DEMOCRITUS
all things are composed of small, indestructible particles of pure matter which move about eternally in infinite empty. This particle is called “atom”.
3
MODERN ATOMIC THEORY
There were more discoveries by several scientists that led to the modern atomic theory.
4 John Dalton J.J. Thomson Ernest Rutherford James Chadwick
ATOM AND CHEMICAL COMBINATION
Law of Constant Composition for a given compound, the relative number and kind of atoms are constant. Law of Conservation of Mass the total mass of materials present after a chemical reaction is the same as the total mass before the reaction. Law of Multiple Proportions if two elements can combine to form more than one compound, the masses of one element that combine with a fixed mass of the second element are in the ratio of small whole numbers.
5
DALTON’S ATOMIC THEORY
Each element is made up of tiny particles called atom. of a given element are identical in mass and other properties. Atoms of different elements are different in some fundamental ways. Compounds result from the chemical combination of a specific ratio of atoms of different elements. Chemical reactions involve reorganization of the atoms. The atoms themselves are not changed in a chemical reaction.
6
Dalton atomic model
CHANGES FROM DALTON’S THEORY
Atom are not indivisible, but rather are composed of smaller , subatomic particles Atoms of an element can have different masses: they are called isotope. Atoms of one element can change into atoms of another element but only by nuclear reaction.
8
THOMSON : DISCOVERY OF ELECTRON cathode ray bends in a magnetic field it must consist of charged particles.
9
https://www.youtube.com/watch?v=O9Goyscbazk
THOMSON : DISCOVERY OF ELECTRON
cathode ray bends toward the positive electrical plate it must consist of negatively charged particles.
10 The rays were also identical no matter what metals were used for the cathode.
THOMSON’S CONCLUSION
Thomson concluded that cathode rays consist of negatively charged particles found in all matter. This particle is later called electron. Thomson could calculate the mass/charge ratio of the electron. Using the apparatus See picture The best value is me/e = -5.686 x 10-12 kg/C
11
The electric and magnetic fields were adjusted so that the cathode ray beam was not deflected. The strengths of the two fields needed to exactly balance the beam was used to calculate the mass to charge ratio of the cathode ray.
12
THOMPSON’S ATOMIC MODEL
Thomson concluded that the cathode ray consists of a beam of negatively charged particles (later we called them electrons) and that these particles are constituents of all matter.
13
Atom is neutral, hence the negatively charged electrons are embedded within positively charged diffuse matter. Thomson proposed his “Raisins in a plum pudding atomic model”.
CHARGE OF ELECTRON: MILLIKAN’S OIL-DROP EXPERIMENT
charge of electron -1.602 x 10-19 C
mass of electron 9.109 x 10-31 kg
14
https://www.youtube.com/watch?v=UFiPWv03f6g&index=3&list=PLPHrUqdEcklOUwzvmYent0onJkSzguLfm
THE NUCLEAR MODEL OF THE ATOM
Ernest Rutherford and his team observed the effect of bombarding thin gold foil with alpha radiation. Most of alpha particles went through with no or little deflections. Some small numbers of the alpha particles were deflected by more than 90°.
15http://cwx.prenhall.com/petrucci/medialib/media_portfolio/text_images/006_RUTHERFORD.MOV
THE NUCLEAR MODEL OF THE ATOM
The alpha particles must hit something massive and positive in the gold atoms.
Rutherford proposed that most of the mass of the atom is concentrated in a positively charged centre, or nucleus, surrounded by space occupied by negatively charged electrons.
16
ATOMIC NUCLEUS : PROTON AND NEUTRON
The positively charged particles in the nucleus are later called “protons”. A proton is a nuclear particle having a positive charge equal to that of the electron but it has a mass so much higher than that of the electron. The mass of protons is not enough to account for the total mass of atom. In 1932, James Chadwick resolved this issue. He discovered neutron, an uncharged massive particle that also resides in the nucleus.
17
ATOMIC VIEW
18
SUBATOMIC PARTICLES
Relative charge
Absolute charge (coulomb)
Mass (amu) Absolute mass (g)
Electron (ē) -1 -1.6022×10-19 0.000548 9.109×10-28 Proton (p) +1 +1.6022×10-19 1.0073 1.673×10-24 Neutron (n) 0 0 1.0087 1.675×10-24
19
amu is defined as 1/12 the mass of the carbon-12 atom. The carbon-12 atom is defined to have a mass of exactly 12.000000 amu. 1 amu = 1.66054 x 10-24 g
ATOM IDENTITY How do we identify an atom?
20
OBJECTIVES
Use a nucleus symbol to identify an atom Give definitions for atomic number and mass number Understand concept of isotope and average atomic weight
21
NUCLEUS SYMBOL
Atoms of every element have the three basic subparticles; proton neutron and electron. How do we identify atom of one element from another? Nucleus symbol is used to identify each element.
22
NUCLEUS SYMBOL
23
symbol of element
atomic number
atomic mass
For carbon atom
ATOMIC NUMBER (Z)
Represents the total number of protons in an atom Every element has its own unique atomic number.
24
Element Atomic number Number of protons Carbon 6 6 Nitrogen 7 7 Oxygen 8 8
MASS NUMBER (A)
The mass number (A) equals to the number of protons plus neutrons in the nucleus. The number of neutrons of an element can be calculated by subtract Z from A.
25
ATOMIC NUMBER AND ELECTRON COUNT
Because an atom is neutral, the number of positively charged protons are cancelled out by an equal number of negatively charged electrons.
26
No. of protons 11 No of electrons 11
Examples:
No. of protons 17 No of electrons 17
ION
Ion are formed only through the loss or gain of electrons; there is no change in the number of protons in the atom. If electrons are lost, no. of protons > no. of electrons therefore the ion has a positive charge. Positively charged ion is called “cation” If electrons are gained, no. of protons < no. of electrons therefore the ion has a negative charge. Negatively charged ion is called “anion” 27
The net ionic charge of an ion is showed by a superscript
Net ionic charge: +1 Na lost one ē
No. of protons 11 No of electrons 10
No. of protons 17 No of electrons 18
Net ionic charge: -1 Cl gained one ē
NUCLEUS SYMBOL AND NO. OF SUBATOMIC PARTICLES
28
ATOMIC MASS
The mass of a single atom is very tiny, so chemists use a unit called an atomic mass unit (amu) to describe atomic masses. This unit is also called a dalton, abbreviated Da. One amu is defined as exactly one-twelfth(1/12) the mass of an atom of carbon-12 and is equal to 1.66054 10–24 g.
30
ATOMIC MASS AND MASS NUMBER
Element Mass of one atom (amu)
Mass of one mole of atom (g)
Carbon-12 12.0107 12.0107
Nitrogen-14 14.0067 14.0067
31
The mass of an atom in amu is approximated by the atom's mass number (A). This number is also the mass in gram of one mole of atoms.
ISOTOPE
Isotopes are atoms whose nuclei have the same atomic number but different mass numbers The nuclei have the same number of protons but different numbers of neutrons. Naturally occurring isotopes exist in different amount depending on their stabilities.
32
Examples
12 13 14 6 6 6 C C C 35 37 17 17 Cl Cl
Example, the amount of different isotopes of Boron.
33
Isotope Isotopic mass (amu)
Fractional abundance Boron-10 10.013 0.1978 Boron-11 11.009 0.8022
The fractional abundance is the fraction of a sample of atoms that is composed of a particular isotope.
AVERAGE ATOMIC WEIGHT
The atomic weight or atomic mass shown in the periodic table of elements is the average value of the isotopes. To calculate the atomic weight of an element, two quantities are required. 1. The atomic mass of each isotope (Isotopic mass) 2. The naturally occurring fractional abundance of each isotope These two quantities are obtained by a “Mass spectrometer”
34
Calculate the atomic weight of boron, B, from the following data:
35
Isotope Isotopic mass (amu)
Fractional abundance Boron-10 10.013 0.1978 Boron-11 11.009 0.8022
average atomic weight (isotopic mass fractional abundance)
atomic weight (10.013 0.1978) (11.009 0.8022) = 10.812 amu
EXERCISE
Chromium, Cr, has the following isotopic masses and fractional abundances
What is the atomic mass of chromium?
36