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Transcript of CHEMISTRY
Everything in the world is made of chemicals. They are not just the colorful bubbly liquids in beakers and test tubes that scientists have in their labs. Grass, rock, cars, buildings, houses -even you - are all made out of sets of basic chemicals that combine in specific ways to form different substances. Chemistry describes the study of how those chemicals join to make everything around us, and how we can combine or split chemicals to make and discover new substances. Chemicals are changing all the time. Chemistry keeps on happening all the time.
Chemistry in action
The best place to see chemistry happening is in the kitchen. We eat a lot of food raw, and we cook some so they're safe to eat, they taste better, or they're easier to digest. Cooking changes the chemicals in food, normally by heat. A raw egg is a runny, slippery, transparent "white" and a thick, yellow yolk. When you boil it, heat is added, causing both parts to become solid, making a hard-boiled egg. Or heat it in a pan with some oil or butter and it becomes yellow lumpy scrambled egg. When you heat a piece of bread in a toaster, it becomes crispy, and its color changes to brown.
Atoms, molecules, elements
An element is a pure substance that cannot be broken into smaller substances by chemical means. Gold, copper, and oxygen are all examples of elements. there are around 100 elements all together. The smallest part of an element that still has the elements properties is an atom. Atoms are not usually found on their own.they combine with other atoms to form molecules. An oxygen atom combines with two other hydrogen atoms to form an H2O molecule or water molecule, the smallest amount of water that can exist.
John Dalton is an English scientist who was the first to develop the ancient Greek idea of atoms and the atomic theory, which states:
Elements are made of tiny particles called atoms
Atoms of an element are all identical to each other
Atoms of a given element are different from the atoms in other elements
Inside an Atom
JJ Thompson was the first person to actually "look" in an atom and find something. He thought that atoms were made of even smaller particles called subatomic particles. He put an element in a cathode ray tube and found that they produced a ray of negative energy. He assumed if they are neutral then they must have atoms that are positively charged as well. He also thought every atom had these particles because he got this result for every element he tried. This lead to the discovery of protons and electrons.
Ernest Rutherford was the one who discovered the nucleus of an atom. He did this when he conducted an experiment where negatively charged particles were fired at high speed through a giant gold foil. He saw that in about every 10 000 particles, one would bounce back instead of going through as if it hit a giant positively charged object, the nucleus. This lead to the second diagram of the atom.
Neils Bohr came up with the idea that electrons surround the nucleus in energy shells called electron shells. He created the Bohr Diagram, which is still used today.
How to Read The Periodic Table?
The periodic table is a table in which all the elements that we know are organized. It helps chemists understand how they react and what elements are similar to each other. The periodic table has groups, or columns, and periods, or rows. Each element has its own symbol so you can easily know which element is which. The symbol is 1 or 2 letter and it comes from it's Latin name ("carbo" meaning charcoal is the Latin name for carbon, so it's symbol is C). Each element is shown with two numbers. The atomic number is the number of protons in the atom. The atomic number of carbon is 6, so it has 6 protons. If you added a proton to carbon, the atomic number would become 7. The atom would no longer be carbon, it would become nitrogen. The second number is the atomic weight or relative atomic mass (RAM). This shows how heavy an atom is relative to other atoms. Carbon has a RAM of about 12, meaning its 12 times heavier than a hydrogen atom, and 2 times lighter than a magnesium atom.
The picture below shows the first ever copy of the periodic table by Dmittiri Mendeleev. He created this model in the 1860s after he saw a pattern in the list of elements other chemists made, which was organized by atomic weight. He was so confident in his work that he left gaps in his model and explained that they were elements that were not yet discovered. He used his table to predict the properties of those elements. He named element number 32 eka-silicon and predicted that it would be a gray, shiny metal, and gave estimates of its density and melting point. His predictions were proven correct when element number 32, later named germanium was discovers in the late 1880s.
periods and groups
The first group on the periodic table is made of elements that have only one electron in their valence shell, like hydrogen, lithium, and sodium. Group 2 has atoms with two valence electrons. That includes magnesium and calcium. This continues all the way to group 8 which is made up of gases with a complete outer shell. This is also called group 0 be some chemists because it doesn`t have any electrons that take part in chemical bonding. They have all the electrons they need to be stable so they don`t react with other elements.
The periodic table has seven periods. The first one has two elements, hydrogen and helium. They are the simplest atoms because they have only one electron shell, making the first period unusual becuase the others don`t have only two elements. The reason there are only two atoms in period one is because the first shell only has space for two electrons. When you add a third one, creating Lithium, it becomes the first member of the next period and creates a second shell. The second row contains elements like nitrogen, carbon, and oxygen. There are eight elements on this row meaning the second shell has room for eight electrons before the third shell is added. The third period, running from sodium all the way to argon, also has eight elements. After that things get more complicated as the shells start accepting more electrons. This creates the block in the middle of the table called a series.
That, however, does not apply to the elements in Group 1, also known as the Alkali metals, which need to lose just one electron, getting rid of the outer shell completely, to become stable. As a result, these elements, such as potassium and sodium, are extremely reactive. Group 7, also known as the haloogens, are also reactive. They have seven electrons in their valence shell, meaning they need one more to become stable. These elements, such as Chlorine, tear away electrons from other elements to become stable negatively charged atoms.
One very common substance on Earth is sodium chloride, the scientific name for table salt. This is found in seawater, or used for seasoning food. A salt molecule forms when an electron from a sodium atom is given to a chloride atom. Both ions hold onto each other strongly making table salt very stable a substance.
Patterns in reactivity
For two or more atoms to bond together, a chemical reaction must occur. When a chemical reaction happens, the electrons of the atoms involved have to rearrange their electrons into a more stable state. For this process to start, energy is needed. The amount of energy is dependant on how easily the atoms can rearrange their electrons. Tthe most reactive elements require only a little amount of enrgy to form bonds and react. The periodic table helps us see which elements are more reactive than others. The atoms in the left side, groups 1 and 2, have only one or two atoms in their outer shell.They would usually get rid of those electrons and their outer shell to create charged ions which later form ionic bonds. It only requires a small amount of energy to let go of one electron, and a bit more to let go of two. Therefore the elments in the first group are more reactive than those in the second one. Tthe pattern continues as you move to the right, making groups 3 and 4 even less reactive. The elements on the right side are reactive as well, just in a different way. The elements in Group 7 and Group 6 need one or two electrons to become stable. They react by trapping the electrons that they need to fill their shells. Flourine, an element in Group 7, requires less energy to trap one electron than the amount the elements in Group 6 would need to trap two, making the elements in Group 7 more reactive. So this tells us that atoms to the left and right require more energy to react therefore are more reactive than those in the middle. Group 8, the Noble gases is the exception to this since they have all the electrons they need to be stable, therefore they don`t react with other elements.
Metallic bond: Electrons that have broken off metal atoms form a ``sea`` of electrons that keeps the atoms together
Ionic Bond: An atom with a few electrons in its outer shell lets go of these electrons, becoming a positive ion. An atom with a few missing spaces takes the electrons, therefore forming a negative ions. The io are attracted to each other, forming a molecule.
Covalent Bond: Two or more non-metals share eletrons so they fill their outer shells. They connect into a molecule.
Metals, NonMetals, and Metalliods
Using the periodic table, we can classify the elements into different groups. A useful way to do so is to group them by whether they aremetals, nonmentals and metalliods.
They have properties that are similar to those of the metals in our everyday:
all solids at room temeperature (except Mercury which is a liquid
shiny, good conductors
ductile and malleable
lose their electrons easily
They're properties are opposite to those of metals:
not malleable or ductile
some are liquids
tend to gain electrons
Metalloids, or semimetals share properties of both metals and nonmetals. They have unique conducting abilites so they are called semiconductors, they only partially conduct heat and electricity. This makes them very useful in the computer chip industry.
By: Salma Elasfar