The periodic table is one of the simplest and most useful tools available to chemists. As mentioned previously, one can predict various chemical properties just by looking at the periodic table! I recommend using this one for convenience:
The periodic table is a table containing information about all of the known elements ranging from common ones like oxygen to rarer ones such as francium. The idea behind the table belongs to Dmitri Mendeleev, a Russian chemist from the 19th century. After the death of his father when Mendeleev was 13 years old, Mendeleev's mother was determined to get Mendeleev educated and walked him all the way from Siberia to Moscow, 2200 miles, in order to make this happen. There, Mendeleev studied chemistry and began to notice trends between elements. To organize these trends, Mendeleev created a chart which was the first attempt at organizing the elements.
Mendeleev's chart organizing elements based off similarities
With this chart, Mendelev was able to describe the properties of different elements and was the first to organize trends between different elements. In fact, he was even able to predict the behavior of 8 elements that had not even been discovered yet!
Columns (vertical) in the periodic table are called groups. Starting from the left, the column with H, Li, K...Fr is known as group 1. The one immediately to the right of it consisting of Be, Mg, Ca...Ra is group 2. This continues until group 18 with He, Ne...Rn. To start you off with understanding the table, let's take a look at a couple different groups.
- Group 1: Alkali Metals. These elements are very reactive.
- Group 2: Alkaline Earth Metals. These elements are similarly very reactive.
- Group 3-12: Transition Metals: These elements have a lot of different properties, but in general are shiny and electrically conductive metals that form solid compounds at room temperature.
- Group 17: Halogens. These elements are very reactive and in general dangerous elements. Not things to be triffled with.
- Group 18: Noble Gases. These elements are all gases that do not react with anything. We sometimes call these "inert" gases.
As you can see, elements in the same group tend to have the same properties. If anyone were to ever ask you whether a halogen (elements in group 17) was likely to be dangerous, you would know to answer that they are dangerous due to their reactivity!
If you recall, the previous post on atomic structure made the claim that atoms are composed of a dense nuclear core, made up of protons and neutrons, with an electron cloud surrounding the core. Right afterwards, I made the claim that the identity of an atom depended solely on the number of protons it had.
It turns out that the periodic table organizes elements by this same metric. The atomic number of an element refers to how many protons it has. This number is on top of its respective element on the periodic table:
The small number on top of each element tells you the atomic number, how many protons the element has. For example, B has 5, C has 6, and S has 16.
The number of electrons for any neutral atoms will be the same as the number of protons. The number of electrons changes as atoms become charged or react with other atoms, but the number of protons must remain the same.
Here's a simulation of how atoms of different elements are made:
Notice that only by adding protons can you change the identity of the element. The addition of neutrons and electrons do not affect the identity of the element.
We've established that atoms are made up of a dense nuclear core, consisting of protons and neutrons, with electrons in orbit around. The question that probably crossed your mind is "why is the nucleus so much denser than the rest of the atoms?" To answer this, we have to introduce the concept of mass. Informally, mass is how much something weighs. Different elements will have different masses because of the difference in the number of protons, neutrons, and electrons:
- Protons have a charge of +1 and a mass of `1 "amu"`, or `m_p = 1.67 * 10^-27 "kg"`.
- Neutrons have a charge of 0 and a mass of `1 "amu"`, or `m_n = 1.67 * 10^-27 "kg"`.
- Electrons have a charge of -1 and a mass of roughly `0 "amu"`, or `m_e = 9.11 * 10^-31 "kg"`
There are a couple things to take away from this. First, protons and neutrons have the same mass. Second, electrons weigh practically nothing. From this information, we can calculate the atomic mass, which is the total mass of all protons, neutrons, and electrons in an atom.
Consider Boron. Boron is element 5. That means it has 5 protons, 5 neutrons, and 5 electrons. The atomic mass of Boron is the sum of all of those masses:
`5m_p + 5m_n + 5m_e = 5(1 "amu") + 5(1 "amu") +5(0 "amu") = 10 "amu"`
This is pretty close to the value of `10.81 "amu"` given on the periodic table! The `0.81` difference can be attributed to the actual mass of electrons: we say that electrons have a mass of `0 "amu"` to simplify calculations, but in reality, electrons just have a very, very small mass compared to protons and neutrons, so we leave them out of the calculations. To figure out exactly how much more "massive" the protons and neutrons are compared to electrons, we can just take the ratio:
`(m_p)/(m_e) = (m_n)/(m_e) = (1.67 * 10^-27 "kg")/(9.11 * 10^-31 "kg") = 1833`
This means that protons and neutrons are nearly 2000 times as massive as electrons are! This is why the nucleus is the densest part of the atom: the proton and neutrons have nearly combined are roughly 4000 times the mass of electrons.
Something to note: all of the atomic mass values on the periodic table were discovered experimentally. However, we can calculate them to a good degree of accuracy just by adding together the approximate mass of the protons, neutrons, and electrons for any given atom. This allows us to know how much a certain atom will weigh.
Hopefully you've noticed by now that the periodic table labels elements in a shorthand notation. For example, element #6, labeled C, is carbon. Most of the symbols are just shorthand for the name of the element, O for oxygen, while others may not be so straightforward such as mercury (Hg).
Unless you get a really mean professor or are preparing for a periodic table recitation competition (don't think these happen too often), you probably won't have to memorize the symbols. It may seem overwhelming at this moment, but in no time you'll know the most common ones by heart.
Here are some of the common ones to get you started:
H: Hydrogen
He: Helium
Li: Lithium
B: Boron
C: Carbon
N: Nitrogen
O: Oxygen
Na: Sodium
Mg: Magnesium
P: Phosphorus
Cl: Chlorine
K: Potassium
Additionally, here's a fun comic done by SMBC using atomic symbols. Warning: very slight inappropriate language.
1. Columns going up and down are called groups. Elements belonging to the same group are likely to have similar properties.
2. The atomic number is the number of protons that a given element has. This is the small number above the elemental symbol in the periodic table. This number is inherent for a given element and cannot change.
3. The atomic mass is the total sum of the mass of all protons, neutrons, and electrons for a given element. This can be found by simply adding all the masses, in amu, together.
4. Protons have a charge of +1 and a mass of 1 amu. Neutrons have a charge of 0 and a mass of 1 amu. Electrons have a charge of -1 and an approximate mass of 0 amu.
1. Where do the atomic symbols come from?
Behind each atomic symbol is a story. Lead is one example. Lead's atomic symbol is Pb as the Latin word for lead is "plumbum." This is because lead used to be used for plumbing. We now know that this is an absolutely terrible idea: by using lead for plumbing, lead was introduced into the water system which resulted in people consuming and drinking lead. Lead is an extremely toxic element to humans, which is why lead paint and leaded gasoline are now banned.
Usually, the element is named after either the discoverer or the location that the discovery was made. Element 97, berkelium, was named after the University of California, Berkeley, where it was discovered. Every so often, the elements will be named in honor of someone unrelated to the project. For example, elements 99 and 100, einsteinium and fermium, are named after the physicists Albert Einstein and Enrico Fermi.
Some of the elements are still not named. As of the writing of this post, elements 113 and 115 have the atomic symbols Uut and Uup, which stand for ununtrium and ununpentium respectively. These are placeholder names until the atomic symbol can be attributed to someone or something justified.