Which one of the following is true of an element? (A) All the atoms of an element have the same number of protons in their nuclei. (B) Elements are made of at least two types of atom bonded together. (C) You can divide up an element into infinitely many pieces. (D) Elements in the periodic table are arranged in order of increasing mass. Or (E) all elements are found in nature in their pure states.
We should start here by reminding ourselves of the definition of an element. In chemistry, one definition of an element is a type of atom or ion specified with a number of protons in their nucleus. A nucleus sits in the middle of an atom or ion and contains protons and potentially neutrons. If a nucleus contains one proton, that means the associated element is hydrogen. If there are two protons, then it’s helium, and so on.
But the word chemical element is sometimes used slightly differently. Sometimes, an element is used to refer to a collection of atoms or ions which all have the same number of protons in their nuclei, although this is sometimes and better described as a sample of an element. There are many definitions in use. Some refer to the periodic table of the elements, and some refer more to the history of the elements. In this case, these definitions will do.
Let’s have a look at the first statement. All the atoms of an element have the same number of protons in their nuclei. In this statement, the word element is being used more in the second stance, referring to a sample of an element — for instance, a lump of graphite, which is made of carbon. Atoms of carbon all have six protons in their nuclei. This statement is therefore true, but let’s have a look at the other four, just in case.
Elements are made of at least two types of atom bonded together. The phrase “types of atom” could be interpreted to mean elements or isotopes. In either case, this statement is wrong because elements are not necessarily bonded to other materials. For instance, helium gas is a sample of an element, but it consists of individual atoms not bonded to anything. So this statement is not generally true for an element.
The third statement is that you can divide up an element into infinitely many pieces. This statement is not true. Elements are made up of protons, neutrons, and electrons. And while some of these can be broken up into slightly smaller pieces, there’s a limit. So this is not a correct answer.
The fourth statement is that elements in the periodic table are arranged in order of increasing mass. While it may be true for some elements that there is a sequence to their masses, like between hydrogen and helium, this relationship breaks down between argon and potassium. And there are a few other examples. The statement is not true because elements in the periodic table are arranged by atomic number, not by atomic mass. So we can move on to the final statement.
This reads, all elements are found in nature in their pure states. For instance, this means that elements like carbon would always be found as graphite or diamond, while metals like iron would be found in their solid form. However, this statement can be easily disproved by looking at some of the more reactive elements — elements like lithium, which are always found in compound ores. While many elements are found in their pure states in nature, that is not true for all elements.
So, of the five statements given, the only one that’s true about elements is that all the atoms of an element have the same number of protons in their nuclei.