Welcome to Introduction of Biology, Bio 105. In this lesson, we will discuss three things. The Bohr model of the atom, not boring, but Bohr. Periodic table and the types of chemical bonds that occur.
First, let's look at the periodic table. The periodic table organizes and displays different elements. Devised by Russian chemist Dmitriy Meldilov, who lived in around 1869, the table groups elements that, through unique, shared certain chemical properties with other elements. The properties of the elements are responsible for their physical state at room temperature, and they may be gas, solids, or liquids.
Elements also have specific chemical reactivity. The periodic table shows the atomic mass and the atomic number of each element, and the atomic number over here on the left appears above the symbol of the element, and the estimated atomic mass appears below it. The second thing we will talk about is the atomic model of Niels Bohr.
In 1913, Danish scientist Niels Bohr developed an early model of the atom. The Bohr model showed that the atom was a central nucleus, example here, containing protons and neutrons with the electrons in circular orbitals at specific distances from the nucleus. These orbits form electron shells or energy shells. So the number of electrons is equal to the number of protons for a neutral atom.
The electrons cannot exist in the other location, and the energy levels were designated by the symbols n. For example, 1n represents the energy level located closest to the nucleus. The electron normally exists in the lowest energy shell.
available, which is the closest one to the nucleus. Energy from the photon of the light can bump up to higher energy shells, but in this situation is unstable and the electron quickly decays back to the ground state in this process. It releases a photon of light.
Electrons fill orbitals in a consistent order. The first fill the orbitus closest to the nucleus, then they continue to fill orbitals of increasing energy further from the nucleus. If there are multiple orbitals of equal energy, they fill with one electron in each energy level before adding a second electron. The electrons of the outermost energy level determine the atom's energetic stability and its tendency to form chemical bonds with other atoms to form molecules.
So under normal conditions, atoms fill the inner shells first, often resulting in a variable number of electrons in the outermost shell. The inner shell has a maximum of two electrons, but the next two electron shells each have a maximum of eight electrons. And this is known as the octet rule.
So the outer shell has what we call valence electrons. and the most stable configuration occurs when the valence shell is filled. The elements in group 18, for example, have the full valence shell and entail the noble gases, which are very stable.
So the octet rule states, with the exception of the innermost shell, the atoms are most stable energetically when they have eight electrons in their valence shell, which is the outermost shell. Mutual atoms and their electron configurations usually have their outer shells filled and it is the outer shell, the valence electron, that reacts with other atoms in order to make bonds. Now the Bohr model had some problems. It didn't take into account several aspects of the atom such as the uncertainty principle of the electron.
It did not work for anything larger than a helium atom and the electrons had a definite radius or momentum. in this model. So what happened then?
Well, Bohr's model was a start, but it was incomplete. So what scientists found is that electrons are not orbiting around the nucleus in a planet-like orbit, but more like a cloud-like sphere with you know a certain percentage of the electrons whirling around in directions. Then scientists figured out that there are such things as complex shapes to describe how the electrons are spatially distributed around the nucleus.
Then Einstein came along with quantum mechanics and showed us how to predict whether the electron might be at any given time. This predicted area is known as the orbital, although useful to explain the reactivity and chemical bonding of certain elements. The Bohr model does not accurately reflect how electrons spatially distribute themselves around the nucleus. They do not circle around the nucleus like the Earth's orbits of the sun, but we do find them in electron orbitals. These relatively complex shapes result from the fact that electrons behave not just like particles, but also like waves.
Mathematical equations from quantum mechanics, which scientists call wave functions, can predict within a certain level of probability. probability where an electron might be at any given time. Scientists call this area where the electron is most likely to be found as its orbital.
Now recall that the Bohr model depicted the atom's electron shell configurations. Within each electron shell are subshells, and each subshell had a specific number of orbitals containing electrons. While it is impossible to calculate exactly an electron's location, scientists know that is most probably located within its orbital path.
The letters S, P, D, F, and F, sorry, the letters S, P, D, and F designate the subshells. The S subshell is spherical in shape and has one orbital. Here you can see the picture here in blue. The P subshell is a dumbbell shape and has three orbitals.
So the valence electrons or the outer Marshall electrons are involved in chemical reactions. And in chemical reactions, you have the reactants, which are the substances used to begin a reaction. And then you have the products, which are the substances formed at the end of the reaction.
A chemical reaction can be irreversible, which means that it proceeds only in one direction until all the reactants are formed. reactants are used up and become products, or it can be a reversible reaction, in which the reactants are converted to the product, but some product can still be converted back to the reactant. In terms of the different types of bonds that atoms formulate, it's about the electrons and whether they're shared or they're donated. And there's four types of bonds that are involved. ionic covalent hydrogen bonds and van der Waals.
We'll briefly touch upon the covalent bond, which has two categories, polar covalent and nonpolar covalent. Ionic bond is a bond resulting from the attraction between oppositely charged ions. One atom gives to another atom its electrons.
For example, is sodium chloride. The sodium is attracted to the negative charge of the chlorine which creates that ionic bond. Then in covalent bonding you have a bond that results from the sharing of paired electrons between two atoms because the outer shell becomes complete.
Hydrogen fluoride is an example. The hydrogen shares its electron with fluorine to complete the outer shell. Examples of covalent bonds are found in ATP, methane gas, and CO2.
If you have unequal sharing of electrons, you have a polar bond. Covalent bonds in which there is unequal sharing of the electron. Water is a great example. Oxygen has a higher electronegativity than hydrogen.
This means it wants the electrons more than the hydrogen, and so the hydrogen will give up its electron almost to the oxygen. Then, if you have equal sharing of electrons, you have non-polar bonds. These are covalent bonds in which there is an equal sharing of electrons. Methane is a great example.
Carbon shares electrons with four other hydrogens equally. And both bond type and the molecular shape determine whether a molecule is polar or nonpolar. An example of mixing a polar compound with a non-polar compound is oil and water.
They just don't mix. Here you can see that water and oil does not dissolve to form, instead you get these little oil droplets, and that's due to being a nonpolar compound. Hydrogen bonds and van der Waals interactions. So ionic bonds and covalent bonds require energy to break.
The weaker bonds are hydrogen bonds, which is an interaction between the positive of the hydrogen ion, and the partial negative charge of the more electronegative atom on another molecule, often occurs between water molecules. Van der Waals interactions are weak attractions or interactions between two or more molecules in close proximity due to changes of electron density.