2.2: Elements and Atomic Number

The modern atomic theory states that atoms of one element are the same, while atoms of different elements are different. What makes atoms of different elements different? The fundamental characteristic that all atoms of the same element share is the number of protons. All atoms of hydrogen have one and only one proton in the nucleus; all atoms of iron have 26 protons in the nucleus. This number of protons is so important to the identity of an atom that it is called the atomic number (Z). The number of protons in an atom is the atomic number of the element. Thus, hydrogen has an atomic number of 1, while iron has an atomic number of 26. Each element has its own characteristic atomic number.

Atoms are electrically neutral, meaning that the overall electric charge is zero. This is because the number of protons (positive charge) equals the number of electrons (negative charge). Therefore, the atomic number also provides the number of electrons. For example, helium has Z = 2, which tells us that there are 2 protons in the nucleus and 2 electrons outside of the nucleus. Sometimes atoms will gain or lose electrons resulting in a difference in the number of protons and electrons, which means the charge is no longer zero. Atoms that have a charge are called ions and will be discussed further in the next chapter.

As we learned in the previous section, protons and neutrons, which are found in the nucleus of an atom, each have a mass of ~1 amu. Because an electron has negligible mass relative to that of a proton or a neutron, the majority of an atom's mass is in the nucleus. The mass number (Ais defined as the total number of protons ($$p^+$$) and neutrons ($$n$$) in an atom:

$A=p^+ + n$

Atoms of the same element always have the same number of protons, same Z, but often have different numbers of neutrons, therefore, different mass numbers.  These atoms are called isotopes and are introduced in next section.

Example $$\PageIndex{1}$$

1. The most common carbon atoms have six protons and six neutrons in their nuclei. What are the atomic number and the mass number of these carbon atoms?
2. An isotope of uranium has an atomic number of 92 and a mass number of 235. What are the number of protons and neutrons in the nucleus of this atom?

Solution

1. If a carbon atom has six protons in its nucleus, its atomic number is 6. If it also has six neutrons in the nucleus, then the mass number is 6 + 6, or 12.
2. If the atomic number of uranium is 92, then that is the number of protons in the nucleus. Because the mass number is 235, then the number of neutrons in the nucleus is 235 − 92, or 143.

Exercise $$\PageIndex{1}$$

The number of protons in the nucleus of a tin atom is 50, while the number of neutrons in the nucleus is 68. What are the atomic number and the mass number of this isotope?

Atomic number = 50, mass number = 118

A simple way of indicating the mass number of a particular atom is to list it as a superscript on the left side of an element’s symbol. Atomic numbers are often listed as a subscript on the left side of an element’s symbol (Figure $$\PageIndex{1}$$). Figure $$\PageIndex{1}$$: Unlike protons, the number of neutrons is not absolutely fixed for most elements. Atoms that have the same number of protons, and hence the same atomic number, but different numbers of neutrons are called isotopes. All isotopes of an element have the same number of protons and electrons, which means they exhibit the same chemistry. The isotopes of an element differ only in their atomic mass, which is given by the mass number (A), the sum of the numbers of protons and neutrons. (CC BY-NS-SA; anonymous by request)

Figure $$\PageIndex{1}$$ shows an easy way to represent isotopes using the atomic symbols where $$X$$ is the symbol of the element, $$A$$ is the mass number, and $$Z$$ is the atomic number. Thus, for the isotope of carbon that has 6 protons and 6 neutrons, the symbol is

$\ce{_{6}^{12}C} \nonumber$

where $$C$$ is the symbol for the element, 6 represents the atomic number, and 12 represents the mass number.

It is also common to state the mass number after the name of an element to indicate a particular isotope. Carbon-12 represents an isotope of carbon with 6 protons and 6 neutrons, while uranium-238 is an isotope of uranium that has 146 neutrons.

Example $$\PageIndex{2}$$:

1. What is the symbol for an isotope of uranium that has an atomic number of 92 and a mass number of 235?
2. How many protons and neutrons are in $$\ce{_{26}^{56}Fe}$$

Solution

1. The symbol for this isotope is $$\ce{_{92}^{235}U}$$
2. This iron atom has 26 protons and 56 − 26 = 30 neutrons.

Exercise $$\PageIndex{2}$$

How many protons are in $$\ce{_{11}^{23} Na}$$

11 protons

Key Takeaways

• Atoms themselves are composed of protons, neutrons, and electrons.
• Each element has its own atomic number, which is equal to the number of protons in its nucleus.
• Isotopes of an element contain different numbers of neutrons.
• Elements are represented by an atomic symbol.

Contributors

• Lisa Sharpe Elles, University of Kansas