Fundamentals of Atomic Theory

The article provides an overview of atomic theory, focusing on the structure of matter, atoms, and the periodic table, as well as the behavior and arrangement of subatomic particles like electrons, protons, and neutrons. It also introduces concepts such as atomic weight, energy levels, electron shells, orbitals, and valence electrons, laying the foundation for understanding how materials like semiconductors interact with energy.

Nearly every study of electronic devices starts with an investigation of atomic theory. Atomic theory describes the structure of matter and the atom. It is necessary for you to learn this information so that you will be able to understand how semiconductor material responds when energy, such as electricity and heat, is applied.

Figure 1. Fundamentals of Atomic Theory for Electronics

Matter, Elements, and Atoms

You should recall from your elementary or high school science classes that anything that occupies space and has weight is called matter. Matter can be a solid, liquid, or gas. It is made of one or more types of elements. An element is the smallest unit of matter that consists of a single type of atom. An atom is the smallest particle to which an element can be reduced and still retain its identity.

Matter can be made of a single type of element, such as copper, or a combination of element types, such as water. Water is a combination of the elements hydrogen and oxygen. Matter that is made of several types of elements is called a compound. A compound is two or more elements that have been chemically combined. The smallest particle to which a compound can be reduced before being broken down into its basic elements is called a molecule.

Atomic Structure

Atoms consist of smaller particles called electrons, neutrons, and protons.

Figure 2. Simple Atomic Structure of an Atom

The nucleus is the core of an atom. The nucleus of every atom is composed of one or more positively charged particles. Each of these particles is called a proton. In addition to the protons, in all elements except the lightest, there are one or more neutrons. A neutron is a particle that has no electrical charge.

The study of electronic devices is primarily concerned with the electron content of an atom. An electron is a negatively charged particle that orbits the nucleus of an atom. An electron is extremely small. It weighs 1/1850 as much as a proton. Therefore, practically, all of the weight of an atom is in the nucleus.

Atomic Weight and Number

The atomic weight of an atom is the sum of protons and neutrons in the nucleus. The atomic weight of an atom is always a whole number, whereas the atomic weight of an element is rarely a whole number because it represents the average value of a large number of atoms. This average value is more accurately referred to as atomic mass. The periodic table lists each element’s atomic mass.

Elements in the periodic table are listed in the order of the number of protons contained in the nucleus of their atoms. This number is known as the atomic number. No two elements have the same atomic number.

Figure 3. Periodic Table of Elements

For example, hydrogen, which is the simplest atom, has one proton and no neutrons in its nucleus. Any atom that has one proton is always a hydrogen atom. The nucleus of a uranium atom has 92 protons. Any atom that has 92 protons is always a uranium atom. Another example is the element zinc from the periodic table. Note that the atomic number of zinc is 30 and the atomic weight is 65.38.

Each period is arranged in a horizontal row. The period is determined by the sequence of outer shell electrons in the row (lithium = 1, beryllium = 2, boron = 3, carbon = 4, etc.). Note that neon on the right of period 2 is filled with 8 outer shell electrons. The atomic numbers, shown for each atom, are consecutive (1−8) from left to right. As new elements are discovered, they are entered into the proper position on the lower portion of the periodic table.

The number of neutrons in the atom of an element can be calculated by subtracting the atomic number from the atomic weight. This is represented by the following formula:

Number of Neutrons = Atomic Weight − Atomic Number

To determine the atomic weight, the atomic mass is rounded to the nearest whole number. This is represented by the following formula:

Atomic Weight = Atomic Mass (Rounded to Nearest Whole Number)

For example, from Figure 3, we see that uranium has an atomic mass of 238.03. To determine the atomic mass of uranium, 238.03 is rounded to the nearest whole number. The nearest whole number of 238.03 is 238. The number of neutrons can then be calculated by subtracting the atomic number from the atomic weight.

Number of Neutrons = Atomic Weight − Atomic Number = 238 − 92 = 46

Example 1

What is the number of neutrons in hydrogen (H)?

Solution

1. Check the atomic mass of Hydrogen, using a periodic table (Figure 3). This comes out to be 1.00794.
2. Round the atomic mass to the nearest whole number to determine the atomic weight. atomic mass = 1.00794 atomic weight = 1.
3. Subtract the atomic number from the atomic weight.

Number of Neutrons = 1 − 1 = 0

Shells

The electrons of an atom are not in an equal distance from the nucleus. They orbit the nucleus in shells, or layers. Neils Bohr, a Danish physicist, identified these shells by the letters K, L, M, N, O, and P. The K-shell is closest to the nucleus. Each shell or layer has a distinct energy level. Electrons do not exist in the space between these energy levels. The energy levels of a shell are identified by the letters s, p, d, f, g, and h. The s-level is closest to the nucleus. A shell may have from one to six distinct energy levels in its structure.

Figure 4. Differences in Shells of an Atom

Energy levels represent two things: distance from the nucleus and the amount of energy possessed by an electron. Energy levels closest to the nucleus are of the lowest value while energy levels furthest away are of the highest value. This is because electrons with more energy occupy the energy levels furthest away from the nucleus. This largely depends on the number of electrons in an atom. The maximum number of electrons in each energy level follows a unique pattern. This pattern is 2, 6, 10, 14, 18, and 22.

This number sequence appears in the other shells but decreases in value with the last level. The complexity of an atom usually dictates its electron shell and energy level assignment.

For example, lithium has three protons and three neutrons in the structure of its nucleus. It also has three electrons to balance the net positive charge of the nucleus. Note that two of these electrons fill the s-level of the K-shell. Since the K-shell only has one energy level, the third electron of the structure must appear in the s-level of the L-shell. The L-shell has s-and p-levels to be filled. The s-level has a capacity of 2, and the p-level has a capacity of 6. The p-level must be filled before the next shell is formed.

The electron in the s-level of the L-shell of lithium is a valence electron. Valence electrons are electrons in the outermost shell and represent the highest energy level of an atom. Valence electrons can effectively enter into some chemical activity with another atom.

Example 2

What is the number of electrons in the energy levels of each shell for aluminum (Al)?

Solution

1. Determine the number of electrons by checking the Periodic Table.
2. Since the number of electrons is equal to the amount of protons, and the amount of protons is equal to the atomic number, the amount of electrons is 13.

Electrons = Atomic Number of the Element

Electrons = 13

Orbitals

The exact path an electron follows in the structure of an atom is unknown. However, this path is generally described as an orbital. An orbital shows the mathematical probability of where an electron will appear in the structure of an atom. It is believed that there are four types of orbitals in atoms. These are identified by the letters s, p, d, and f. The f-type of orbital is not shown in this sequence because there is very little evidence that it is as active as the others in atomic bonding. This type of orbital is more meaningful in chemical analysis.

Atomic Theory Review Question

1. The number of protons in the nucleus of an atom is an indication of the _____.
2. The atomic weight of an atom is the sum of the number of _____ and _____ in the nucleus of an atom.
3. The atomic _____ represents the average value of protons and neutrons in a large number of atoms.
4. A negatively charged particle that orbits the nucleus of an atom is a(n) _____.
5. The (K-L-M-N-0-P-Q)-shell of an atom is closest to the nucleus. _____
6. The energy levels of a shell are identified by the letters ___, ___, ___, ___, ___, and ___.
7. The _____ shell of an atom has six energy levels.
8. A(n) _____shows the mathematical probability of where an electron will appear in the structure of an atom.

Answers

1. atomic number
2. protons, neutrons
3. mass
4. electron
5. K
6. s, p, d,e ,f, and g
7. P
8. orbital

Atomic Theory Key Takeaways

Understanding atomic theory and the structure of atoms is essential because it forms the basis for analyzing and designing electronic components, especially semiconductors. Knowledge of how electrons behave, how elements interact, and how energy levels function allows engineers and scientists to manipulate materials for practical applications such as transistors, diodes, and integrated circuits.