1.1.5 Electronic Structure

In this lesson, we will explore the sizes of atoms and their nuclei, the distribution of mass within an atom, and the relative masses of protons, neutrons, and electrons. We will also learn how to calculate the numbers of protons, neutrons, and electrons in an atom or ion based on its atomic number and mass number.

Sizes of Atoms and Nuclei

Atoms are incredibly small particles, with an approximate radius of about 0.1 nanometers (1 x 10^-10 m). Comparatively, the nucleusA membrane-bound organelle in eukaryotic cells that contains DNA. of an atom is much smaller, with a radius that is less than 1/10,000 of the atom's size, approximately 1 x 10^-14 m. The nucleus occupies a minuscule portion of the total volume of an atom.

Mass Distribution in Atoms

Almost all of the mass of an atom is concentrated within its nucleus. The nucleus contains protons and neutrons, collectively known as nucleons. Electrons, although having negligible mass, contribute to the overall charge and chemical behaviour of the atom.

Relative Masses of Subatomic Particles

The relative masses of subatomic particles are as follows:

• Proton: 1 (relative mass)
• Neutron: 1 (relative mass)
• Electron: Very small (negligible mass compared to protons and neutrons)

Protons and neutrons have approximately the same mass, while electrons are significantly lighter.

Mass Number and Isotopes

The sum of the protons and neutrons in an atom is known as its mass number. It represents the total number of nucleons in the nucleus. Atoms of the same element can have different numbers of neutrons, leading to variations called isotopes. Isotopes have the same number of protons (atomic number) but differ in the number of neutrons, thus resulting in different mass numbers.

Atomic Representation

Atoms can be represented using a notation that indicates their mass number and atomic number. For example:

In this example, "23" represents the mass number (sum of protons and neutrons) of the sodium atom, and "11" represents the atomic number (number of protons) of sodium.

Calculating Protons, Neutrons, and Electrons

Given the atomic number and mass number of an atom or ion, we can calculate the numbers of protons, neutrons, and electrons. The atomic number gives the number of protons, which is also equal to the number of electrons in a neutral atom. The difference between the mass number and the atomic number provides the number of neutrons.

Relating Atom Size to Objects in the Physical World

To gain a better understanding of the size of atoms, let's compare them to objects in the physical world. Consider the following example:

A typical atom has a diameter of about 0.1 nanometers. Comparatively, the width of a human hair is around 50,000 to 100,000 nanometers. This means that the diameter of an atom is approximately 500,000 to 1,000,000 times smaller than the width of a human hair.

Conclusion

In conclusion, atoms are incredibly small, with most of their mass concentrated within the nucleus. Protons and neutrons have similar relative masses, while electrons have negligible mass compared to nucleons. The sum of protons and neutrons in an atom gives its mass number. Atoms of the same element can have different numbers of neutrons, resulting in isotopes. Atom representation includes the mass number and atomic number. By knowing the atomic number and mass number, we can calculate the numbers of protons, neutrons, and electrons in an atom or ion.