Using the Periodic Table to Diagram Atoms

On its most basic level, you can think of the periodic table as a simple listing of all of the different types of atoms we have discovered or created. As I write this, there are currently 118 elements listed in the periodic table. [An element is a pure substance made up of a single type of atom. Water (H2O) is not an element because it is made up of both oxygen and hydrogen atoms. Hydrogen is an element because it is made up of only a single type of atom… hydrogen atoms. Basically, each element corresponds to a type of atom.]

Atomic number and mass number

Elements (or atoms) are listed in the periodic table according to their atomic number. The atomic number is the number of protons an atom has in its nucleus. Remember, an atom is identified based on its number of protons.

hydrogen element
periodic table
atom/element atomic number mass number number of protons number of neutrons number of electrons atomic mass (u)
hydrogen (H) 1 1 1 0 1 001.0078
helium (He) 2 4 2 2 2 004.0026
carbon (C) 6 12 6 6 6 012.0000
nitrogen (N) 7 14 7 7 7 014.0031
oxygen (O) 8 16 8 8 8 015.9949
sodium (Na) 11 23 11 12 11 022.9898
aluminum (Al) 13 27 13 14 13 026.9815
chlorine (Cl) 17 35 17 18 17 034.9689
calcium (Ca) 20 40 20 20 20 039.9626
iron (Fe) 26 56 26 30 26 055.9349
copper (Cu) 29 63 29 34 29 062.9296
gold (Au) 79 197 79 118 79 196.9666
plutonium (Pu) 94 244 94 150 94 244.0642

Copper has an atomic number of 29 and a mass number of 63. The atomic number is the number of protons in the nucleus, and the mass number is the number of protons and neutrons in the nucleus. This means that the copper atom has 29 protons and 34 neutrons in its nucleus (29 + 34 = 63). A neutrally charged copper atom will then have 29 electrons in orbit around its nucleus (29 − 29 = 0). Because almost all of an atom’s mass is in its nucleus, we can estimate an atom’s mass just by counting its protons and neutrons. Both protons and neutrons have masses approximately equal to 1 u (1 proton ≈ 1.0073 u, 1 neutron ≈ 1.0087 u, and 1 u ≈ 1.66 × 10-27 kg), so the mass of a copper atom will be approximately 63 u, and the mass of 1 mole (6.022 × 1023 atoms) of copper will have a mass of approximately 63 g.

VIsotopes, Mass Numbers, and Molar Masses
VThe Discovery and Creation of New Elements
Energy levels and Bohr diagrams

As you have learned, the electrons in an atom must have specific and discrete energy levels. The lowest energy level that an electron can have is n = 1. The next lowest energy level is n = 2. An electron cannot have an energy level of n = 1.5. (This is a bit like saying that a cat can weigh 9 lb or 10 lb, but not 9.5 lb!) Although this may seem very, very strange (perhaps even impossible), it accurately describes the behavior of atoms and is one of the key differences between classical mechanics and quantum mechanics.

energy level electron shell maximum number of electrons number of atomic orbitals
n = 1 1 2 1
n = 2 2 8 4
n = 3 3 18 9
n = 4 4 32 16
n = 5 5 50 25
n = 6 6 72 36

Only a specific number of electrons can exist with a specific energy level. For example, only two electrons can have an energy level of n = 1 and only eight electrons can have an energy level of n = 2. Sometimes it is convenient to think of the electrons with the same energy level as sharing an “electron shell” in the electron cloud surrounding an atom’s nucleus. The technical reason why the first electron shell can only hold two electrons is because only two quantum states can have an energy level of n = 1. But that reason is very abstract unless you have a deep understanding of quantum mechanics. There is a classical mechanical explanation I can give for why only a specific number of electrons can have the same energy level, but it is not the complete reason.

electron shells

The higher the energy level of an electron, the farther away it is likely to be from the atom’s nucleus. Since electrons in the first electron shell have the lowest energy level (n = 1), they will be closest to the nucleus. However, while negatively charged electrons are strongly attracted to the positively charged nucleus, they are also strongly repelled by each other. This means that electrons will push each other away if they get too crowded together. Trying to add a third electron to the first electron shell would make the electron shell too crowded, so the electron gets pushed into the second electron shell with a higher energy level (n = 2). The second electron shell can hold more electrons before it gets too crowded because it is farther away from the nucleus and has more physical space to fill.

One way to represent the structure of an atom is with a Bohr diagram.

symbol: Al
atomic number: 13
mass number: 27
To draw an atom’s Bohr diagram, find the atom’s symbol in the periodic table and its atomic number and mass number.
protons: 13
neutrons: 27 − 13 = 14
electrons: 13
Use the atomic number and mass number to find the number of protons, neutrons, and electrons in the atom.
Bohr diagram of an Al atom
Draw the atom’s nucleus, and label the atom and the number of protons and neutrons.
Bohr diagram of an Al atom
Draw the first electron shell and start adding electrons until the shell is filled or you run out of electrons.
Bohr diagram of an Al atom
Draw the second electron shell and start adding electrons until this shell is also filled or you run out of electrons.
Bohr diagram of an Al atom
Continue drawing electron shells and adding electrons until you have added all 13 electrons to the Al atom.

While a Bohr diagram is easy to draw and conveys useful information about an atom’s structure, it can reinforce certain misconceptions. First, electrons do not orbit an atom’s nucleus in concentric circles. This is a holdover from when we used to model electrons like planets in a solar system. Second, electrons do not have specific positions at any given time. And third, electrons do not occupy electron shells. Electron shells are a construct that we created to help us think about atoms. To really understand atoms and atomic behavior, you need to think in terms of atomic orbitals.

The thirteenth electron of an aluminum atom actually occupies an atomic orbital that looks something like this. This is known as a “p-orbital.” This particular orbital has a quantum state of n = 2, l = 1, and m = 0; and is shaped a bit like a dumbbell, with one region in front of the nucleus and a second region behind the nucleus. Up to two electrons can occupy any one atomic orbital: one electron with a spin of s = +½ and one electron with a spin of s = -½. There is one atomic orbital with an energy level of n = 1, four atomic orbitals with an energy level of n = 2, and nine atomic orbitals with an energy level of n = 3 (see the table above). Atomic orbitals are sometimes referred to as “subshells.”

Having a detailed understanding of atomic orbitals is not important at this point, but it is important to recognize that the Bohr diagram grossly oversimplifies the structure of an atom and that the Bohr model of an atom (with electrons traveling in concentric, circular orbits) was replaced by a more modern model of an atom based firmly on quantum mechanics decades ago. Having some familiarity with atomic orbitals will also help you better understand the structure of the periodic table and valence electrons later in this unit.

Energy level transitions and the ground state

Electrons in an atom can transition to a different energy level by gaining or losing energy, as long as there is room at that energy level for another electron. (Remember, the number of electrons that can have any given energy level is limited because no two electrons can share the same quantum state.) In general, electrons will occupy the lowest energy levels available. This is why, when drawing our Bohr diagram of the aluminum atom, we started by completely filling the first electron shell before moving on to the second electron shell. This electron configuration is called the ground state of the atom, and it represents the lowest energy state an atom can have.

Bohr diagram of an excited Al atom

But an electron does not have to occupy the lowest energy level available. If an electron absorbs the energy from a photon, it can get kicked up to a higher energy level even though there is still room at a lower energy level. In this Bohr diagram of an aluminum atom, there are five electrons with an energy level of n = 3 even though there is still enough room in the second electron shell for two more electrons with a lower energy level of n = 2. When an atom is not in the ground state, it is said to be in an excited state.

It takes energy to move an electron from a low energy level to a higher energy level. And with enough energy, an electron can be completely removed from an atom. This corresponds to an energy level of n = ∞ (infinity). The energy required to remove an electron from an atom is called the ionization energy. Removing an electron turns an atom into an ion because the number of protons and electrons is no longer equal and the atom now has a positive charge. It takes 9.59 × 10-19 J of energy to remove the thirteenth electron from an aluminum atom, turning it into an ion (Al+). Removing a second electron, turning the atom into the ion Al2+, then takes even more energy (3.02 × 10-18 J).

VThe Physics of Firelight
VUsing Atomic Line Spectra to Identify Atoms

We have just touched the surface of what the periodic table can tell us. Next, you will learn how chemical properties of an atom can be predicted from the atom’s structure and how the periodic table is organized to group elements with common properties together.