Who is it n Have you ever tried

Who is it? n Have you ever tried to guess the identity of someone without looking? n What are some things you know exist but can’t see? n How do you know they exist? n What kinds of things might scientists study that they can’t see? 1

Atomic Structure 2

Atomic Structure Learning Targets: n I will be able to: 1. explain how scientists discovered subatomic particles. 2. explain how today’s model of the atom was developed. 3. diagram the structure of the nuclear atom. 3

Atomic Structure n n n Our understanding of atoms took many centuries. The 4 th century Greek philosopher, Democritus, suggested that the universe was made of invisible units called “atoms”. The word atom means “that which cannot be divided”. The Greek idea of atoms had to wait 2000 years before it became accepted. 4

Atomic Structure n n n In the 1700’s, chemists found out that new substances could be made by mixing some things together and by taking other things apart. When substances can no longer be broken apart into simpler substances what is left is an element: element substance that cannot be broken down into simpler substances and whose atoms are exactly alike 5

John Dalton n n In 1808, an English schoolteacher named John Dalton proposed his own atomic theory. He used evidence such as the “law of definite proportions”. His theory proposed: 1. Every ELEMENT is made of tiny, unique particles called atoms that cannot be subdivided. 2. Atoms of the same element are exactly alike. 3. Atoms of different elements can join to form molecules. Dalton thought the atom was in the shape of a solid sphere that was the same inside and out. https: //www. youtube. com/watch? v=Cd 2 sh. PDHMlk&list=PLTSop. UOAV 3 NQiv_VKWzux. Dwyd 8 i. Jdezx. R&index=99 6

William Crookes n n n In 1870, William Crookes conducted an experiment that proved the existence of small particles with a cathode ray tube (CRT) When electric current was applied to the cathode, a glowing beam appeared in the tube. The glowing beam could be bent by a magnet, proving the glow wasn’t light but negatively charged particles. 7

n n How did the English scientist William Crookes know that the greenish glowing beam in the CRT was not just a beam of light? What was the greenish glowing beam made up of? 8

J. J. Thomson n n In 1897, British scientist, J. J. Thomson repeated Crookes experiment using different metals and different gases in the CRT. His experiments showed charged particle beams occurred regardless of which metal or gas was used. Thomson concluded the charged particles were bits of matter that are part of every atom and were later named electrons https: //www. youtube. com/watch? v=oddjd. B 0 qf. Mg 9

J. J. Thomson n Since atoms are neutral, Thomson reasoned that atoms must contain some sort of positive charge. (Cathode Ray Tube experiment) His model, like a plum pudding, had negative electrons (plums) embedded in a positive sphere (pudding). Thomson changed Dalton’s atom model from a solid sphere the same throughout to a sphere with electrons evenly distributed throughout the sphere (Plum Pudding Model). 10

J. J. Thomson n n Thomson’s discovery led other scientists to wonder about the rest of the atom. If electrons are negatively charged and atoms have no charge, then the center of an atom must be positively charged. It wasn’t until 1920 that scientists identified protons. They found a proton is 1836 times heavier than an electron. 11

Ernest Rutherford n n n In 1911, Ernest Rutherford conducted new experiments to support Thomson’s atomic model. He shot alpha particles (particles that come off of unstable atoms) at a thin film of metal. Rutherford was surprised! He expected the alpha particles to go straight through but they didn’t, they went many different ways, even bouncing back, after hitting the thin metal. 12

Ernest Rutherford n n To make sense of his results, he hypothesized that instead of electrons and protons being evenly spaced in an atom, most of the mass and all of the positive charge is crammed into a small space in the center of an atom. This nucleus is surrounded by empty space where the electrons are found. 13

The Nuclear Atom (The Rutherford Model) n n In his model, atoms were mostly empty space with negatively charged electrons moving around a small, very dense, positively -charged nucleus in the center of the atom. Later (1919) Rutherford named the positive particles “protons”. 14

Ernest Rutherford n n n Many scientists were very interested in Rutherford’s model of the atom but had questions. There was too much mass for the number of protons in many atoms. Where does the extra mass come from? Rutherford thought there must be another particle in the nucleus. This new particle: 1. has the same mass as a proton and 2. is electrically neutral (no charge). 15

Rutherford’s Revised Atomic Model n n 1. 2. After Rutherford’s experiments, the model of the atom was changed again to include neutrons. The nuclear atom has: a tiny nucleus with protons and neutrons tightly packed together and electrons in the space surrounding the nucleus 16

James Chadwick n n Having no charge made it difficult to prove the existence of the new particle (it took 20 more years). In 1932, British scientist, James Chadwick, discovered this other particle in the nucleus of an atom. The particle was nearly the same mass as a proton. This particle is called a “neutron”, because it is electrically neutral. 17

n In Rutherford’s experiment, why wouldn’t the electrons in the atoms of the gold foil affect the paths of the alpha particles? n How does the nuclear atom differ from the uniform sphere model of the atom? 18

Comparing Atom & Nucleus n n n An atom is much larger than its nucleus. E. G. If the nucleus was the size of a poppy seed, the atom would be as large as a football stadium. An atom is mostly empty space. 19

What About Electrons? n n n Physicists tried to determine how electrons are arranged around a nucleus. The problem was how to keep negatively charged electrons from crashing into the positively charged nucleus. It was found that electrons are constantly moving. 20

Niels Bohr n n n In 1913, Niels Bohr, a Danish scientist, revised Rutherford’s model. Bohr showed that electrons could only have specific amounts of energy, leading them to move in certain orbits. Bohr proposed that electrons are found only in specific orbits around the nucleus. 21

Planetary Model n n n This model resembled planets orbiting the sun. The positive charge of the nucleus keeps the negatively charged electron orbiting around the nucleus instead of shooting off into space. The orbitals are different distances from the nucleus and each orbital can only hold a certain number of electrons. 22

Erwin Schrödinger n In 1926, Erwin Schrödinger used mathematical models to show that electrons do not have exact paths. n He and Werner Heisenberg proposed the electron cloud as the area surrounding the nucleus where electrons are most likely to be found. n Schrödinger and Heisenberg revised the model. They found that electrons can be anywhere in a cloudlike region around the nucleus. This region is called an orbital. 23

Electron Cloud Model n Electrons behave more like waves on a vibrating string than like particles. n The different energy levels contain different numbers of electrons depending on the energy level size and electron energy. 24

n How did Rutherford’s findings change Thomson’s model of the atom? n Why did Rutherford change his model? n What is the structure of the nuclear atom? n In Rutherford’s experiment, why wouldn’t the electrons in the atoms of the gold foil affect the paths of the alpha particles? 25

The Structure of Atoms n Three main subatomic particles are distinguished by mass, charge and location in the atom. Electrons Protons Neutrons Charge -1 Charge +1 No Charge In Orbitals Outside Nucleus In Nucleus mass = 9. 11 x 10 -31 mass = 1. 67 x 10 -27 mass= 1. 67 x 10 -27 26

Atoms n Each element has a unique number of protons. n Each atom of the element will have the same number of protons. n Atoms are electrically “neutral” overall. Atoms have the same number of protons as electrons. These charges exactly cancel leaving zero charge. n Positive and negative charges attract each other with a force known as “electric force”. 27

Atomic Nucleus n The atomic number tells you how many protons are in an atom. n Every element has a different atomic number. n Atoms may or may not have the same number of neutrons as protons. n The mass number tells you how many protons plus neutrons are in the nucleus of an atom all together. 28

n What distinguishes the atoms of one element from the atoms of another element? n Describe three kinds of particles found in atoms. Where are they located in the atom and what are their charges? n If an atom has 49 protons, how many electrons does it have? n How do we determine the number of neutrons in an atom? 29

n n The Atom Song http: //www. youtube. com/watch? v=v. Uz. TQW n-wf. E&feature=related 30