ATOMS The Building Blocks of Matter History of

ATOMS The Building Blocks of Matter

History of the Atom • Around 400 B. C. Democritus was the first person to use the term atom. – Atom is from the Greek word “indivisible” • 1704 Isaac Newton proposed a mechanical universe with small solid masses in motion • 1808 John Dalton proposed his atomic theory based upon measurable properties of mass

Properties of Mass • There are several basic laws which drive Atomic Theory • Law of Conservation of mass – Matter is never created nor destroyed during ordinary physical and chemical changes • Law of Definite Proportions – States that a chemical compound contains the same elements in exactly the same proportions by mass regardless of the size of the sample or source of the compound • Law of Multiple Proportions – If two or more different compounds are composed of the same two elements, then the ratio of the masses of the second element combined with a certain mass of the first element is always a ratio of small whole numbers

Dalton’s Atomic Theory • In 1808, an English schoolteacher named John Dalton proposed an explanation for the different properties of mass. • Reasoned that elements were composed of atoms and combine to form compounds

Daltons Atomic Theory 1. 2. 3. 4. 5. All matter is composed of extremely small particles called atoms. Atoms of a given element are identical in size, mass, and other properties; atoms of different elements differ in size, mass, and other properties Atoms cannot be subdivided, created, or destroyed Atoms of different elements combine in simple wholenumber ratios to form chemical compounds In chemical reactions, atoms are combined, separated, or rearranged

Dalton’s WRONG!!!! • We now know that atoms can be divided into smaller particles. • An element can have atoms of different masses, called isotopes.

Structure of the Atom • Nucleus – Dense, positively charged center of the atom – Composed of protons and neutrons – Makes up the mass of an atom • Protons – Positively charged • Neutrons – No charge • Electrons – Negatively charged, located outside the nucleus – Makes up the size of the atom 75047

Electrons • In 1897, J. J. Thomson, using cathode ray tubes, found negatively charged particles called electrons – https: //www. youtube. com/watch? v=O 9 Goyscbazk • In 1909, Robert Millikan measured the charge of the electron – Oil drop experiment • https: //www. youtube. com/watch? v=XMf. YHag 7 Liw

Plum Pudding Model • Proposed by Thomson • Negative electrons were spread throughout the positive charge of the atom • Like plums in plum pudding, thus the name…get it • He was wrong!!!

Glowing Matter • Around 1900, Max Planck explained the phenomenon of hot glowing matter. • Explained this using discrete units of energy he called “quanta” or “quantum” • This idea would later be used to “correctly” place electrons in an atom.

Behold…The Nucleus • In 1911, Ernest Rutherford found a dense, positively charged bundle of matter he called the nucleus • Used the Gold Foil Experiment to find the nucleus • https: //www. youtube. com/watch? v=Zd 6_z. Vd. Mg. Jk&feature=related • https: //www. youtube. com/watch? NR=1&v=k. Ha. R 2 rs. FNhg • Nuclear forces are short range forces that hold protons and neutrons very close together

Rutherford’s Model • Ernest Rutherford used his research to develop his own model of the atom • Dense positively charged center, nucleus • Electrons traveled around the nucleus in empty space • He was Wrong!!!!

STAY TUNED To see if someone got the correct model of the atom!!!

The Power of the Proton • In 1914, H. G. J. Moseley using x-ray tubes, determined the charges on the nuclei of most atoms. • He wrote "The atomic number of an element is equal to the number of protons in the nucleus". • This work was used to reorganize the periodic table based upon atomic number instead of atomic mass.

The rest of the Nucleus • In 1932, James Chadwick, using alpha particles discovered a neutral atomic particle with a mass close to a proton. • Thus was discovered the neutron

Counting Atoms • Atomic number (Z) is the number of protons an atom contains – Change the number of protons, change the element • Isotopes are atoms with the same number of protons but with varying numbers of neutrons – Two was to symbolize isotopes, hyphen notation and nuclear symbol notation • Mass Number is the total number of protons and neutrons in an atom • Nuclide is a general term for a specific isotope of an element

Masses of Atoms • Relative atomic mass uses units called “atomic mass units (amu)” – An amu is defined as the 1/12 th the mass of a carbon 12 atom – Since a Carbon-12 atom contains 6 protons and 6 neutrons, 1 amu equals the approximate mass of either a proton or neutron • Atomic Weight is the weighted average of atomic masses of all known isotopes – The most abundant isotopes contribute more to the overall mass of the element