EARLY MODELS OF THE ATOM

Models of Matter a model is a tentative description of a system or theory that accounts for all of its known properties models are invented (usually) to interpret the invisible and relate experimental results

1. The Four Elements An Ancient Greek model (5 BCE), based on logic Matter is composed of four elements: air, fire, earth and water An atom is a single indivisible particle

2. John Dalton’s Atomic Theory (1803) based on experimentation, Experimental data showed: - elements combine in the same percent by mass each time the same compound is formed: atoms of different elements combine in fixed proportions John Dalton was an English school teacher that lived in the early 1800's.  At the age of 12, he began teaching.  He came up with his theory of the atom based on experimental data of the time.

Dalton’s Theory: 1. all matter is made of small spherical particles called atoms 2. each element is made of its own type of atom 3. atoms of different elements have different properties 4. atoms of different substances can combine in constant ratios to form new substances

Law of Constant Composition: atoms of different elements can be chemically combined in a fixed whole number ratio to form compounds, ex. H2O

Law of Conservation of Mass: 5. atoms cannot be created or destroyed during chemical or physical changes Law of Conservation of Mass: - the mass of the reactants in a chemical reaction is equal to the mass of the products 6. Atoms can neither be subdivided nor changed to one another according to Dalton’s theory atoms cannot be created or destroyed. Therefore when a chemical reaction occurs, atoms can only rearrange to form new substances. This means that the number and type of atoms are the same after the reaction as they were at the beginning

atom -the smallest particle of an element that has the properties of that element BUT studies on static charging of objects were not falling into line with this model static charge was shown to be the result of charged particles

3. Thomson’s Atomic Theory (1897) used a cathode ray tube to energize electrons experimental data showed: charged particles originated from the gas inside the tube charged particles are deflected away from a negatively charged object, but towards a positively charged object. John Joseph Thomson was a physics professor at Cambridge University, in England.  A cathode-ray tube is a sealed tube that contains a low pressure gas.  At either end of the tube is an electrode.  When powered, a beam of charged particles originates from the negative electrode, or the cathode as it is called.

Thomson’s Experiment Voltage source - +

Thomson’s Experiment Voltage source - +

- + Thomson’s Experiment Voltage source Passing an electric current makes a beam appear to move from the negative to the positive end.

Thomson’s Experiment Voltage source By adding an electric field

Thomson’s Experiment Voltage source + - By adding an electric field, he found that the moving pieces were negative

No matter what the gas used in the cathode-ray tube, or the metal used for the electrodes, the beam of particles reacts the same way. (indicates that the particles are the same for all atoms) Atoms are usually neutral, or not charged

Thomson’s Model Found the electron. Couldn’t find positive (for a while). Said the atom was like plum pudding. A bunch of positive stuff, with the electrons able to be removed.

Theory: “Plum Pudding Model” Atoms are composed of smaller particles. These particles are the same for all different types of atoms. These particles are negatively charged and are called electrons. Electrons are embedded throughout the uniform sphere of positive charge to make up a neutral atom. Matter is naturally neutral except when it gains or loses electrons

4. Rutherford’s Gold Foil Experiment (1911) Rutherford (at McGill) used very small positive particles (alpha particles: positive helium nuclei) and a fluorescent screen detector to see the particles Since the mass is evenly distributed in gold atoms alpha particles should go straight through. Rutherford was born and educated in New Zealand.  From 1898 to 1907, he was a physics professor at McGill University in Montreal.  From 1907 to 1919, he worked at the University of Manchester, and afterwards, finished hiss brilliant career at Cambridge University.  He was awarded the Nobel Prize in Chemistry in 1908, and knighted in 1914.

experimental data showed: Alpha particles (+ve) were shot at a thin sheet of gold Most alpha particles passed through unaffected Some alpha particles were deflected at large angles.  Some are even reflected back. - Thomson's model did not explain the deflection, so …

Fluorescent Screen Lead block Uranium Gold Foil

What he expected

What he got

Because

Because, he thought the mass was evenly distributed in the atom.

How he explained it Atom is mostly empty space Small dense, positive region at center. Alpha particles are deflected by this region if they get close enough.

+

+ Theory 1. an atom is made of mostly empty space 2. all the positive charge and most of the mass are in a tiny center of the atom: the nucleus 3. electrons surround but do not fill the rest of the atom

Modern View The atom is mostly empty space. Two regions Nucleus- protons and neutrons. Electron cloud- region where you might find an electron.

Symbolic representation of elements Name Symbol Charge Mass Location proton p +1 1 a.m.u. nucleus neutron n electron e- -1 1/1837 a.m.u. energy levels around nucleus

Sub-atomic Particles Z - atomic number = number of protons determines type of atom. A - mass number = number of protons + neutrons. Number of protons = number of electrons if neutral.

Symbols A X Z 23 Na 11

Elements are represented by the symbols found on the periodic table The atomic number of the element is written as a subscript before the symbol, it represents the number of protons (which is equal to the number of electrons in a neutral atom)

The atomic mass number is written as a superscript before the symbol ie Carbon atomic no. = 6 atomic mass no. = 12 The atomic mass of the element represents the umber of protons plus the number of neutrons in the atom The number of neutrons in an element can be determined by subtracting atomic no. from atomic mass no. (Carbon has 12 - 6 = 6 neutron.)