MondayOctober 4, 2010 • (Review of Important Concepts)
Bell RingerMonday, 10-4-10 List the five major points of Dalton’s Atomic Theory. • All matter is composed of extremely small particles called atoms. • Atoms of a given element are identical in size, mass and other properties: atoms of different elements differ in these ways. • Atoms cannot be subdivided, created or destroyed. • Atoms of different elements combine in simple whole-number ratios to form compounds • In chemical reactions, atoms are combined, separated or rearranged, not created nor destroyed.
Announcements • ??
Steps in the Scientific Method • ask a question • do background research • construct a hypothesis • test your hypothesis by doing an experiment • analyze your data and draw a conclusion • communicate your results
Reading a Graduated Cylinder • How much liquid is in each cylinder?
Density of Liquids • Less dense liquids float on top of more dense liquids. Honey has the highest density of all of the liquids in this column. Karo syrup is less dense than honey, so it floats on top of the honey. The liquids progressively get less dense as you go up the column. Which liquid has the lowest density?
Physical vs. Chemical Properties • Physical Properties • phase – solid, liquid, gas, plasma • phase changes – melting point, boiling point, freezing point, condensation point • polarity and non-polarity of molecules • Chemical Properties • burning • rusting • tarnishing • digesting
Flying around throughout the container Particles in Phase • Particles in the liquid phase • Particles in the solid phase • Particles in the gas phase • In a tight, rigid structure • Occupying the bottom portion of the container
Differences in Physical Properties • Recall the saltwater solution that you had in your mixture separation lab. • If you didn’t have a way to boil the water off, you could have set the beaker on a table for several days and the water would have eventually evaporated out of the beaker, leaving the salt. • In order for this to happen, what category of matter must saltwater be? • Answer: a mixture!
Independent vs. Dependent Variables • Things that change are called “variables.” • In an experiment, there can only be two variables: the independent variable and the dependent variable. • At the beginning of the experiment, the experimenter changes the independent variable to kick things off. • As the experiment proceeds, another variable will change, the dependent variable, as a result of the independent change.
Physical vs. Chemical Changes • When a physical change occurs, the material that changes does not change its chemical identity. • For example, when a log is cut in half, it’s still a log. No new substances are produced. • When a chemical change occurs, the material that changes does change its chemical identity. • For example, when a log is burned, it’s no longer a log – it’s now ash, smoke, carbon dioxide, and water vapor. New substances are produced.
Density • A sample of material is in the shape of a cube. The cube is 1.26 cm long, 1.43 cm wide, and 0.650 cm high. The cube has a density of 4.26 g/cm3. What is the mass of the cube? • Density = mass/volume • Therefore, mass = density x volume • mass = 4.26 g/cm3 x (1.26 cm x 1.43 cm x 0.650 cm) • mass = 36.4 g
Important Safety Equipment • Some safety equipment is meant to prevent an accident from happening. • Some safety equipment is meant to be used after an accident has occurred and is designed to lessen the accident’s effect. • Determine whether each of the following is preventative or is designed to lessen an accident’s effect after it’s already happened. • Aprons, goggles, first aid kit, fume hood, shower, fire extinguisher, eyewash
The Strong Nuclear Force Generally, particles that have the same electromagnetic charge repel one another, therefore, we would expect a nucleus with more than one proton to be unstable. However, when two protons are extremely close to each other, there is actually a strong attraction between them. A similar attraction exists when neutrons are very close to each other, or when protons and neutrons are very close together.
The Structure of the Atom The Nucleus • The Strong Nuclear Force • These short-range proton-neutron, proton-proton, and neutron-neutron forces hold the nuclear particles together and are referred to as nuclear forces.
The Structure of the Atom Together, protons, neutrons and electrons are referred to as subatomic particles.
The Structure of the Atom Charge and Mass of the Sub-atomic Particles
Some substances are made up of only one kind of atom - Elements Exactly how does one element differ from another? It is the number of protons in an atom that distinguishes an atom of one element from the atom of another element
All atoms of the same element will have the same number of protons, and atoms of different elements will have different numbers of protons Most atoms are very stable. Even if atoms bond or break apart during chemical reactions, the number of protons in each atom always remains the same. The atoms themselves are only rearranged in different combinations Adding or removing a proton from an atom usually takes (or releases) huge amounts of energy
We can refer to each element by the number of protons its atoms contain Atomic numbers start at 1, with the element hydrogen, and go up by ones to The synthetic element number 114. The heaviest elements have been created in a laboratory and have not been seen in nature This unique number is called the atomic number
In addition to the atomic number, every atomic nucleus can be described by its Mass Number Atoms of the same element have the same number of protons The mass number is equal to the total number of protons plus neutrons in the nucleus of an atom Atoms of the same element can have different numbers of neutrons
In the Periodic Table, the atomic number increases by one whole number at a time. This is because you add one proton at a time for each element. The atomic masses however, increase by amounts greater than one. This difference is due to the neutrons in the nucleus. Neutrons add mass to the atom, but do not change its charge
The total number of protons and neutrons in the nucleus of an atom is called the mass number isotope notation Sometimes, the mass number of an element is included in the symbol. By convention, the mass number is written as a superscript above the symbol and the atomic number as a subscript below the symbol You can find the number of neutrons by subtracting the atomic number from the mass number. How many neutrons does the carbon atom above have?
Many elements have atoms with different numbers of neutrons. These different forms of the same element are called Isotopes Any given Isotope of an element is called a nuclide. Isotopes are atoms of the same element that have different numbers of neutrons The three Isotopes of Hydrogen Mass # 1+0=1 Mass # 1+1=2 Mass # 1+2=3
Atomic Mass Units are used to assign masses to an atom. One AMU is exactly 1/12 the mass of a carbon-12 atom. Average atomic mass is the weighted average of the atomic masses of the naturally occurring isotopes of an element. Cu: 69.17% Cu-63with an atomic mass of 62.929 598 amu, and 30.83% Cu-65, with an atomic mass of 64.927 793. 0.6917 x 62.929 599 amu + 0.3083 x 64.927 793 amu = 63.55 amu.
Example How many neutrons are present in an atom of carbon that has a mass number of 14? Solution The mass number is the number of protons plus the number of neutrons. (1) You are asked for the number of neutrons. (2) You are given that it is carbon-14. Carbon has 6 protons. (3) The relationship is n + p = mass number (4) Solve for n n= mass number -p (5) Plug in numbers and get answer n = 14 - 6 = 8 There are 8 neutrons in a carbon-14 nucleus.
Why aren’t there infinite numbers of elements, each with an atomic number greater than the one before it? The answer may lie in the forces that keep a nucleus together. Because the protons are repelling each other, they (and the nucleus) should fly apart! Remember that positive charges repel each other. In the nucleus, however, positive protons and neutral neutrons sit side by side.
The nucleus stays together because there is another force acting that is stronger than the repulsion of the protons for each other. Because it is stronger than the electromagnetic force, scientists call it the Strong Nuclear Force. Unlike gravity, which can reach millions of miles, the strong force only acts on very short distances. The effective distance for the strong force is so short, we do not feel it outside the nucleus of an atom.
Atoms are electrically neutral. Are these electrons randomly placed or are they organized in some way? An atom of helium has an atomic number of 2 and two protons in its nucleus. A neutral atom of helium would therefore have two electrons, which stay close to the nucleus because the positive protons and the negative electrons attract each other. An atom of silver has an atomic number of 47 and 47 protons in its nucleus. A neutral atom of silver would therefore have 47 electrons. Electrons are never all in one place at the same time. Instead, they literally buzz around the nucleus at a very fast rate, or frequency. Because of this behavior, we can refer to the entire space that electrons occupy as the electron cloud
The current model of the atom describes the area of the electron cloud that each electron occupies as an Energy State. Electrons that occupy this level are at a lower energy state than electrons that occupy the second energy level, which is farther from the nucleus. Each energy level can hold up to a specific number of electrons The farther away from the nucleus the electron is found, the higher its energy state. Therefore, the electron cloud is divided into energy levels. The first energy level is closest to the nucleus and has the lowest energy.
Sometimes, when energy is added to an atom, electrons can absorb enough energy to “jump” to a higher energy level. When they fall back to their normal energy level, they release light in a characteristic frequency. Like the layers of an onion, as the energy levels extend farther from the nucleus, they get larger in diameter and can hold more electrons. The maximum number each level can hold is shown.
It is important to note that some energy levels can overlap. In fact, each energy level is subdivided into smaller regions calledOrbitals. Some orbitals in the third energy level may have higher energy than some in the fourth and so on. Scientists have found out exactly which orbitals are occupied, and by how many electrons, in all 114 elements.
The Bohr Model of the Atom • Electrons circle the atom only in allowed paths, or orbits. • Orbits have a definite, fixed energy level. • Energy level is lowest in the orbit closest to the nucleus. • Energy of electrons is higher in orbits successively farther from the nucleus. • Analogy: Rungs of a ladder – you cannot stand between the rungs!
The Bohr Model of the Atom • While in a given orbit, electrons can neither gain nor lose energy – but can move to higher energy level by gaining energy in some form. • When the electrons drops back down to its original, lower energy level, it emits a photon of light (of a characteristic color for that particular element.)
Arrangement of Electrons in Atoms(The Race Around the Track) Electron Configurations
Describing the arrangement of electrons in an atom Carbon 1s22s22p2 Electron Configurations Number of electrons in sublevel main energy level (n) energysublevel (l)
Sample Problem Boron 1s22s22p1 How many electrons are present in a boron atom? Atomic number for boron? Orbital notation for boron?
Sample Problem Boron 1s22s22p1 # e- : 2+2+1=5 e- Atomic number: #p+=#e-=5
Sample Problem Boron 1s22s22p1 Orbital notation: _____ _____ _____ _____ _____ 1s 2s 2px 2py 2pz
The Relative Energies of Orbitals Energy level overlap.
Electrons Accommodated in Energy Levels and Sublevels
The Basic Chemical Laws • The following Chemical Laws can be explained on the basis of Dalton’s Atomic Theory: • The Law of Conservation of Mass • The Law of Definite Proportions • The Law of Multiple Proportions
The Law of Conservation of Mass Mass is neither created nor destroyed during ordinary chemical reactions or physical changes. The number of oxygen atoms and the number of carbon atoms are the same before and after each reaction.
The Law of Definite Proportions A chemical compound contains the same elements in exactly the same proportions by mass regardless of the size the sample or source of the compound – given compound always composed of the same combination of elements.
The Law of Definite Proportions • Example: Table salt is always table salt. • No matter how many or how few NaCl crystals you look at, the mass % of sodium and chlorine remains unchanged. • (NaCl) is always 39.34% Na by mass and 60.66% Cl.