Problem 54 - Chemistry An Atoms First Approach

Question

Which of the following statements is/are true? For the false statements, correct them. a. All particles in the nucleus of an atom are charged. b. The atom is best described as a uniform sphere of matter in which electrons are embedded. c. The mass of the nucleus is only a very small fraction of the mass of the entire atom. d. The volume of the nucleus is only a very small fraction of the total volume of the atom. e. The number of neutrons in a neutral atom must equal the number of electrons.

Step-by-Step Solution

Answer

a. False. Correct statement: "Not all particles in the nucleus of an atom are charged." b. False. Correct statement: "The atom is best described as a central nucleus containing protons and neutrons, surrounded by electrons in orbitals." c. False. Correct statement: "The mass of the nucleus is the majority of the mass of the entire atom." d. True. e. False. Correct statement: "The number of protons in a neutral atom must equal the number of electrons."

Step 1: a. All particles in the nucleus of an atom are charged.

This statement is false. The nucleus of an atom contains protons and neutrons. Protons have a positive charge, but neutrons have no charge (i.e., they are neutral). So, not all particles in the nucleus are charged. A correct statement would be: "Not all particles in the nucleus of an atom are charged."

Step 2: b. The atom is best described as a uniform sphere of matter in which electrons are embedded.

This statement is false. It refers to the plum pudding model, which is an outdated model of the atom. The modern atomic model, known as the Rutherford or Bohr model, consists of a dense central nucleus containing protons and neutrons, surrounded by electrons in orbitals. A correct statement would be: "The atom is best described as a central nucleus containing protons and neutrons, surrounded by electrons in orbitals."

Step 3: c. The mass of the nucleus is only a very small fraction of the mass of the entire atom.

This statement is false. In fact, the nucleus contains most of the mass of the atom, as protons and neutrons are much more massive than electrons (electrons have a negligible mass compared to protons and neutrons). A correct statement would be: "The mass of the nucleus is the majority of the mass of the entire atom."

Step 4: d. The volume of the nucleus is only a very small fraction of the total volume of the atom.

This statement is true. The nucleus is relatively small compared to the size of the atom, and the volume it occupies is only a very small fraction of the total volume of the atom. Most of the atom's volume is occupied by the electron cloud surrounding the nucleus.

Step 5: e. The number of neutrons in a neutral atom must equal the number of electrons.

This statement is false. In a neutral atom, the number of protons must equal the number of electrons in order to balance the positive and negative charges. However, the number of neutrons can vary, leading to the existence of isotopes for each element. A correct statement would be: "The number of protons in a neutral atom must equal the number of electrons."

Key Concepts

Nuclear Particles

The nucleus is the dense central core of an atom where most of its mass is concentrated. It is composed of two types of nuclear particles: protons and neutrons. Protons are positively charged particles, and they determine the chemical identity of the atom, or what element it is. Neutrons, on the other hand, have no charge and contribute to the mass of the nucleus but do not affect the atom's electrical properties.

While protons repel each other due to their like charges, the nucleus stays intact because of the strong nuclear force. This force acts between nuclear particles and is incredibly strong at very short distances, effectively gluing protons and neutrons together.

As seen in the exercise solution, an accurate statement would be that not all particles in the nucleus are charged, since neutrons are neutral. Understanding this concept is crucial for students as it lays the foundation for comprehending more complex topics like isotopes and nuclear reactions.

Atomic Models

Throughout history, atomic models have evolved as scientists discovered more about the nature of matter. The initial model, akin to a 'plum pudding,' was proposed by J.J. Thomson, where the atom was thought to be a sphere of positive charge with electrons dotted throughout like plums in a pudding. However, this model was disproved by Rutherford's gold foil experiment, which led to the planet-like model featuring a central nucleus.

The Rutherford model was refined into the Bohr model, which introduced the concept of electrons orbiting the nucleus in specific layers or shells. Modern quantum mechanics later expanded on this by describing electron positions as probability clouds rather than fixed orbits, revealing that atoms have complex structures that can only be accurately described using quantum mechanics.

In the context of the textbook exercise, it's important to correct the misconception that an atom is a uniform sphere of matter. It's more appropriate to consider the modern understanding of atomic structure, which includes a dense nucleus surrounded by a cloud of electrons occupying a vast volume.

Atom Mass and Volume

The mass of an atom is overwhelmingly concentrated in its nucleus, which contains the heavy protons and neutrons. Electrons, while critical to an atom's properties and reactions, have a mass that's approximately 1/1836 that of protons and neutrons, rendering their contribution to the total atomic mass negligible.

It's a common misconception, as highlighted in the exercise, to think that the nucleus accounts for just a small fraction of the atom's mass. In reality, the nucleus makes up almost the entire mass of the atom. When discussing volume, however, the situation is reversed. The nucleus occupies a minuscule portion of the atom's volume, with the electron cloud taking up the bulk of the space. This disparity leads to a mostly empty structure where the solid core is tiny in comparison to the overall size of the atom. This concept is paramount in understanding the low density of the atomic structure, explaining properties like the compressibility of gases and the probability-based nature of electron positions.

Protons, Neutrons, Electrons

Atoms are the fundamental building blocks of matter, and they consist of three primary particles: protons, neutrons, and electrons. Protons and neutrons form the nucleus at the center of the atom, while electrons inhabit the space around the nucleus.

Protons carry a positive electric charge, and their number defines the atomic number, which determines the element's identity in the periodic table. Neutrons, as stated earlier, are neutral and add to the atomic mass without changing the elemental identity. Electrons are negatively charged and are responsible for most of the chemical reactions and bonds, as they are the parts of the atom that interact the most with other atoms.

In a neutral atom, the number of protons equals the number of electrons, ensuring the overall electrical neutrality. However, the neutron number can vary even among atoms of the same element, leading to various isotopes with different physical properties. This variation does not affect the charge but can significantly impact the stability and behavior of the atom, as was clarified in the exercise solutions.