Being able to read the Periodic Table provides you with a “legal cheat sheet”. Therefore, Episode 3 recaps the set-up of the periodic table and how to read it.
Being able to read the Periodic Table provides you with a “legal cheat sheet”. Therefore, Episode 3 recaps the set-up of the periodic table and how to read it.
Dimitri Mendeleev is known as the Father of the Periodic Table (1:25). He arranged the elements with increasing relative atomic mass as well as elements with similar properties together (2:25). Today’s periodic table arranges elements by increasing atomic number (3:13). The horizontal rows in the periodic table are called periods, vertical columns in the periodic table are called families or groups (3:40). Several groups are described in greater detail: alkali metals, alkaline earth-metals, transition metals, metalloids, halogens and noble gases. The periodic table can also tell you the number of electrons (4:05), valence electrons (6:30), protons (7:11) and neutrons (7:55).
An experiment calls for the substance KBr, but you are out of it. Which of the following can you use instead? K2S, NaCl, MgBr2 (9:25)
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Hi and welcome to the APsolute Recap: Chemistry Edition. Today’s episode will recap the Periodic Table.
Let’s zoom out
Chemistry is special, in many ways. One of the characteristics that sets it apart from other subjects at school is that students get to use a legal "cheat sheet": the periodic table! In the periodic table, the elements are organized in a specific way, enabling you to get information about the atomic structure and the reactivity of an element. Therefore, being able to read and interpret the periodic table will be essential to your success in AP Chemistry. So, in today's episode, we will recap the periodic table and how to read it.
Let’s zoom in!
Elements have been known to humankind for thousands of years and by 1860 around 60 elements were identified. There was, however, no coherent information about the elemental properties and, with an increasing number of elements, the need for organization arose.
Let’s meet the father of the periodic table: Dimitri Mendeleev. According to history, Mendeleev wrote the names of the elements with their relative mass and their properties on individual cards and spent days trying to see a pattern. His task was especially challenging, because at that time, his set of cards was incomplete, since not all elements had been discovered. Imagine beginning a game of poker without the twos or kings - not so easy. Allegedly Mendeleev worked without a break for several days and nights, but to no avail. Legend has it that when he finally fell asleep he dreamed about how the elements could be arranged. I’m not sure if this is REALLY how he made his discovery, but maybe you’ve experienced something similar: you’ve been working on a project or an especially challenging problem and you just can't figure it out. And then all of a sudden, when your mind is doing something completely different, a lightbulb goes on and it all falls into place. It might have been similar for Mendeleev. He arranged the elements with increasing relative atomic mass as well as elements with similar properties together. There were two pairs causing problems. One of them is the placement of Tellurium and Iodine. Organized by increasing relative atomic mass, Tellurium is heavier than Iodine. But when it comes to properties, Tellurium fit much better with group 16 and Iodine in group 17! Gasp – are these the black sheep in the family? It must have been like trying to make a puzzle, and two pieces just don’t fit. You might decide to use your fist and hammer them into place, but something will always be off. Square peg - round hole.
Mendeleev was missing a crucial piece of knowledge: he didn’t know about atomic structure or subatomic particles. Today’s periodic table still groups elements with similar properties together, but it is no longer organized by relative atomic mass, but rather by atomic number, which is the number of protons in the atom’s nucleus. This actually solved the entire puzzle and the Iodine and Tellurium found their rightful spot! Grab a periodic table and see if you can find the second pair!
Now, let’s take a closer look at the elements in the periodic table. The horizontal rows in the periodic table are called periods – some properties repeat in periodic patterns; therefore, we use the term periodic table. Vertical columns in the periodic table are called families or groups. Similar to you and your family, the elements within a group share certain characteristics and are sometimes referred to with a “family” name. For example, the elements in group 1 are alkali metals. They are highly reactive and soft, you can even cut them with a knife. The elements in group 2 are alkaline earth metals. They are a bit less reactive than the group 1 metals, but the elements in both groups cannot be found in pure form in nature, only in compounds.
The metals in group 3-12 are transition metals. These contain some of the metals you might think of when you hear the word metal: like gold, silver and iron. Their reactivity varies, but they are less reactive than group 1 and 2 metals. When looking at the right side of the periodic table you’ll see a “staircase”. The staircase separates metals and nonmetals. Some elements directly adjacent to the staircase are classified as metalloids: they have some properties of metals and some of the properties of nonmetals. Metals below the staircase are called post-transition metals (nope, unfortunately not Harry Potter metals). Group 17 elements are called halogens, which means “salt former”. Together with alkali metals, they are forming compounds that we summarize under the term salt. The elements in group 18 are noble gases. Their name is due to them being very unreactive and therefore “too noble” to interact with other elements. There are two rows of elements in the bottom of the periodic table: lanthanides and actinides. Many of these elements are radioactive or artificial. Their position on your periodic table is purely for convenience: They actually fit after the 2nd group in periods 6 and 7, but it would make the periodic table so wide that it would be a nuisance to work with it in that format.
Let’s take a closer look at how the periodic table tells us more about the atomic structure of the element. I’ve already mentioned that it is organized by increasing atomic number, which is the number of protons in the nucleus. The number of protons actually determines the identity of an element: For example an atom with 8 protons is always oxygen. The number of neutrons and electrons can vary without changing the identity, but the number of protons cannot. A neutral atom has the same number of electrons as it has protons, so that the charges are balanced. Therefore, knowing that oxygen has eight protons it also has to have eight electrons. In chemistry we are mostly interested in valence electrons. These are the electrons that are in the highest energy level of the atom – meaning, the furthest from the nucleus. They actually determine how reactive an element is. To determine the number of valence electrons, you can use the group number in the periodic table: Group 1 has 1 valence electron, group 2 has 2 valence electrons. Skipping the transition metals, group 13 has 3 valence electrons, group 4 has 4 valence electrons and so on. See? A built in cheat sheet.
Now what about the number of neutrons? To be able to determine those, we have to clarify relative atomic mass. The weight of a proton, a neutron or an electron is obviously unimaginably small: a proton, for example, weighs 1.67 x 10-27 kg. Aside from maybe your math teacher no one enjoys working with those numbers, so chemists use relative atomic weights: relatively speaking protons and neutrons weigh 1; by comparison an electron weighs
5.5 x 10-4 – which is pretty much nothing. So if you know the relative atomic mass of an element, in our example oxygen with a relative atomic mass of 16 and we know that it has eight protons, where each weighs 1, then the other 8 atomic mass units have to be the neutrons, in our case eight of them. The number of neutrons therefore is the relative atomic weight minus the number of protons. When looking for the relative atomic weight on the periodic table, you’ll see that for most elements, the number has decimal places. How can that be? Do they have a fraction of a neutron? No! There are actually atoms of the same element with the same number of protons but different numbers of neutrons – These are called isotopes. (Bonus points if you know which cartoon baseball team is called the isotopes!) Each isotope differs in relative atomic mass. The periodic table gives us the average atomic mass, taking into account the abundance of an isotope and its weight. If you, for example, look at bromine with an average atomic mass of 79.90 you know that the majority of natural occurring bromine atoms have 35 protons and 45 neutrons – but there also has to be a small percentage that has 35 protons and 44 neutrons.
To recap……
The periodic table is structured in groups, whose elements share some characteristics, and periods. The elements are furthermore organized by increasing atomic number, which is equal to the number of protons. Neutral atoms have the same number of electrons as they have protons. The number of neutrons can be derived by subtracting the relative atomic mass by the number of protons.
Coming up next on the Apsolute RecAP Chemistry Edition: Chemical Bonding
Today’s Question of the day is about the periodic table.
Question: An experiment calls for the substance KBr, but you are out of it. Which of the following can you use instead?
A. K2S
B. NaCl
C. MgBr2