Atoms and the periodic table
Definitions & Key takeaways
The periodic table is a chart that organizes all the known elements according to their atomic number (number of protons in the nucleus). The elements are arranged in rows and columns, with each element occupying a specific place on the table. Each element has a unique symbol that is used to represent it. For example, hydrogen is represented by H, lithium by Li, and oxygen by O. The atomic number of an element is represented by a superscripted number to the left of the element's symbol. For example, the atomic number of hydrogen is 1, so it is represented as H1.
The elements are arranged in increasing order of atomic number. As you move from left to right across a row, the atomic numbers increase. As you move down a column, the atomic numbers increase.
One of the first models of the atom was the “plum pudding” model, which was proposed by the physicist J. J.
Thomson in the nineteenth century. In the “plum pudding” model, atoms were thought to have a cloud of positive charge, and within that cloud there were tiny electrons with negative charge.
Sort of like the mid-nineteenth century british dessert — plum pudding --- where the plums were the electrons sitting within a delicious positive charge of pudding.
However, in the 20th century the physicist Ernest Rutherford realized that the plum pudding model wasn’t quite right. He did a famous experiment where he took positively-charged alpha particles, and shot them at a piece of gold foil.
Most of these alpha particles passed straight through the gold foil and hit the detector, but some of them actually bounced off of the gold foil and hit the sides of the detector.
His explanation for this was that the atoms are mostly empty space, and that the majority of the alpha particles travel straight through the foil and strike the back wall of the detector, but that on very rare occasions, the alpha particles bounce off of something.
He visualized a very small but very dense object in the middle of the atom, that he called the nucleus, which the alpha particles were bouncing off of on those rare occasions.
Since the alpha particles are positively charged, Rutherford figured that the nucleus must also be positively charged since like charges repel.So Rutherford gave us the idea that the modern atom consists of mostly empty space containing electrons, which are miniscule in size compared to the atom overall.
But in the middle of that empty space there is a very small but very dense nucleus. Later, scientists showed that the nucleus actually consists of two types of particles: protons and neutrons.
Protons have a positive charge but neutrons have no charge.Now, when looking at the periodic table you can find the simplest atom, which is hydrogen.
Hydrogen has the symbol H, and it has an atomic number equal to one. Atomic numbers tell you the number of protons in the atom which means that Hydrogen has one proton in its nucleus.
The number of protons in the nucleus of an atom is unique to each element. A typical hydrogen atom has a nucleus with one proton and one electron outside the nucleus.
Hydrogen is an electrically neutral atom because the electron and proton have the same magnitude of charge, but different signs, so their charges cancel out.
However, there are variants of hydrogen atoms called “isotopes,” which all have one proton and one electron, but varying numbers of neutrons in their nuclei.
The most common isotope of hydrogen is protium, which does not have any neutrons in its nucleus. Another isotope, deuterium, has one neutron in the nucleus.
And a third isotope is tritium, which has two neutrons in the nucleus.We can distinguish between these isotopes by writing 11H for protium, 21H for deuterium, and 31H for tritium.
Here, the subscript one refers to the atomic number. For an electrically neutral atom, it also tells you the number of electrons as well.
The superscript refers to the mass number, which is the number of protons plus neutrons. Since protium has one proton and no neutrons, the mass number is one.
Deuterium has one proton and one neutron, so the mass number is two. Tritium has one proton and two neutrons, so the mass number is three.These rules apply to any element in the periodic table.
For example let’s look at carbon on our table. Carbon has an atomic number of six, which tells you that it has six protons, and it’s electrically neutral, so it has six electrons as well.
An example of a carbon isotope can be written as 136C.The superscript of 13 is a mass number, which equals the total number of protons and neutrons.
If we subtract the number of protons, which is 6, then we’re left with 13 minus 6 or 7 neutrons. Another way to represent isotopes is to write the name of the element followed by a hyphen and then the mass number.
So carbon-13 refers to this carbon isotope.Let’s get our periodic table back up again. Now as you can see, hydrogen has an atomic number of one, which means one proton and one electron.
The electron for hydrogen can be symbolized by drawing a dot next to the H symbol. This is called Lewis dot notation.
The next element on the periodic table, helium, has an atomic number of 2, which means that it has two protons and two electrons.
Since the neutral helium atom has two electrons, the Lewis dot notation for helium has two dots next to the He symbol.Atoms have shells around them that they like to have “filled” with electrons.
Both hydrogen and helium have a shell that can hold a maximum of two electrons. Since helium has two electrons, Helium already has a full shell and is chemically unreactive.
In fact, helium is an example of a “noble gas”, which are elements on the far right side of the periodic table that are unreactive because their shells are filled with electrons.
In contrast, hydrogen is very reactive since only has one electron and is therefore much more willing to form a bond with another element so that it can share two electrons and have a full shell.
The next element on the periodic table is lithium, which has an atomic number of 3. So it has three protons and three electrons.
Two of the electrons fill up the first shell. The two electrons in the first shell are considered to be inner shell electrons and are called core electrons.
The third electron must go into a second shell. This electron in the second shell is considered to be an outer shell electron and is called a valence electron.
Valence electrons are important because they are the electrons that participate in chemical reactions. When we draw Lewis dot notations, we are only drawing valence electrons.
So to draw the dot notation for Lithium, we draw a single dot next to lithium to represent its one valence electron. The next element is beryllium, and it has an atomic number of 4, which means it has four protons and four electrons.
Two of the electrons fill up the inner shell, and the remaining two electrons are in the outer shell. Since valence electrons are the electrons in the outer shell, beryllium has two valence electrons.
So to draw the dot notation, we draw two dots next to the symbol for beryllium.Next up is boron, with five protons and electrons.
Two core electrons fill up the inner shell, and the remaining three valence electrons are in the outer shell. So Boron has three dots for its three valence electrons.
Then there’s carbon with four dots, nitrogen with five dots, oxygen with six dots, and fluorine with seven dots. Finally, there’s neon with eight valence electrons which completes its second shell.
Just like helium, this is a noble gas because it’s not looking to chemically react.Each horizontal row of the periodic table is called a period, and moving across the period from left to right, we are adding one additional proton and electron for each element.
Hydrogen and helium are in the first period, and moving from Lithium to Neon includes the elements in the second period.
Now that the second shell in the second period is full, we have to move to a new period.Next let’s look at sodium, which is in the third period.
The atomic number is 11. Therefore, sodium has eleven protons and eleven electrons.
The first two electrons fill the first shell, the next eight electrons fill the second shell, which leaves one electron that has to go into a third shell.
The ten electrons in the two inner shells are core electrons and the one electron in the outermost shell is a valence electron.
We keep adding valence electrons to the outer shell as we move across a period and when we completely fill a shell, we move to a new period.
Now, the vertical columns on the periodic table are called groups. The leftmost column of elements is group 1A, and they all have one valence electron.
Likewise, all the elements in group 2A have two valence electrons, and so on. So the group number tells you how many valence electrons an atom has.
And because they all share the same number of valence electrons, each group of elements has similar chemical properties.
For example, group 7A are the halogens, and since they all have seven valence electrons, and are just missing one electron to fill their shells, they’re very chemically reactive.
A final pattern, is that the left side of the periodic table has the metals. One exception to this is hydrogen, which is a nonmetal.
And the right side of the periodic table has the nonmetals, elements like carbon, nitrogen, and oxygen. A little step ladder line is drawn on the periodic table, which divides the metals from the nonmetals.
Some of the elements that sit directly on the stepladder line, like boron and silicon, are called metalloids.##SummaryAs a quick recap, early experiments revealed that atoms consist of a nucleus that has protons and neutrons.
The nucleus is surrounded by negatively-charged electrons. Atoms of each element have a unique number of protons; however, isotopes of the same element have different numbers of neutrons.
In a neutral atom, the number of protons is equal to the number of electrons. Elements in the same group on the periodic table have the same number of valence electrons and therefore react in similar
No notes for this video yet
Try adding a note below