Atomic Number

much larger than the nucleus because electrons expand the total size of the atom. Figure one depicts what an atom might look like structurally:

Figure 1.

So, basically, every atom has a nucleus in the center that has at least one proton in it and at least one electron, which is “orbiting” the nucleus at a fixed energy level with a certain quantum spin value. We will talk more about this quantum spin value and about “orbitals” of electrons in the next chapter. Electrons contribute much to the charge of the atom but are elementary particles of their own.

As you may have surmised, an elementary particle is one that cannot be further broken down. Quarks are elementary particles, gluons are elementary particles, and the different gauge bosons (beyond the scope of this chapter) are all elementary particles, while neutrons and protons are not elementary particles.

ATOMIC NUMBER

As mentioned, the atomic number is defined as the number of protons in an atom. In chemistry, it is represented by the letter Z. An element is a substance that has a specific atomic number. For example, hydrogen, the smallest element, has just one proton, having a Z value of 1. The number of protons or the atomic number defines the element.

Atoms, by definition, have a neutral charge so that the number of protons is equal to the number of electrons. Carbon has six protons and six electrons, so its atomic number is

six. Similarly, oxygen has an atomic number of 8 because it has 8 protons and 8 electrons. An atomic number is always going to be a whole number as it reflects an actual number of protons and ranges from 1 to 118 (although only the first 98 actually exist as relatively stable elements in nature).

The atomic number helps identify the element and will form the basis of arranging the elements in the periodic table, which is arranged according to increasing order of atomic number. While the atomic number determines some aspect of the properties of the element, the electrons help determine the actual bonding behavior of the element as you will soon learn.

You can learn the atomic number of any element by looking at its symbol on the periodic table of the elements. For example, next to the letter describing the element, such as Magnesium or Mg, there will be a number that defines the atomic number. Figure 2 shows the elemental symbol of Magnesium along with its atomic number 12:

Figure 2.

Another way of determining the atomic number is by looking at the isotope symbol. An isotope is defined as any of two or more forms of the same element that contain equal numbers of protons but different numbers of neutrons in their nuclei, and therefore differ in relative atomic mass but not in chemical properties. Isotope notation consists of both the atomic mass (to be discussed next) and atomic number written to the left of the chemical symbol. The atomic mass is written as a superscript, and the atomic number as a subscript. For example, carbon -14 is an isotope of carbon and is written as the carbon symbol “C,” with the atomic mass, 14, written as a superscript, and its atomic