral subatomic particles found in the nucleus of an atom. They carry no electrical charge, distinguishing them from protons and electrons. Neutrons contribute to the mass of an atom but do not affect its chemical identity.

• Alongside protons, neutrons add weight to the nucleus, influencing the atom's mass number.
• Neutrons help to stabilize the nucleus; too few or too many can lead to radioactive decay.

In isotopes, variations in neutron number among atoms of the same element create different isotopic forms: the primary distinctions being mass number and stability rather than chemical properties. Therefore, neutrons play a vital role in the structure and stability of atoms across the periodic table.