The Atom and Elements

Development of Atomic Theory: Democritus (460-370 BC) Proposed that matter was made of small particles he called atoms. In Greek this means indivisible or cannot be divided. He believed different atoms would vary in size and would be in constant motion. Extremely small particles in motion John Dalton (1766-1844) Published the atomic theory: Elements were composed of atoms. Atoms of a given element are identical. Atoms of different elements have different properties. Atoms don’t change, but can be combined and rearranged with other atoms. Compounds are formed when atoms of more than one element combine. Atom as solid object Joseph John Thomson (1856-1940) Atom as solid object with a positive center and electrons embedded in the atom Identified the electron, which carries a negative charge. He thought that electrons were embedded in the atom like raisins in raisin bread.

Development of Atomic Theory: Ernest Rutherford (1871-1937) Used high speed lightweight atoms called alpha particles to bombard very thin gold foil. Most of these alpha particles passed through the gold foil. The fact that these particles went through the foil lead to his theory that atoms have mostly empty space. Alpha particles went through spaces.  Neils Bohr (1885-1962) Described the electrons moving around the nucleus in fixed orbits. Each orbit has a set amount of energy. We use this model for a basic understanding of the atom’s structure. However, more recent research has shown that electrons move around the nucleus in waves rather than elliptical orbits. Electrons are better represented as an “electron cloud.” Electrons move so fast that scientists prefer the atom model where electrons are represented by a cloud. James Chadwick (1891-1974) Solved the problem of “missing mass” in the atom by discovering the neutron. 

Electrons do not have much mass compared to protons and neutrons. Basic Structure of an Atom Proton Positive Charge 1 AMU 1 AMU + Neutron + 1 AMU 1 AMU No Charge Many students likely know this information. While going through this slide be sure ask students to tell you what they know before “telling” them the answers. Page down, page down: Direct students to write the notes as they appear. Explain of a proton and it’s placement in the atom Explanation of a Neutron and it’s placement in the atom Page down (4 times): Explanation of Electron and it’s placement in the atom Page down (5 times) - display the charges of sub-particles Page down: Talk about the relative masses of each of the sub-particles. “If you weighed yourself once and then took off your watch, would it make much difference in your weight?” is a good illustration of how miniscule the mass is of an electron. Be sure to point out that protons and neutrons weigh about 1 AMU. This helps later when trying to figure out the number of neutrons there are in an atom. Electron AMU is an atomic mass unit. Electrons do not have much mass compared to protons and neutrons. Negative Charge

Notice the Pattern of the Periodic Table. Have them watch the next slide carefully and then page down.

What Do You Notice? H He Li Be B C N O F Ne Na Mg Al Si P S Cl Ar K Ca 1 H 1.01 2 He 4.0 3 Li 6.94 4 Be 9.01 5 B 10.81 6 C 12.01 7 N 14.01 8 O 16.00 9 F 19.00 10 Ne 20.18 11 Na 22.99 12 Mg 24.31 13 Al 26.98 14 Si 28.09 15 P 30.97 16 S 32.07 17 Cl 35.45 18 Ar 39.95 19 K 39.10 20 Ca 40.08 Page down if necessary. Have them look at the top numbers (atomic numbers) as they go across. It is likely that they will not be able to see them very well so you might have to read them aloud. Page up to do it again if necessary. Explain that the atomic number tells us how many protons an atoms has. You might ask the question (using the classroom periodic table) How many protons does carbon have. And them switch back and say, “If an atom has 5 protons, what is it it?” Continue questioning until you feel that they understand. Explain the atom mass is the total weight (on average…). Ask them how much a proton weighs (1 amu), how much an electron weighs (too small to weigh) and neutron weighs (1 amu). Page down to the next slide.

(# of protons and electrons) (# of neutrons and protons) Element Name Beryllium 4 Atomic Number Be (# of protons and electrons) Element Symbol Average Atomic Mass 9.01 (# of neutrons and protons) Atomic Mass - Atomic Number = # of neutrons Have the students Draw the box “of Be” on their page. Have them refer to a periodic table. Ask them to find “Be” on the table. Ask someone to give the element name. Page Down: Beryllium, The element name, Page Down: Atomic Number, Page Down: Atomic Symbol, Page Down: Atomic mass. Have students copy this down. If their periodic table has them listed in a different places within the “box”, ask them how they might be able to determine difference between each. Page Down: Explain that the atomic number tells us the number of protons and electrons an element has. Page Down again. Ask “if there are the same number of protons and neutrons, then what is the overall charge of an atom?” Page Down:Have them write down “tell us the number of neutrons” and then say, “But it does it indirectly. We know that each proton weighs 1 AMU. How many protons does beryllium have? (4). So how much do the protons weigh? (4). So if the protons take up 4 of the 9.01, what is left over? (5.01) Do electrons take up any of that mass? (no) So what else is taking up that mass? (the neutrons) So if the neutrons weigh 1 AMU, then how many neutrons are there? (5) Page down 3 times Make sure that the students have copied everything down. 9 - 4 = 5 + + + 5 Neutrons +

What are the numbers of protons, electrons and neutrons? Nitrogen 7 N P = E = N = 7 14.01 Have the students figure out the number of protons, electrons and neutrons. There is a 90 second delay until the answers appear. You can use page down to have them come in sooner.

What are the numbers of protons, electrons and neutrons? Fluorine 9 F P = E = N = 9 10 19.00 Have the students figure out the number of protons, electrons and neutrons. There is a 90 second delay until the answers appear. You can use page down to have them come in sooner.

What are the numbers of protons, electrons and neutrons? Chlorine 17 Cl P = E = N = 17 19 35.55 Have the students figure out the number of protons, electrons and neutrons. There is a 90 second delay until the answers appear. You can use page down to have them come in sooner.

H Isotopes 1    Elements usually have differing amount of neutrons. These different forms are called isotopes. Let’s take hydrogen as an example: 1 Hydrogen H 1.008 As you can see from the atomic mass (1..008) the most naturally abundant form of hydrogen does not have a neutron. However, in rare instances isotopes form. Below are the isotopes of hydrogen:  Protium  Deuterium  Tritium …. are isotopes of hydrogen.

Calculate the AVERAGE ATOMIC MASS Mg: 23.9924 amu at 78.6 % 24.9938 amu at 10.1 % 25.9898 amu at 11.3 % Multiply Mass X Percent for each isotope Add all

Solution to AAM 23.9924 amu X 78.6 % =18.86 24.9938 amu X 10.1 % = 2.52 + 25.9898 amu x 11.3 % = 2.94 24.32 amu

Energy Level Max # of Electrons 15 18 1 2 4 3 2 8 17 14 3 8 1 11 P = 5 2 12 N = 8 10 7 Page Down: In making a diagram of an element scientists used to use a model called the Bohr Model. Although it does not completely describe the structure of an atom, it is a useful tool for understanding the interactions of matter. First you would write down the # of protons and neutrons the atom has. Page Down: First energy level - can hold up to 2 electrons. Page down Page Down: Second energy level - can hold up to 8 electrons. Page Down: Because the atoms are spinning around the nucleus it is important to put the out of each other’s path. The first two are paired. The next three are distributed around the energy level one at a time. The next three are paired with the previous three. Teacher Note: The rationale for teaching this is to help them understand the Lewis Dot diagrams later. Page Down: Third energy level - Similar to the 2nd level… however in reality the students will learn more about this level in chemistry. Page down. Make sure that the students have copied this slide. 9 6 16 The Bohr Model 13 Valence Electrons – The electrons on the outer shell

Ions Charged Particle p+ and e- are not equal.

Lost e- Positive Charge Smaller than the atom Cation 27Al +3 P= N= E= 13

Gained e- Negative charge Larger than the atom Anion 80Se -2 P= N= E= 34

Symbol 34S -2 P= N= E= 16

Symbol 26Mg +2 P= N= E= 12 21 21