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Chapter 12 Introduction to Atoms.

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1 Chapter 12 Introduction to Atoms

2 Development of the Atomic Theory
Atom - the smallest particle into which an element can be divided and still be the same substance It still retains its characteristic properties Atoms make up elements Elements combine to form compounds Because all matter is made up of elements or compounds, atoms are considered the building blocks of matter.

3 Democritus Proposes the Atom
In 440 B.C. Democritus believed that there was a point where a particle could not be divided any further. This was against popular belief held by Aristotle He called this particle an atomos (Greek for atom), which means indivisible He believed these particles were constantly moving and they form different materials by joining together

4 Dalton Creates an Atomic Theory Based on Experiments
John Dalton, a British chemist, wanted to know why elements combine in specific proportions to form compounds i.e. water (H2O) always has two hydrogen and one oxygen atoms bound together He performed experiments on compounds and concluded that they combined in specific proportions because of atoms


6 He stated the following:
After many experiments and observations Dalton published his atomic theory in 1803. He stated the following: 1) All substances are made of atoms. ATOMS Are small particles that cannot be created, divided, or destroyed 2) Atoms of the same element are exactly alike, and atoms of different elements are different 3) Atoms join with other atoms to make new substances This was the first step in our understanding of the atom and our current atomic theory

7 Thomson Finds Electrons in the Atom
In 1897, a British scientist named J. J. Thomson discovered that atoms are made of small particles. This was contrary to Daltons atomic theory Thomson experimented with a cathod ray He noticed that the ray was affected (bent) by a positive charge He concluded that the ray contained negatively charged particles

8 These negatively charged particles are now called electrons
Thomson revised Dalton's atomic theory to include the presence of electrons even though he did not know how they were arranged in the atom He came up with the plum pudding model Model - is a representation of an object or system Thomson’s model illustrated positively charged material with negatively charged particles located throughout.



11 Rutherford Opens an Atomic “Shooting Gallery”
In 1909, a former student of Thomson’s named Ernest Rutherford tested Thomson’s theory He aimed positively charged particles, larger than protons, at gold foil Most particles passed straight through But to Rutherford’s amazement; Some were deflected at various angles And a few were bounced straight back It became obvious to Rutherford that Thomson’s model was wrong


13 Rutherford Presents a New Atomic Model
In 1911 Rutherford revised the atomic theory He concluded that atoms are mostly empty space with lightweight negative electrons moving around And in the center of the atom is a tiny, extremely dense, positively charged region - Nucleus He calculated that the diameter of the nucleus was 100,000 times smaller than the atom itself

14 Bohr States That Electrons Can Jump Between Levels
In 1913 a Danish scientist who worked with Rutherford, Niels Bohr, suggested that electrons travel around the nucleus in definite paths These paths are located at levels at certain distances from the nucleus He also believe that electrons could jump from one level to the next Bohr’s model was valuable in predicting some atomic behavior, but was too simple to explain all atomic behavior


16 The Modern Theory: Electron Clouds Surround the Nucleus
Many scientists have contributed to our current understanding of the atom This led to the following change to the atomic theory; Electrons do not travel in definite paths around the nucleus The exact paths cannot be predicted The regions around the nucleus where electrons are likely to be found are called - Electron Clouds


18 Chapter 12 Quiz 1 1) __________is the smallest particle into which an element can be divided and still be that element 2) __________ is a unifying explanation for a broad range of hypothesis and observations that have been supported by testing 3) __________ are the negatively charged particles found in all atoms 4) ___________ discovered the nucleus 5) ___________ are regions where electrons are likely to be found Bonus) __________ are atoms that have the same number of protons, but different numbers of neutrons


20 Section 2: The Atom

21 How Small Is an Atom? An average sized atom (i.e. aluminum) has a diameter of cm That’s three hundred millionths of a centimeter It would take a stack of 50,000 aluminum atoms to equal the thickness of a sheet of aluminum foil There are 2 x 1022 atoms in one penny! 20,000,000,000,000,000,000,000 atoms That’s twenty thousand billion billion atoms That is 4,000,000,000,000 times more atoms than people on Earth!

22 What’s Inside an Atom? As tiny as an atom is, it consists of even smaller particles Protons, Neutrons, and Electrons Protons and Neutrons make up the nucleus, which is at the center of the atom

23 Protons - the positively charged particles of the nucleus
All protons are identical Each has an approximate mass of 1.7 x grams or 1 amu Neutrons - particles of the nucleus that have no charge Neutrons have a slightly greater mass than protons The nucleus of the atom is small, but very dense

24 Outside the Nucleus Electrons - negatively charged particles in atoms
Electrons are found moving around the nucleus within electron clouds Electrons are very small in mass compared to protons and neutrons ( amu) It takes more than 1,800 electrons to equal the mass of one proton

25 Positively charged atoms are Cations
The charges of protons and electrons are opposite, but equal in strength Therefore if there are equal amounts of protons and electrons, the overall charge of the atom is zero - neutral i.e. 10 protons - 10 electrons = 0 If the number of protons and electrons differ, the atom becomes a charged particle - Ion Positively charged atoms are Cations Negatively charged atoms are Anions


27 How Do Atoms of Different Elements Differ?
There are 117 different elements, each made of different atoms The number of Protons determines the element This is called the Atomic number Each element is composed of atoms with the same atomic number

28 Are All Atoms of an Element the Same?
Isotopes - are atoms that have the same number of protons, but have different number of neutrons Atoms that are isotopes of each other are the same element because they have the same number of protons Some isotopes have unstable nuclei and become radioactive

29 How Can You Tell One Isotope from Another?
You can identify one isotope from another by its mass number Mass number - is the sum of the protons and neutrons in an atom Electrons are not included in this calculation because their mass too small to affect it To identify a specific isotope write the name of the element followed by a hyphen and the mass number i.e. carbon - 12 (C-12) has contains 6 protons and 6 neutron, while carbon - 13 has 6 protons and 7 neutrons

30 How Do You Calculate the Mass of an Element
Most elements found in nature contain a mixture of different isotopes i.e. All copper is composed of copper-63 and copper-65 atoms Atomic Mass Unit (amu) - is the weighted average of the masses of of all naturally occurring isotopes To calculate the amu of an element, multiple the mass of each element by its percentage of abundance (in decimal form)

31 Copper consists of copper-63 at 69% abundance, and copper-65 at 31% abundance. What is the amu of copper? (63 x .69) = (65 x .31) = 63.62 amu * So the amu of copper is amu

32 Now You Try One Chlorine consists of chlorine-35 at 76%, and chlorine-37 at 24%. What is the amu of chlorine (35 x .76)= 26.6 (37 x .24)= 35.5 amu

33 What Forces Are at Work in Atoms?
There are four basic forces at work everywhere, including in atoms. They include: 1) Gravity - the attraction between two objects based on their masses Because the masses of the particles in atoms is so small, the gravity between them is small 2) Electromagnetic Force - particles of the same charge repel, while opposite charges attract. The electromagnetic force holds electrons around the nucleus

34 3) Strong Force - At the close distances between protons in the nucleus, the strong force is greater than the electromagnetic force between the protons This keeps the nucleus together 4) Weak Force - In certain unstable, a neutron can change into a proton and a electron. It is an important force in radioactive atoms

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