By Ron B. Davis Jr., Georgetown University
Why is there a periodic table at all? What is it that makes some materials elements—and most not? And what is it about the elements that make some of their properties so predictable and ordered that they can be organized in this way? The answer to these questions comes down to atoms, the fundamental units of matter.
Compelling Evidence for the Existence of Atoms
John Dalton offered the most compelling evidence for atoms yet in the early 1800s. His argument was fairly simple. He noted that compounds of elements always seemed to combine in the same proportions by mass.
For example, tin and oxygen combined to make a compound that was 88.1% tin by mass, and another that was 78.8 percent tin, but never in between. He called this the ‘Law of Multiple Proportions’ and he argued that this would only be possible if elements consisted of discrete, indivisible particles. If not, a continuum of proportions should be possible.
Dalton was right about the existence of atoms. And yet, atoms, it turns out, are not really indivisible, as everyone had assumed.
This article comes directly from content in the video series Understanding the Periodic Table. Watch it now, on Wondrium.
The Complex Structure of Atoms
Dividing an atom changes it into a whole new element. Atoms have a complex structure, a structure built up from particles even smaller than atoms—the sub-atomic particles.
Atoms have a dense nucleus, made of positively-charged protons and un-charged neutrons. Atoms also have a much larger, but much less dense region of negatively-charged electrons surrounding the nucleus.
The number of protons in an atom is called its ‘atomic number’. This number of protons gives each atom its identity. All atoms of carbon, for example, contain six protons in their nucleus—atomic number 6. One proton less, and that atom isn’t carbon, it is boron at atomic number 5.
The atomic nucleus also contains a second type of particle called the neutron. One neutron gives an atom additional mass nearly identical to the mass of a proton.
But neutrons are neutral—they lack charge—so the number of neutrons does not change an atom’s identity as an element. For example, the carbon atom can have six protons and six neutrons, but it could just as easily have six protons and seven neutrons. We call these atoms ‘isotopes’ of one another, and often indicate which isotope we want to discuss by indicating the total number of protons and neutrons—carbon-12 or carbon-13, for example. .
Protons and neutrons have similar masses and make up the vast majority of an atom’s mass. This is why we often refer to an atom’s total mass simply in terms of ‘atomic mass units’ or amu, where one amu is roughly the mass of one proton or one neutron.
Roles of Electrons
Electrons are the third type of sub-atomic particle, but they are very much smaller—about 1000 times less massive than protons and neutrons.
Electrons play two critical roles in an atom. The first is that electrons balance out the positive charge of protons in the nucleus. Our carbon atom with six protons needs six orbiting electrons to remain neutral overall. Removing one of those electrons leaves our carbon with a net positive charge. It’s no longer in balance. Conversely, adding an additional electron into a neutral carbon produces a net negative charge.
In both cases, we say that the carbon atom has become a carbon ion, meaning that it has a positive or negative overall charge. Ions can behave quite differently than their corresponding neutral atoms.
The second, even more consequential, role of the electrons—which orbit the nucleus, on the outer edge of each atom or ion—is that electrons interact directly with the world around each atom.
Electron Shells and Periods on the Periodic Table
Electrons occupy regions of space around a nucleus that are called shells. These shells form complex layers of electrons around the nucleus. It is the outermost layer of electrons, called the ‘valence shell’, that primarily determines how an atom interacts with the world around it.
When one shell is filled to capacity, a new shell begins filling, and a new period on the table begins. It is the structure of these electron shells that drives the organization of the periods, giving the periodic table its name and its distinctive shape.
The periodic table got its start when Dimitri Mendeleev lined up all known elements of his day in order of increasing atomic mass. When he did this, he noticed recurring patterns of chemical behavior—in particular, the ratios in which elements combined with oxygen or hydrogen—repeated at regular intervals.
He found that by wrapping the table and creating periods, these elements combining with oxygen or hydrogen in the same ratios—known as their ‘valence’—could often be aligned within a column (or group).
The Secret to Grouping Elements
The secret to the ability of Mendeleev’s table to group elements with similar properties so long ago is that those properties he used, atomic mass and valence, are closely related to certain features of atomic structure—specifically atomic number and the structure of the electron cloud that drive the organization of the table today.
Each element’s place on the table is determined by its atomic structure, and an atom’s structure influences its properties. The table’s systematic organization of elements in terms of their atoms offers to share tremendous insights when it comes to the abundance, distribution, and properties of the elements and compounds in the world around us.
Common Questions about the Structure of the Fundamental Units of Matter
Atoms have a dense nucleus, made of positively-charged protons and un-charged neutrons. Atoms also have a much larger, but much less dense region of negatively-charged electrons surrounding the nucleus.
Atomic mass units or amu is a unit of measurement that represents the total mass of an atom. One amu is roughly the mass of one proton or one neutron.
Electrons play two critical roles in an atom. The first is that electrons balance out the positive charge of protons in the nucleus. The second is that electrons interact directly with the world around each atom.
