Question

Where in an atom is nearly all of its mass concentrated? Explain your answer in terms of the particles that contribute to this mass.

Short answer

Nearly all of an atom's mass is concentrated in its nucleus, which is composed of protons and neutrons, the particles that contribute most to the atom's mass.

Step-by-step solution

Identifying the Atomic Nucleus

The majority of an atom's mass is concentrated in its nucleus, which is the central part of the atom.

Understanding Atomic Particles

The nucleus contains protons and neutrons, which are referred to as nucleons. These particles are much heavier than electrons, which orbit the nucleus.

Comparing Particle Masses

Protons and neutrons each have a mass of approximately 1 atomic mass unit (amu), while electrons have a mass of about 0.0005 amu. Thus, since electrons contribute negligibly to the total mass, nearly all the mass is due to protons and neutrons in the nucleus.

Key concepts

Atomic Nucleus

The atomic nucleus is the very heart of an atom, a dense region residing at its center. It was discovered by Ernest Rutherford in 1911 through his famous gold foil experiment which revealed that atoms have a small, positively charged core surrounded by a cloud of negatively charged electrons. This central core, or nucleus, is incredibly dense and holds nearly all of an atom's mass.

Inside the nucleus, we find two types of subatomic particles—protons and neutrons—that largely determine an atom's mass and its chemical properties. While electrons do play a crucial role in chemical reactions and bonds, their contribution to the atom's mass is minimal, making the nucleus the major contributor to the mass of an atom.

Protons and Neutrons

Protons and neutrons, collectively known as nucleons, are the fundamental components of an atomic nucleus. Protons are positively charged particles, and their number in the nucleus determines the chemical element—for instance, hydrogen has one proton, while carbon has six. Neutrons, on the other hand, carry no charge and thus are neutral. However, they play a crucial role in providing stability to the nucleus by mitigating the electrostatic repulsion between protons.

Both protons and neutrons have a similar mass, which is substantially greater than that of electrons. Without the presence of neutrons, protons would repel each other due to their like charges, leading to an unstable nucleus. Neutrons help to 'glue' these protons together, allowing the nucleus to exist in a stable state.

Nucleons

The term 'nucleons' refers to the particles contained within the nucleus of the atom, namely protons and neutrons. These particles are bound together by the strong nuclear force, which, despite being powerful, acts over an extremely small range.

The number of nucleons in an atom's nucleus, known as the mass number, is key to understanding an element's atomic mass. The mass number is the sum of the number of protons and neutrons. For most elements, it's possible to have varying numbers of neutrons, resulting in different isotopes of the same element. Isotopes may have different physical properties but share chemical characteristics because they have the same number of protons.

Atomic Mass Unit

When discussing the mass of nucleons, we often use the atomic mass unit (amu), a standard unit that expresses the mass of atoms and subatomic particles. One atomic mass unit is defined as one-twelfth the mass of an atom of carbon-12, which has six protons and six neutrons.

By this convention, the mass of a proton and a neutron is approximately 1 amu, and the electron's mass is so small (about 0.0005 amu) that it's often considered negligible when calculating the mass of an atom. Therefore, the atomic mass of an element, usually found under the symbol of the element on the periodic table, closely approximates the sum of the masses of the protons and neutrons in the atomic nucleus, reflecting the mass number of the most common isotope of the element.