Atoms: Development of the Atomic Theory

1. Atoms:Development of the Atomic Theory

2. Ancient Belief Ancient Greeks believed that all things were made up of earth, wind, fire, and water.

3. Democritus • 460 BC - Greek philosopher proposes the existence of the atom • said that all matter was composed of tiny particles that he called atomos. Atomos is Greek for “not to be divided”. • His theory: • all atoms are small hard particles • made of a single material formed into different shapes and sizes • always moving, and that they form different materials by joining together

4. John Dalton • 1803 - British chemist; elements combined in specific proportions to form compounds. • Rediscovered Democritus’ theory and added four parts. • His theory: • all substances are made of atoms that cannot be created, divided, or destroyed • atoms join with other atoms to make new substances called compounds • Atoms of different elements are different • atoms of the same element are exactly alike, and atoms of different elements are different in mass and size (elements)

5. Edward Frankland • 1852 - English chemist developed the valence theory • His theory: • every atom has a fixed number of bonds (chemical links) that it can form • for the atom to be stable, all of these bonds must be used.

6. J.J. Thomson • 1897 - English chemist and physicist discovered 1st subatomic particles • His theory: used a cathode-ray tube • negatively charged particles called electrons andpositively charged matter • Assumed since there was a negatively charged particle there must be a positively charged particle: the proton but he did not know how they were arranged • created a model to describe the atom as a sphere filled with positive matter with negative particles mixed in • Referred to it as the plum pudding model

7. Cathode Ray Tube

8. Ernest Rutherford • 1912 - New Zealand physicist • Uses the gold-foil experiment and discovers the nucleus • Said the atom was mainly a solid core at the center (nucleus) • His theory: • small, dense, positively charged particle present in nucleus called a proton • electrons travel around the nucleus,but their exact places cannot be described

9. Niels Bohr • 1913 - Danish physicist discovered energy levels • It is the form we know today • His theory: • electrons travel around the nucleus in definite paths and fixed distances • electrons can jump from one level to a path in another level • Is called the planetary model

10. Erwin Shrodinger • 1924 - Austrian physicist developed the electron cloud model • His theory: • electrons exact path cannot be predicted • regions, referred to as the electron cloud, are areas where electrons can likely be found.

11. James Chadwick • 1932 - English physicist discovered neutrons • His theory: • neutrons have no electrical charge • neutrons have a mass nearly equal to the mass of a proton • unit of measurement for subatomic particles is the atomic mass unit (amu)

12. Demitiri Mendeleev 1869-Russian chemist The creator of the first Periodic Table of Elements Predicted the properties of the elements yet to be discovered His periodic table is based on atomic mass.

13. Henry Moseley 1915- English scientist that developed the periodic table that we use today Sorted the chemical elements of the periodic table of the elements in a logical order based on their physics- on their atomic number (how many protons an element has)

14. Modern Theory of the Atom Atoms are composed of three main subatomicparticles: the electron, the proton,and the neutron. Most of the mass of the atom is concentrated in the nucleus of the atom. The protons and neutrons are located within the nucleus while the electrons exist outside of the nucleus. In stable atoms, the number of protons is equal to the number of electrons.

15. Modern Theory of the Atom Cont. The type of atom is determined by the number of protons it has. The number of protonsin an atom is equal to the atomic number The sum of the number of protonsand neutrons in a particular atom is called the atomicmass Valence electrons are the outermost electrons and are where bonding takes place

16. Isotopes An element that has the same number of protons but different number of neutrons. Example: C12, C14

17. Chemical Symbol The shorthand way of writing an element. 1st Letter is always capitalized; the second and third letters are lower-cased.

18. Proton (p+)-Positively charged particle in the atom and is found in the nucleus. It has a mass 1840 times greater than an electron. A proton has a charge of +1 and a relative mass of 1amu. Neutron (no)-Neutral particle in the atom found in the nucleus. It has a mass nearly equal to that of a proton, but it carries no electrical charge. A neutron has 0 (zero) charge and a relative mass of 1 amu. Electron (e-)-Negatively charged particle in the atom found on the outside of the nucleus (the y surround the nucleus in an electron cloud). Its mass is 1/1840 that of a hydrogen atom. An electron as a charge of -1 and has a relative mass of 0 (zero).

19. Figuring out the Neutron Number of any element Subtract the proton number from the atomic mass number. To calculate mass number, you add the proton number plus the neutron number. Mass#=Proton # + Neutron # Neutron #=Mass# - Proton#

21. Atoms

22. Isotopes and average atomic mass-Elements come in a variety of isotopes, meaning they are made up of atoms with the same atomic number but different atomic masses. These atoms differ in the number of neutrons. The average atomic mass is the weighted average of all the isotopes of an element.

23. Calculating Average Atomic Mass of an Element Example: A sample of cesium is 75% 133Cs, 20% 132 Cs, and 5% 134Cs. What is the average atomic mass? (1st, change all percentages to decimals) Answer: .75 x 133=99.75 .20 x 132=26.4 .05 x 134= 6.7____ 132.85 133 a.m.u. (atomic mass units)

24. Radioactivity-the process by which some elements spontaneously emit radiation. • Radiation is the rays and particles emitted by a radioactive material. There are 3 types of natural radiation: • a-alpha radiation • b-beta radiation • g-gamma radiation

25. Alpha radiation consists of particles containing two protons and two neutrons. Alpha particles are identical to a helium nucleus. The symbol is a or 4HeThey are large but do not penetrate. The relative mass is 4 and the charge is +2. Beta radiation consists of fast-moving electrons. The symbol for a beta particle is B or 0B or 0e. They are small and fairly penetrating. The relative mass is 0 and the charge is -1. Gamma radiation (or gamma rays) is not particles but extremely high-energy electromagnetic radiation. They are very penetrating and dangerous. They have no mass and no charge.

26. The neutron to proton ration (n0:p+) determines the stability of a nucleus. Unstable nuclei will continue to decay until a stable ration is achieved. Nuclear equations show the changes involved with the atomic number and the mass number using nuclear symbols.

27. The total mass numbers of the reactants must equal the total mass numbers of the products. The total atomic numbers of the reactants must equal the total atomic numbers of the products.

28. Nuclear equations show the changes involved with the atomic number and the mass number using nuclear symbols. The total mass numbers of the reactants must equal the total mass numbers of the products. The total atomic numbers of the reactants must equal the total atomic numbers of the products.