Question

The _________ number represents the sum of the number of protons and neutrons in a nucleus.

Short answer

The \(\textbf{atomic mass number}\) represents the sum of the number of protons and neutrons in a nucleus.

Step-by-step solution

Understanding the structure of an atom

An atom is the smallest unit of matter. It consists of a nucleus, which is the central part, and electrons that revolve around the nucleus. The nucleus contains protons, which are positively charged particles, and neutrons, which are neutral particles (no charge).

Define the term representing the sum of protons and neutrons

The term that represents the sum of the number of protons and neutrons in a nucleus is called the atomic mass number (also referred as mass number). The atomic mass number is denoted by the symbol "A."

Fill in the blank with the correct term

Now that we know the term representing the sum of the number of protons and neutrons in a nucleus, we can fill in the blank with the correct term: The atomic mass number represents the sum of the number of protons and neutrons in a nucleus.

Key concepts

Protons

Protons are fundamental particles found in the nucleus of an atom. They have a positive electrical charge, which is denoted by a plus sign (+). This positive charge plays a crucial role in the atom's interactions and stability.
Due to their charge, protons help to balance the charge of electrons, which are negatively charged. This balance is essential for atomic structure and influences chemical behavior.

• Protons are part of the mass of an atom, contributing significantly to the atomic mass number.
• The number of protons defines the element; for example, hydrogen has one proton, while helium has two.

This means that the number of protons differentiates one element from another. In a neutral atom, the number of protons equals the number of electrons, ensuring the atom has no overall charge.

Neutrons

Unlike protons, neutrons have no electrical charge, which is why they are called 'neutral' particles. Neutrons are also found in the nucleus and, along with protons, they make up most of an atom's mass.
Neutrons play a fundamental role in stabilizing the nucleus. They create a buffer between the positively charged protons, reducing repulsive forces that would otherwise push protons apart.

• Neutrons contribute to the atomic mass number, just like protons.
• The number of neutrons can vary in atoms of the same element, leading to different isotopes.

This variability in neutron number explains why some isotopes are more stable than others and affects atomic behavior.

Atom Structure

Atoms are the basic building blocks of all matter. They are composed of a nucleus surrounded by electrons. The structure of an atom determines its properties and behaviors.
The nucleus, sitting at the heart of the atom, contains protons and neutrons, contributing to the atom's mass. Electrons circle this dense core in various energy levels (or shells), determining the atom's size.

• Electrons in outer shells are involved in chemical reactions and bonding.
• The arrangement of neutrons affects the stability of the nucleus.

Understanding atom structure helps explain the behavior of elements in reactions and the periodic table's arrangement.

Nucleus

The nucleus is the atom's central part, containing protons and neutrons. It is extremely dense compared to the rest of the atom, with most of the atom's mass concentrated here.
The nuclear forces between protons and neutrons keep the nucleus intact despite the repulsive forces between protons.

• The nucleus defines critical properties like the atom's identity (via protons) and isotopic nature (via neutrons).
• The strong nuclear force is crucial to holding the nucleus together, balancing these electric charges.

Because of its composition, the nucleus plays a key role in nuclear reactions and stability, as well as energy production.

Mass Number

The mass number, often called the atomic mass number, is notable in characterizing and identifying elements. It is the sum of the number of protons and neutrons present in the nucleus of an atom, symbolized by 'A'.
This number provides significant insight into the atom's isotopic identity and is crucial for calculating atomic mass.

• Mass number is not the same as atomic weight, as it does not involve electrons.
• Each isotope of an element has a distinct mass number.

Using mass numbers helps differentiate atoms that have the same number of protons but different numbers of neutrons, offering a deeper understanding of atomic structure and periodic trends.