History of Atomic Theory Figuring Out Atoms

Dalton’s Atomic Theory 1.Matter is composed of extremely small particles called atoms. 2.Atoms are indivisible and indestructible. 3.Atoms of a given element are identical in size, mass, and chemical properties.

Dalton’s Atomic Theory 4.Atoms of a specific element are different from those of another element. 5.Different atoms combine in simple whole- number ratios to form compounds. 6.In a chemical reaction, atoms are separated, combined or rearranged.

Dalton’s Model Atoms are the smallest division of matter. They cannot be divided.

Modern Advancements It wasn’t until almost 80 years after Dalton’s theory that we saw further discoveries.

J.J. Thomson and the Cathode Ray Tube

Some sort of particles were hitting the Zinc-Sulfide coating If you bring a magnet towards the beam, the beam moves (towards the positive and away from the negative) These particles were in all sorts of elements J.J. Thomson and the Cathode Ray Tube

These small particles are now called electrons. Thomson designed a new model of an atom by changing Dalton’s model. This model is called the plum pudding model. J.J. Thomson and the Cathode Ray Tube

Plum Pudding Model A uniformly positive atom with small negative particles mixed in

Rutherford’s Gold Foil Experiment To test the plum pudding model, Ernest Rutherford designed a simple experiment.

Rutherford’s Gold Foil Experiment

Each atom has a small, dense nucleus Most of the atom is empty space Within the nucleus are particles that have a positive 1 charge. He called these particles protons. He was able to improve the plum pudding model Rutherford’s Conclusions

Rutherford’s Model Small, dense, positively charged nucleus with smaller negative charges surrounding it.

Last Piece of the Puzzle One of Rutherford’s colleagues, James Chadwick discovered that there must be another particle in the nucleus. This particle is the same mass as a proton but does not have a charge. He called these particles neutrons.

Atomic Particles So now we know…. Electrons are small, negatively charged particles Protons are positively charged particles in the nucleus Neutrons are neutral particles in the nucleus

Atomic Structure

In the nucleus there are two types of particles: Protons and Neutrons Around the nucleus there are electrons

Practice The number of protons determines the element How many protons are in each of the following elements? He: F: Ga: Be: The number of electrons = the number of protons

Periodic Table Atomic Number = Number of Protons or Number of Electrons

Elemental Notation Rather than writing out the element’s information each time, we use a special notation to organize information. – Mass Number – Atomic Number – Elemental Symbol

The Numbers The mass number of an element is the number of protons + neutrons. The atomic number of an element is the number of protons the element has. To find the number of neutrons, subtract the atomic number from the mass number. Rn Mass Number Atomic Number Element Symbol 136 Number of Neutrons

Isotopes Isotope- Atoms with the same number of protons but different number of neutrons. Atoms of the same element have the same number of protons Atoms of the same element can have different numbers of neutrons 11 6 C 12 6 C 13 6 C 14 6 C

Ions In the nucleus there are protons and neutrons – Protons are positive – Neutrons are neutral If the nucleus has 3 protons it has a charge of 3+

Neutral Atom In a neutral atom, there are equal numbers of protons and electrons Each positive is cancelled by a negative.

Ions In an ion, there are unequal numbers of protons and electrons 3 + charge 4 – charge 1 – charge

Ions In an ion, there are unequal numbers of protons and electrons 3 + charge 2 – charge 1 + charge

What is the charge of an ion that has 29 protons and 32 electrons?

What is the charge of an ion that has 12 protons and 13 electrons?

What is the charge of an ion that has 54 protons and 52 electrons?

Modern Model Finding out more about atoms

Do Now Get one of each paper Keep your homework Complete the do now on your notes page!

What we know… We know that there are protons, neutrons and electrons. Protons and Neutrons are in the nucleus. The nucleus has a positive charge. Electrons are in the space around the nucleus. But scientists were learning more about the behavior of atoms while they were studying light.

Photoelectric Effect Scientists continued to explore the properties of matter and light. High EnergyLow Energy

Properties of Light Studies showed that light behaved as packages of energy. We call these packages photons. Low Energy High Energy

Photoelectric Effect e-e- e-e- e-e- e-e- e-e- e-e- e-e- e-e- Experiment: Shining light on a piece of metal With low energy photons, no electrons are removed.

Photoelectric Effect e-e- e-e- e-e- e-e- e-e- e-e- e-e- e-e- Experiment: Shining light on a piece of metal With many low energy photons, no electrons are removed.

Photoelectric Effect e-e- e-e- e-e- e-e- e-e- e-e- e-e- e-e- Experiment: Shining light on a piece of metal With high energy photons, electrons are removed.

Niels Bohr Bohr reviewed these results and came up with a new model of the atom. If only certain types of energy could remove electrons, he thought maybe there were energy levels for electrons.

Bohr’s Model e-

Bohr’s Model e-

Bohr’s Model This idea of energy levels in an atom were represented by rings.

Bohr’s Model In the first energy level there are two electrons. e-

Bohr’s Model In the second energy level there are 8 electrons. e-

Bohr’s Model In the third energy level there are 18 electrons. e-

Energy With Rings Electrons in the outer rings have more energy than those in the inner rings Low Energy High Energy

Electron Configuration On our periodic table, the electron levels are represented by the electron configuration. Fluorine: 2-7 Boron: 2-3

Electron Configuration What is the electron configuration for the following elements? Oxygen (O): Beryllium (Be):

Drawing Bohr’s Model Fluorine

Drawing Bohr’s Model Boron

Drawing Bohr’s Model Oxygen

Drawing Bohr’s Model Beryllium

Wave-Mechanical Model The modern structure of the atom has maintained many of the same features we discussed with one small difference… The Electron Cloud

Electron Cloud Rather than defined rings, the electrons have areas they are most likely found (highest probability) that are similar to Bohr’s Rings.

Do now Get one of each paper Complete the “What we know” section of 2-9.

Valence Electrons and Lewis Dot Diagrams

What we know To find mass number, add protons and neutrons The number of protons always equals the atomic number To find neutrons, subtract protons from mass number For an atom, the number of electrons equals the number of protons

Valence Electrons The outermost electrons are responsible for all of the chemical properties of the element. These outermost electrons are called valence electrons. 2-5 Valence Electrons

Periodic Table The last number in the electron configuration are the valence electrons.

Lewis Electron Dot Diagrams G. N. Lewis was a college professor in 1902 teaching chemistry. He wanted a way to represent the valence electrons. He decided to develop his own method.

Lewis Electron Dot Diagrams Each of the valence electrons are represented by a dot. Start by putting one dot on each side of the symbol then pairing them up. S 2-8-6

Lewis Electron Dot Diagram CClNBr

Excited vs. Ground State

Ground State When an atom is at its lowest energy, the electrons are in the ground state. Fluorine P=9 N=9 e- 2 7

Excited State When an atom absorbs energy, an electron is moved up a level. This is called the excited state. Fluorine P=9 N=9 e

Energy of Electrons Specific amounts of energy is gained when electrons go to an excited state and move up an energy level Specific amounts of energy is lost when electrons return to ground state.

Absorption Spectrum Boron P=5 N=6 e- Only certain photons of light can be absorbed or released by an atom based on its electron configuration.

Emission Spectrum Emission spectrum show which colors of light are released.

Emission Spectrum Boron P=5 N=6 e- Light is produced when electrons down an energy level and energy is released. This is when an electron goes from excited to ground state.

To determine what elements are in a mixture, match up the lines in the mixture to the lines in the known spectra.

Vocabulary 1.Proton 2.Neutron 3.Electron 4.Valence Electron 5.Mass Number 6.Ion 7.Isotope 8.Emission/Bright line Spectrum 9.Excited State 10.Ground State 11.Wave-mechanical model 12.Electron Cloud 13.Orbital 14.Lewis Dot Diagram 15.Bohr’s Model