1. Atomic Theory 1

2. The Atom 2

3. Early Theory Democritus (400 BC) thought the world was made of tiny indivisible particles Aristotle believed that all matter was made up of 4 elements: Fire Earth Water Air 3

4. John Dalton Believed that… 1. All matter is composed of extremely small, indivisible particles called atoms 2.All atoms of a given element are identical 3.Atoms cannot be created or destroyed 4.Atoms combine to form compounds 4

5. J.J Thompson The discovery of electrons led to Thompsons plum pudding model of the atom (blueberry muffin). Positive background speckled with negative particles Atom is seen as divisible. 5

6. Ernest Rutherford Suggested the Nuclear Model The atom consists of a nucleus containing most of the mass and a strong positive charge in the middle of the atom Nucleus= protons + neutrons Electrons form a cloud around the nucleus 6

7. The Atomic Structure Atoms have no charge and are made up of several subatomic particles. They are: 1. Neutrons: are found in the center of the atom (nucleus). Have no charge or are neutral. 2. Protons: are also found in the center of the atom (nucleus). Have a positive charge. 3. Electrons: are found revolving around the nucleus of the atom in shells or electron clouds. Have a negative charge. The nucleus: is the center of the atom and is made up of protons and neutrons. (also has a positive charge) 7

8. Terms: Electrons: negatively charged particles around the nucleus, “cloud” formation. Protons: positively charged particles found in the nucleus Neutrons: no charge, or neutral particles also found in the nucleus Nucleus: dense middle of the atom, has a positive charge due to protons 8

9. Neils Bohr Believed electrons to be found in orbital paths around the nucleus (like planets around the sun). Electron close to the nucleus were low in energy and as the orbit got larger the energy of the e - increased. Mapped out where the electrons are most likely to be found 9

10. What makes atoms different from one another? The number of protons an atom has determines what kind of atom it is. Example: The oxygen atom has 8 protons therefore we know it is an oxygen atom. You can determine the number of protons an atom has by looking at its atomic number found on the periodic table. 10

11. Atomic Number The atomic number of an element is found on the periodic table. They are arranged in increasing order going from left to right. So Hydrogen has an atomic number of 1, Helium is 2 and so on. The atomic number tells us how many protons an atom has. This also tells us how many electrons an atom has, because the number of protons and electrons are balanced. So an atom has the same number of protons as electrons. 11

12. Finding the number of electrons and protons an atom has: To find the number of protons and electrons an atom has we must look at the atomic number. Ex: Find the number of protons and electrons a Titanium atom has. 12

13. Atomic Mass and Mass Number The atomic mass of an atom also appears on the periodic table as the weighted average (because of the isotopes). The mass number of an atom is equal to the number of protons and neutrons it has in the nucleus. Formula: Mass # = # protons + # neutrons 13

14. Atomic number = number of protons (Z) Mass number = p + + n° (A) In an atom #p + = #e - 14

15. Calculating the number of neutrons an atom has: To calculate the number of neutrons an atom has we must use the previous formula: Mass # = # protons + # neutrons Or # neutrons = Mass # - # protons (atomic #) Ex: Calculate the number of neutrons that the sodium atom has. 15

16. Practice Nuclear Symbol A (mass number) E (element symbol) Z (atomic number) Z (atomic number) Find theFind the number of protons number of protons number of neutrons number of neutrons number of electrons number of electrons atomic number atomic number mass number mass number W 184 74 F 19 9 Br 80 35 16

17. Isotopes All atoms of the same element have the same number of protons and electrons, but they don’t have to have the same number of neutrons. This can result in an isotope: atoms of the same element that have different numbers of neutrons, (changes the mass number and atomic mass). There are three different isotopes of Hydrogen. 17

18. Isotopes Isotopes: Different forms of the same element based on the number of neutrons in the nucleus Protium 18