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Atoms Unit 3 Introduction to the Atom + + + - - -

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2 Atoms Unit 3 Introduction to the Atom

3 Some of the First Discussions about the Atom…. Democritus (400 BC) named the atom, which means indivisible, and said it was the smallest particle and could not be broken down. He was pretty much correct in that if you did break the atom down, it wouldn’t be that “element” any more. But he wasn’t aware that the atom was made up of particles.

4 Greeks Philosophers Continue the Discussion and Disagree Aristotle (350 BC) believed matter could always be broken down into smaller and smaller parts. And this is true to a certain extent… but eventually you come to the small basic particles that can no longer be broken apart.

5 Dalton's Atomic Theory In 1808, John Dalton proposed that elements were composed of atoms & that only whole numbers of atoms can combine to form compounds His ideas are now called the Atomic Theory of Matter Dalton’s Atomic Theory ( AD) was widely accepted but not totally correct

6 1. All matter is composed of extremely small particles called atoms 2. Atoms of a given element are identical in size, mass, and other properties; atoms of different elements differ in size, mass, & other properties ELEMENT 2 ELEMENT 2 ELEMENT 3 ELEMENT 3 ELEMENT 1 ELEMENT 1 ELEMENT 4 ELEMENT 4 Dalton’s Atomic Theory

7 3. Atoms cannot be subdivided, created, or destroyed 4. atoms of different elements combine in simple whole # ratios to form chemical compounds 5. in chemical reactions, atoms are combined, separated, or rearranged ++ ++

8 Not all of Dalton’s claims held up to the scrutiny of experimentation Not all of Dalton’s claims held up to the scrutiny of experimentation Atoms CAN be divided into even smaller particles Atoms CAN be divided into even smaller particles Not every atom of an element has an identical mass (isotopes are atoms of the same element, but with different mass numbers) Not every atom of an element has an identical mass (isotopes are atoms of the same element, but with different mass numbers) Atomic Theory

9 Dalton’s Atomic Theory of Matter has been modified. Dalton’s Atomic Theory of Matter has been modified. What remains is… What remains is… 1. All matter is composed of atoms 2. Atoms of any one element differ in properties from atoms of another element One of the disputed statements of Dalton was that atoms are indivisible (we now know about protons, electrons, and neutrons) Atomic Theory

10 In the 1800’s it was determined that atoms are actually composed of several basic types of smaller particles It’s the number and arrangement of these particles that determine the atom’s chemical properties. A new definition of an atom is the one use today: the smallest particle of an element that retains the chemical properties of that original element. Atomic Theory

11 Plum Pudding Model JJ Thomson’s cathode ray tube experiment in the late 1800s showed that atoms had smaller parts, called negative corpuscles; he developed the “plum pudding model.” The plum pudding model showed electrons (the plums) mixed in together with protons (the cake batter)

12 Negative particles embedded in a sphere of positive plasma-like matter. THINK… Chocolate Chip Cookie Negative particles embedded in a sphere of positive plasma-like matter. THINK… Chocolate Chip Cookie

13 Atomic Structure Scientists still didn’t really understand how the particles were put together in an atom. This was a difficult question to resolve, given how tiny atoms are. Most thought it likely that the atom resembled Thomson’s model

14 Rutherford’s gold foil experiment In 1911, Ernest Rutherford provided a more detailed picture of the internal structure of the atom He showed that atoms had a nucleus where most of the mass resided. His model had a nucleus with a hard dense positively charged mass, negatively charged electrons outside, and that the atom was actually mostly empty space.

15 Rutherford Model

16 Bohr Model of the Atom Neils Bohr put electrons into different energy levels or shells. Neils Bohr put electrons into different energy levels or shells. (This model is not correct either… electrons do not travel in orbits or paths like the model suggests) (This model is not correct either… electrons do not travel in orbits or paths like the model suggests)

17 Modern Day Theory (Electron Cloud Theory) The Modern Theory suggests that electrons are located somewhere in a cloud. The Modern Theory suggests that electrons are located somewhere in a cloud.

18 Basic and important fact to remember: All atoms contain the same basic parts, but atoms of different elements have different numbers of protons. The Periodic Table lists atoms in consecutive order by their Atomic Number The atomic number is directly related to the number of protons in the nucleus of each atom of that element

19 Atoms have: A nucleus small, dense part of the atom consists of protons and neutrons An electron cloud large part of the atom that is empty space except for the electrons that are moving very fast and very randomly around the nucleus Nucleus Electron Cloud

20 The total number of protons & neutrons determines the mass of the atom Called the “Mass Number” (atomic mass is the averaged mass of the isotopes and is given on the periodic table. Simply round the atomic mass to get the mass number) Called the “Mass Number” (atomic mass is the averaged mass of the isotopes and is given on the periodic table. Simply round the atomic mass to get the mass number) A Carbon atom, has 6 protons and 6 neutrons, so its mass number is 12 A Carbon atom, has 6 protons and 6 neutrons, so its mass number is 12 If you know the atomic number & mass number of an atom of any element, you can determine the atom’s composition and the number of neutrons.

21 The Proton The protons are what give the atom its charge (+) They add mass to the atom as well. Each proton is equal to one AMU (atomic mass unit) The number of protons in an atom determines what element it is. The atomic number signifies the number of protons Protons are held together in the nucleus by the “Strong Force” otherwise they would repel each other.

22 The Neutron The neutron adds mass (1 amu) to an atom but has NO charge The neutron adds mass (1 amu) to an atom but has NO charge Atoms of the same elements are identical due to the number of protons, but there can be different numbers of neutrons (we call those isotopes) Atoms of the same elements are identical due to the number of protons, but there can be different numbers of neutrons (we call those isotopes) To find the number of neutrons, subtract the atomic number from the mass number. To find the number of neutrons, subtract the atomic number from the mass number.

23 Electrons Electrons have almost no mass, and we DO NOT count their mass. Electrons have almost no mass, and we DO NOT count their mass. Located outside the nucleus in the electron cloud (aka: shells, orbitals, energy levels) moving at incredibly high speeds. Located outside the nucleus in the electron cloud (aka: shells, orbitals, energy levels) moving at incredibly high speeds. Electrons have a negative (-) charge. Electrons have a negative (-) charge. Electrons found in the outermost shells of the atom are responsible for chemical reactions. Electrons have different amounts of energy depending what energy level they are at. Electrons found in the outermost shells of the atom are responsible for chemical reactions. Electrons have different amounts of energy depending what energy level they are at. Electrons can be removed and added to atoms quite easily, unlike protons. Electrons can be removed and added to atoms quite easily, unlike protons.

24 Subatomic Particles ParticleSymbol Charge Relative Mass Electron e Proton p Neutron n0 1

25 Location of Subatomic Particles cm cm electrons electrons protons protons neutrons neutrons cm cm nucleus

26 Atomic Number Counts the number of protons in an atom and determines what element it is

27 Atomic Number on the Periodic Table 11Na Atomic Number Symbol The symbol represents the element. RULE: The first letter is always capitalized, and IF there is a second letter, it is lower case.

28 All atoms of an element have the same number of protons 11Na 11 protons Sodium

29 Atomic Mass on the Periodic Table 11Na22.99 Atomic Number Symbol Atomic Mass Atomic mass is the weighted average mass of all the atomic masses of the isotopes of that atom. That is why there is a decimal.

30 Mass Number Counts the number of protons and neutrons in an atom (note: Atomic Mass is different from Mass Number. On your periodic table of elements, the atomic mass is usually given and you need to round it to the nearest whole number to use to figure Neutrons)

31 Atomic Notation Show the mass number and atomic number Show the mass number and atomic number Give the symbol of the element Give the symbol of the element mass number 23 Na sodium Na sodium-23 atomic number 11 atomic number 11

32 Number of Electrons An atom is neutral when no charge is indicated. An atom is neutral when no charge is indicated. The net charge is zero The net charge is zero Remember: Atomic number = Number of protons Remember: Atomic number = Number of protons and therefore…. and therefore…. Number of protons = Number of electrons when the atom is neutral. Number of protons = Number of electrons when the atom is neutral.

33 Subatomic Particles Showing the P E N O P Zn O P Zn p + 15 p + 30 p + 8 e - 15 e - 30 e - 8 n16 n35 n

34 Isotopes Atoms with the same number of protons, but different numbers of neutrons. Atoms with the same number of protons, but different numbers of neutrons. Atoms of the same element (same atomic number) with different mass numbers Atoms of the same element (same atomic number) with different mass numbers Isotopes of chlorine Isotopes of chlorine 35 Cl 37 Cl 1717 chlorine - 35 chlorine - 37 chlorine - 35 chlorine - 37

35 Learning Check Naturally occurring carbon consists of three isotopes, 12 C, 13 C, and 14 C. State the number of protons, neutrons, and electrons in each of these carbon atoms. Naturally occurring carbon consists of three isotopes, 12 C, 13 C, and 14 C. State the number of protons, neutrons, and electrons in each of these carbon atoms. 12 C 13 C 14 C #p _______ _______ _______ #e _______ _______ _______ #n _______ _______ _______

36 Solution 12 C 13 C 14 C #p #e #n 6 7 8


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