How to use Github



About GitHub

GitHub is a git repository hosting service founded in 2008 which allows users to work together to create source code repositories and indexes. It is the largest host of source code in the world with over 24 million users and 67 million different repositories. GitHub has been used for many video games, applications, and browser extensions.

How to Create an Account

GitHub requires that you make a username, give a valid email address and create a password associated with your username. You can set up your account here. You will also need to set an email address which you will use to commit (publish) pieces of program code from your computer. GitHub recommends that you verify your account with an email address as well.

Basic User Features
- Repository Issues
- Repository Pull Requests
- Branch Management
- User Profiles
- Organizations

Programming/Adding Repositories

To create your first repository, navigate your way to the top left of the GitHub menu bar and click "+". Now click "New Repository" and fill in the details on that page. Please note you have to have a confirmed email address to create a new repository. Now, you can download Git and clone your repository. Now you can start to code! GitHub will support any language you can think of, so write a simple program, start tracking your repository and then push your changes!

Conclusion

Learning GitHub isn't that hard once you know your way around the website and it is an amazing program that supports open-source code. If you are truly interested in being involved in this community, then you will probably need to learn much more than we could summarize in this article. Therefore, we recommend that you look to the Github Help page for support in your future ventures. If you are a total beginner, then please look at these Github guides.

Written by MakeTheBrainHappy and -Vuton-. The banner was created by bigpuppy.
Edited by WolfCat67 and jromagnoli. This was originally posted in the Scratch Open Source forums.

How many Valence Electrons does Carbon have

Answer: Carbon has four valence electrons located in its second energy level. Because Carbon only has four electrons, it can form up to four other bonds which tend to be covalent in nature.

Carbon is Element #6 on the periodic table and has an average mass of 12 amu. The model shown to the left differentiates the electrons based off of whether they are in the s vs. p orbital. Still, the two orbitals which are closer together constitute the second energy level and are all considered to be valence electrons. Source.
Carbon has an average mass of 12 amus which means that other isotopes. Indeed, carbon has three known isotopes: Carbon-12, Carbon-13, and Carbon-14. The last of these is known to be radioactive. Carbon's four valence electrons allow it to form a variety of bonds and compounds which exist in nature. It is also one of the key ingredients in molecular processes going on in your cells. Let's explore some of the compounds which carbon forms to carry out these vital processes.

Carbon can form some pretty interesting "Covalent Network Solids." Learn more about this topic in this post. They are some of the most durable materials on earth due to the fact that in order to melt them you need to break covalent bonds. Diamond is one of the hardest materials on earth, while graphite is more soft due to its more complex electronic structure. Source
One class of compounds which carbon forms, especially with itself, it a Covalent Network Solid (shown above). Carbon can also form many bonds with other compounds such as hydrogen and chlorine to form molecular solids. Methane and Carbon Dioxide, two greenhouse gases, are common compounds in which carbon is the central atom connecting all other component structures together.

Carbon is also an important component of sugars which we ingest every day. Source
Carbon is essential to holding together large complex polysaccharides which we consume on a daily basis to gain energy (ATP). Common examples of sugars are glucose, fructose, galactose, sucrose and maltose. Carbon is usually near the center of the molecule. Its four valence electrons allows it to form the maximum amount of bonds in compounds.

To summarize, carbon can form four bonds because it has four valence electrons in its second energy level. This allows it to form a wide variety of compounds and molecules which form the building blocks of many everyday materials and of life itself.


How many valence electrons are in an atom of Phosphorus

Answer: A neutral Phosphorus Atom has five valence electrons. These are contained in the third energy level of the atom. 


Phosphorus Atom Properties
The Phosphorus Element as it would be represented on a modern periodic table. Source
Phosphorus is an element which is part of Group 15 (formally known as the Pnictogen group) and is directly below the nitrogen atom. As was mentioned before, a neutral Phosphorus Atom contains five valence electrons in the third shell. You can see a Bohr Model of Phosphorus below this paragraph. It also supports the original answer of five valence electrons.
Bohr Model of a Phosphorus Atom
Bohr Model of a Phosphorus Atom.
Due to the number of valence electrons, Phosphorus is capable of forming three bonds with other elements. Yet since it has a third-energy level, it can form more than three bonds if it is favorable to do so. Phosphorus tends to form an anion even though it has a low electronegativity. Due to its high reactivity, phosphorus is never found a pure element on earth.

Electron Configuration of Phosphorus with a Lewis Diagram on the side as well. Source
The Valence Electrons are found in different types of orbitals. Two are in the s-orbital (a circular orbital) while the other 3 are in p-orbitals (bell-shaped orbitals). There are a total of 4 orbitals in the valence shell to minimize electron-electron repulsion. Since their is less electron-electron repulsion, this formation is actually more stable than one with an extra electron. Therefore Phosphorus has a higher ionization than its Group-16 (Chalcogen) neighbors Sulfur.

Phosphorus as it moves in a biochemical process. Source
Phosphorus is necessary for some functions in life and is an essential component of fertile soil. It moves as a compound (not as a pure element) through what is called the Phosphorus cycle from the soil to plants and then back into the air before returning to the soil. Phosphorus is also essential component of matches. It was discovered in 1669 by Hening Brand.

Sources:
http://thechemistryguru.com/phosphorus/

When an atom loses an electron, it becomes

Answer: When an atom loses an electron, it becomes a cation (positive ion). When an atom gains an electron, it becomes an anion.


This diagram displays the difference between a positively charged ion (cation), a neutral atom, and a negatively charged atom (anion). Source
Ions are created when an atom loses or gains an electron. Atoms can never lose or gain protons, as this would change the fundamental identity of the atom. The overall charge is determined by the balance between protons and electrons. In the diagram below, the negatively charged atom is said to have a -1 charge while the more positively charged atom has a +1 charge.

As was stated before, when an atom loses an electron, it becomes a cation. What kinds of substances can form cations?

Table Salt  (NaCl). Image Source
Substances which undergo ionic bonding create a positive cation and a negative anion which are held together by these charge attractions. Salts, such as the NaCl shown above, are a type of solid which is held together by ionic bonding. These ions disassociate in polar solvents, such as water. This is why table salt dissolves in water. In this aqueous state, the Na+ and Cl- ions begin to move around freely in the solution and become ions. These free moving cations and anions are the reason why this new solution is a good conductor of electricity.

Silver (Ag) Bars. Image Source
Metals, which usually form metallic bonds are also prone to ionizing. Silver, shown above, almost always ionizes with a +1 charge. There metallic bonds are slightly weaker than the ionic bonds before and that it why they can sometimes lose electrons without much trouble. The electrons in metals are also delocalized, which means that they can move around freely. This makes it very difficult to accurately predict what charge most metals will form. The metals with form known charges are Ag (+1), Zn (+2) and Sc (+3).

Periodic Table with electronegativies. Darker = Higher Electronegativity. Source
To summarize, when an atom loses an electron it becomes a cation. You will generally elements as cations on the left-hand side of the periodic table, since these elements have a low electronegativity. The same logic can be applied to the other side of the periodic table, where you will generally see elements as anions since they have relatively high electronegativities.



What is a Catalyst?

Answer: A catalyst is a substance which changes the rate of a reaction without being used up by the reaction. 
This diagram explains the effect of a catalyst as lowering the activation energy needed for a reaction to occur. Source
A catalyst essentially lowers the amount of energy (called the activation energy) needed to run a chemical reaction. This will allow for more of this reaction to occur which will have the outcome of usually speeding up your entire experiment. There are two different types of catalysts:

Heterogeneous Catalysts- When the Catalyst is in a different phase when compared to the other reactants.

Homogeneous Catalysts- When the Catalyst is in the same phase when compared to the other reactants.

This paper describes an interesting use of catalysts, where the scientists formed Methane based on an Iron Catalyst and some CO2. Catalysts are used in many reactions where the activation energy is to high and prevents the reaction from occurring under normal circumstances.

How can I identify a Catalyst?

You will usually find a catalyst at the beginning (with the reactants) of a multi-step process and at the end (with the products). Then you can identify a substance as being a catalyst, since it was unchanged throughout the entire process.

A biological catalyst which breaks down maltose sugars into glucose. Source

What is a biological Catalyst called?


A biological catalyst is known as an enzyme, since these are materials are proteins which were molded so that they could help speed up reactions which take place inside of the cell. They do this by supporting or hindering reactions through their active sites, which allows cells to regulate the reactions inside of them. Inorganic molecules can also serve as catalysts in biological systems, but these are not unique to biological systems.

The Lewis Dot Structure for H2O

H2O Lewis Structure
Created by MakeTheBrainHappy.

This is the Lewis Dot Structure for H2O. You could alternatively also draw the structure by including two dots for every bond. While oxygen's octet seems to have been filled, hydrogen only has two electrons for its valence shell. Based on our previous discussion surrounding the covalent bonds which hydrogen forms, we know that hydrogen's valence shell only requires two electrons because it only has a 1s orbital. Therefore its valence electron is filled by oxygen sharing another electron.  

H2O's Lewis Dot Structure gives it many unique properties mostly due to the two lone pairs on the central oxygen atom. This increases electron-electron repulsion and therefore creates a bent structure as opposed to CO2's linear structure. This "bent" molecular structure gives it many unique properties such as being polar. One of the most fascinating phenomena is the idea of "hydrogen bonding" which influences water's properties dramatically.

Hydrogen bonding is a type of intermolecular force where the electronegativity difference in the oxygen atom causes a partial permanent dipole to form between the hydrogen and oxygen atoms (shown in the dashed lines). The bonds in the water molecule themselves are covalent bonds. You can learn about the difference in this post. Source

Due to the strength of these hydrogen bonds, water has a relatively high melting and boiling point, although they are not as high as network covalent solids. Those are bonded by intramolecular forces which involve the actual sharing of electrons vs. partial dipole forces in hydrogen bonds. There are only three types of bonds which can hydrogen bond. These are N-H, O-H, and F-H bonds due to the large electronegativity differences between the molecules.

NH3 is another molecule which creates similar hydrogen bonds as H2O. Source

How does hydrogen bonding impact the melting and boiling point of water?

Common knowledge indicates that the melting point of water is 0˚C whereas the boiling point is 100˚C. This is due to the exceptional strength of hydrogen bonds as an intermolecular force. Comparing it to the two other "simple" structures which undergo hydrogen bonding; namely, HF (hydrogen fluoride) and NH3 (ammonia), we see that H2O actually has a higher melting and boiling point than either of these two. 

The reason for this can be found by examining the lewis dot structure for H2O. When you look at the structure you notice that it has two lone pair electrons and 2 hydrogen bonds. When you visualize the structures for HF and NH3 (you can see the lewis structure of NH3 in this article) you see that they either have three lone pairs and one hydrogen or three hydrogens and one lone pair. This does not optimize the amount of potential hydrogen bonds; whereas H2O's 2:2 ratio does. 

Therefore on average H2O is able to form more hydrogen bonds with itself as indicated by the lewis dot structure of H2O, allowing for an overall stronger intermolecular force. This means that more energy is required the break these intermolecular forces to allow for either melting or boiling (phase transitions) and therefore H2O has a higher freezing/boiling point. 

What other properties does water have?

Water is one of the only elements to be found in all three states (solid, liquid and gaseous) naturally on earth. Hydrogen bonding causes a crystalline structure which makes it less dense than liquid water. This is why ice floats in the oceans. Water has also a high surface tension and adhesion. Cohesion and adhesion are essential for plants who manipulate these processes to move water from the roots to the leafs to support the growth of the plant. All this is caused by the simple structure of H2O, represented by the Lewis Dot Diagram above.

Water molecules in all three states of matter. Source

How does the lewis dot structure for water relate to acids and bases?

As you can see in the lewis dot structure for H2O, water can be broken up into two constituent ions: H+(aq) and OH-(aq). It may also be helpful to mention that H+ ions, i.e. individual protons, do not actually exist naturally within solution (they exist as H3O+ or hydronium ions). However for the purposes of this article we will consider H+(aq) as equivalent to H3O+(aq). These groups are incredibly important in formulating various environments and reacting to different species. So important in fact that they are very prominent in the definitions for acids and bases. 

A specific definition including both was the first developed for acids and bases in 1884 by the Swedish chemist Svante Arrhenius. He stated that acids produce H+ ions and bases produce OH- ions. However this theory does not account for molecules that don't contain these ions and rather interact with water to produce them (such as ammonia - NH3). Nevertheless, more expansive theories dealing with these limitations center around these two very important ions. 

So to what extent are these ions produced in pure water?

Pure water here is defined as just containing H2O molecules. As mentioned in our previous analysis of the lewis dot structure of H2O, the intermolecular forces governing behavior are very strong. So are the intramolecular bonds within the molecule which are governed by the large electronegativity difference between hydrogen and oxygen (2.20 vs. 3.44, respectively). Given that the electronegativity scale (originally developed by famous twentieth century chemist Linus Pauling) goes up to four this difference is incredibly significant. As you can imagine therefore the amount of ionization is very low: the solubility constant is approximately 1.00 * 10^-14 at 25˚C. 

From this solubility constant you can calculate the approximate molar concentration based on the following equation (the autoionization of water):

2H2O(l) => H3O+(aq) + OH-(aq)

Kw = [H3O+(aq)][OH-]

Since the H3O+(aq) and OH-(aq) are produced in a 1:1 ratio we can replace these with x representing the amount of each ion produced.  

1.00*10^-14 = [x][x]

x = 1.00*10^-7 M

This means that .00001% of a pure water solution contains H3O+(aq) with another .00001% containing OH-(aq). On the whole however, this is an incredibly small proportion of the total water solution. 

How does this process depend on temperature?

As we mentioned before in our discussions of the lewis dot structure for H2O the intermolecular forces between H2O molecules are very strong. These need to be broken in addition to the intramolecular forces. When you increase the temperature you increase the average amount of energy available to break bonds within the environment. Therefore you would expect there to be a higher molar concentration of these ions in the pure water solutions as you increase the temperature. 

Kw decreasing as temperature increases => increases molar solubility. Source

As you can see in the graph above this trend does bear out since the Kw is decreasing as temperature increase. Since we are talking about concentrations that are very small decimals decreasing how negative the power is is like increase the concentration. You can perform a similar molar solubility calculation as we did above in order to figure out the concentration at any temperature above (estimating the y-value based on the x-value).

Is H2O polar or nonpolar?

Based on our previous discussions surrounding the lewis dot structure for H2O you could take a pretty good guess what the answer to this question is. However if you are interested in learning more about the answer to this question you can check out our article on the polarity of H2O. The article also goes over the phases of water and the concept of water as a "universal solvent", both what is implied by the term and what the limitations are of water as a solvent.  

Scratch 101: Winter Trees

Hello and welcome to Scratch 101! In this tutorial we will be creating a "Winter Tree" that is customizable after we complete it.
Objective:
To create an animation using data structures, angles, and pen features. Think about how the image can be customized.
Final Project:

Graphical Setup:
The graphical template is located here: https://scratch.mit.edu/projects/197133814/
Scripts:
Common Mistakes:
  • Make sure that you type in the correct numbers!
  • Make sure to correctly create the "Repeater", "X", and "Y" variables. You can learn more about variables here.
  • Make sure to set the colors correctly to either "green" or "brown". Learn more about this pen block here.

What Type of Bond is joining two Hydrogen Atoms

Answer: Hydrogen atoms in compounds such as H2 and H2O are joined to the molecule with covalent bonds. This is where electrons are shared between the atoms in order to fill there valence shells. 


Representation of the covalent bond joining two hydrogen atoms in the H2 bond. Source

As we mentioned before, hydrogen can only form one covalent bond because its 1s electron orbital holds a maximum of two electrons. Hydrogen can form either a polar covalent bond or a nonpolar covalent bond (want to know the difference? Click here). H2 (shown above) is an example of a nonpolar covalent bond (because there is no electronegativity difference between the two molecules). Water (shown below) contains polar covalent bonds because the oxygen has a greater electronegativity than the hydrogen atoms.
Water is a good example of hydrogen bonded in a polar covalent molecule. Source

Why can I not call this "hydrogen" bonding?

Hydrogen bonding refers to a phenomena which occurs when an H is bonded to either nitrogen, oxygen or fluorine. Water (shown above) is a molecule which can exhibit hydrogen bonding. Due to the large electronegativity difference, strong dipoles are created on either end of the molecule (as shown below). These can then attract one another and create substances with higher boiling points.

Hydrogen bonding between water molecules. Source
 Hydrogen bonding occurs between these dipoles and these so called "H-bonds" are actually not permanent bonds and can be broken quite easily. Covalent bonds are much stronger and are used to build the molecule itself. Hydrogen-bonding could not occur between two hydrogen atoms anyway because the hydrogen always has the partial positive charge. If the hydrogen's were to get close to one another, then these like charges would repel eachother.

Therefore, hydrogen atoms are joined to themselves or other elements with covalent bonds.