Remember how we compared atoms and elements to the alphabet? Well, the periodic table would be like organizing all the letters of the alphabet by a bunch of different criteria, such as vowels vs. consonants, their sounds, etc. Only elements have a lot more criteria, so its a little more complicated. Let's start by breaking down each little tile on the periodic table.
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At the top of the tile, you will often find the name of the element.
Right below that, you'll find a number. Remember when we talked about how every element has a specific number of protons? That number of protons is known as the atomic number. So the number at the top of the tile tells you how many protons you will find in the nucleus of an atom of that particular element.
Below the atomic number, you will find a letter or two. This is the symbol for the element. This is like an abbreviation for that element. We use these abbreviations when we create compounds and chemical formulas. Keep in mind, each element symbol will only have one capital letter, even if it has two letters. If you see two capital letters, that means two different elements.
At the very bottom of the tile, there is another number. This number is the atomic weight. Basically, this number tells you about how much mass is in an atom. Remember how we talked about protons and neutrons each have a mass of about 1 AMU? Well these combine to make the atomic mass. You can actually use this number to figure out how many neutrons are in an atom of a given element. You will need to round the atomic mass to the nearest whole number to make this work. So lets take lithium as an example. Its atomic mass, rounded, is 7. We know this is made up of protons and neutrons, each with a mass of 1. Since Lithium has atomic number of 3, we know it has 3 protons, which means a mass of 3. We still have 4 AMU left to get to 7. That means 4 neutrons. Cool, huh?
Let's make it a nice, easy to remember formula:
Right below that, you'll find a number. Remember when we talked about how every element has a specific number of protons? That number of protons is known as the atomic number. So the number at the top of the tile tells you how many protons you will find in the nucleus of an atom of that particular element.
Below the atomic number, you will find a letter or two. This is the symbol for the element. This is like an abbreviation for that element. We use these abbreviations when we create compounds and chemical formulas. Keep in mind, each element symbol will only have one capital letter, even if it has two letters. If you see two capital letters, that means two different elements.
At the very bottom of the tile, there is another number. This number is the atomic weight. Basically, this number tells you about how much mass is in an atom. Remember how we talked about protons and neutrons each have a mass of about 1 AMU? Well these combine to make the atomic mass. You can actually use this number to figure out how many neutrons are in an atom of a given element. You will need to round the atomic mass to the nearest whole number to make this work. So lets take lithium as an example. Its atomic mass, rounded, is 7. We know this is made up of protons and neutrons, each with a mass of 1. Since Lithium has atomic number of 3, we know it has 3 protons, which means a mass of 3. We still have 4 AMU left to get to 7. That means 4 neutrons. Cool, huh?
Let's make it a nice, easy to remember formula:
Ok, now lets talk about why the periodic table is laid out the way it is. There are actually reasons behind why each tile is in its particular spot. Take a look at the table below:
See all the different colors? The each represent something different. The key is down at the bottom of the image. Most of the periodic table are made up of metals, but not all the metals are alike, so they get different categories. Also, notice that metals are to the left on the periodic table. As you move right, you get metalloids and then further right you get nonmetals and finally the noble gases are furthest right.
The modern periodic table is based upon the original work of a guy named Dmitri Mendeleev. In 1869, Mendeleev and the scientific community only knew about 65 elements, but even then Mendeleev noticed similarities between certain elements. So he arranged those 65 elements with two rules:
1. Each element must have a higher atomic weight than the element to its left. Or, in simple terms, atomic weight increases moving left to right, the way you read a book. Notice how Boron's atomic weight (11 when rounded) is lower than Carbon's (12 when rounded) which is lower than Nitrogen's (14 when rounded). The weights just keep going up.
The modern periodic table is based upon the original work of a guy named Dmitri Mendeleev. In 1869, Mendeleev and the scientific community only knew about 65 elements, but even then Mendeleev noticed similarities between certain elements. So he arranged those 65 elements with two rules:
1. Each element must have a higher atomic weight than the element to its left. Or, in simple terms, atomic weight increases moving left to right, the way you read a book. Notice how Boron's atomic weight (11 when rounded) is lower than Carbon's (12 when rounded) which is lower than Nitrogen's (14 when rounded). The weights just keep going up.
2. Similar chemical properties go in the same column. This rule is a little harder to see but pretty easy to understand. If elements are in the same column, the have the same reactivity in chemical reactions. For instance, Carbon likes to react with Oxygen. In fact, Carbon Dioxide is one of the most common gases on earth. Because Silicon and Germanium are in the same column, we know that they also like to react with Oxygen.
When you look back at the colored periodic table, you might see that some columns are all the same color, like Alkali Metals or Noble Gases. This is the perfect example of how columns have the same chemical properties. Noble Gases are all non-reactive, meaning they don't want to react with anything. They just want to be left alone. Alkali Metals, on the other hand are all highly reactive. Okay, so we keeping seeing this word "reactivity" pop up. Let's explain what exactly that means.
Reactivity
Reactivity is simply a measure of how likely it is that an element will react with another element. Like we talked about before, there are certain elements that are desperate to react (Alkali Metals) and others that just want to be left alone (Noble Gases).
![Picture](/uploads/2/0/1/1/20114429/4913451.jpg?146)
Notice how those two groups are on opposite sides of the periodic table? The periodic table is actually laid out by how reactive elements are. In general, as you move left to right on the table, you decrease in reactivity. Once you get all the way to the right, you hit the Noble gases, who have no interest in reacting with anybody.
The reason behind the reactivity wasn't discovered until long after Mendeleev's time. It all has to do with electrons; you know, the tiny little negatively charged particles from the Atom's page. Remember how we talked about the electron levels? Well those levels are important, because an element's reactivity is directly related to how many electrons are in the very outer shell. These are known as valence electrons.
Take the Noble gases for instance. They don't want to react with anybody. That's because they all have 8 electrons in their outer shell. In other words, they are full! There are no empty spots left, so they are good to go just like they are.
Alkali Metals, on the other hand, are just desperate to react. That's because they only have one valence electron. They are lonely! They want some other cool electrons to talk to, so they react with any element they can.
Remember how we talked about the periodic table increase in reactivity left to right? That because the number of electrons in the outer shell increases left to right, for the most part. We tend to skip over the yellow metals in the middle because they don't follow the rule. But, for the rest, the number of electrons in the outer shell goes like this:
The reason behind the reactivity wasn't discovered until long after Mendeleev's time. It all has to do with electrons; you know, the tiny little negatively charged particles from the Atom's page. Remember how we talked about the electron levels? Well those levels are important, because an element's reactivity is directly related to how many electrons are in the very outer shell. These are known as valence electrons.
Take the Noble gases for instance. They don't want to react with anybody. That's because they all have 8 electrons in their outer shell. In other words, they are full! There are no empty spots left, so they are good to go just like they are.
Alkali Metals, on the other hand, are just desperate to react. That's because they only have one valence electron. They are lonely! They want some other cool electrons to talk to, so they react with any element they can.
Remember how we talked about the periodic table increase in reactivity left to right? That because the number of electrons in the outer shell increases left to right, for the most part. We tend to skip over the yellow metals in the middle because they don't follow the rule. But, for the rest, the number of electrons in the outer shell goes like this: