Atoms and Their Structure

History of the Atom Original idea (400 B.C.) came from Democritus, a Greek philosopher Democritus expressed the belief that all matter is composed of very small, indivisible particles, which he named atomos.

Who’s Next? John Dalton (1766-1844), an English school teacher and chemist, studied the results of experiments by other scientists.

Dalton’s Atomic Theory Dalton proposed his atomic theory of matter in 1803. Although his theory has been modified slightly to accommodate new discoveries, Dalton’s theory was so insightful that it has remained essentially intact up to the present time.

Dalton’s Atomic Theory All matter is made of tiny indivisible particles called atoms. Atoms of the same element are identical; those of different atoms are different.

Dalton’s Atomic Theory, cont. Atoms of different elements combine in whole number ratios to form compounds Chemical reactions involve the rearrangement of atoms. No new atoms are created nor destroyed.

Parts of the Atom Because of Dalton’s atomic theory, most scientists in the 1800s believed that the atom was like a tiny solid ball that could not be broken up into parts. In 1897, a British physicist, J.J. Thomson, made discoveries that required Dalton’s 1st postulate to be modified He discovered that atoms are made of smaller (subatomic) particles.

Parts of the Atom Thomson’s experiments used a cathode ray tube. It is a vacuum tube - all the air has been pumped out.

Thomson’s Experiment Voltage source - + Vacuum tube Metal Disks

- + Thomson’s Experiment Voltage source At each end of the tube is a metal piece called an electrode, which is connected through the glass to a metal terminal outside the tube.

- + Thomson’s Experiment Voltage source When the electrodes are charged, rays travel in the tube from the negative electrode, which is the cathode, to the positive electrode, the anode.

- + Thomson’s Experiment Voltage source Because these rays originate at the cathode, they are called cathode rays.

Thomson’s Experiment Voltage source - +

Thomson’s Experiment Voltage source - +

Thomson’s Experiment Voltage source - +

Thomson’s Experiment Voltage source - +

Thomson’s Experiment Voltage source + - By adding an electric field,

Thomson’s Experiment Voltage source + -

Thomson’s Experiment Voltage source + - Thomson found that the rays bent toward a positively charged plate and away from a negatively charged plate.

Thomson’s Experiment Voltage source + - He knew that objects with like charges repel each other, and objects with unlike charges attract each other.

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

Thomson’s Experiment Voltage source + - Thomson concluded that cathode rays are made up of invisible, negatively charged particles.

Cathode Ray Tube

Thomson’s Model From Thomson’s experiments, scientists had to conclude that although atoms are neutral, some of the subatomic particles have a charge (positive or negative).

Thomson’s Model Matter is not negatively charged, so atoms can’t be negatively charged either. If atoms contained extremely light, negatively charged particles, then they must also contain positively charged particles — probably with a much greater mass than electrons.

Thomson’s Model J.J. Thomson said the atom was like plum pudding, a popular English dessert.

Thomspon’s Model In 1886, scientists discovered that a cathode-ray tube emitted rays not only from the cathode but also from the positively charged anode. Years later, scientists determined that the rays were composed of positively charged subatomic particles.

Isotopes In 1910, J.J. Thomson discovered evidence that atoms of the same type (elements) can have different masses. Neon consisted of atoms of two different masses.

Isotopes Atoms of an element that are chemically alike but differ in mass are called isotopes of the element.

Ernest Rutherford In 1909, a team of scientists led by Ernest Rutherford in England carried out the first of several important experiments that revealed an arrangement far different from the plum pudding model of the atom.

Rutherford’s Experiment The experimenters set up a lead-shielded box containing radioactive polonium, which emitted a beam of positively charged subatomic particles through a small hole.

Rutherford’s Experiment The sheet of gold foil was surrounded by a screen coated with zinc sulfide, which glows when struck by the positively charged particles of the beam.

Florescent Screen Lead block Polonium Gold Foil

What Rutherford Expected The alpha particles to pass through without changing direction very much.

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

What Rutherford Observed

How Rutherford Explained It To explain the results of the experiment, Rutherford’s team proposed a new model of the atom: Because most of the particles passed through the foil, they concluded that the atom is nearly all empty space.

How Rutherford Explained It Because so few particles were deflected, they proposed that the atom has a small, dense, positively charged central core, called a “nucleus.” +

Alpha particles are deflected by it if they get close enough to the nucleus. +

The Nuclear Model of the Atom The new model of the atom as pictured by Rutherford’s group in 1911 is shown below.

Explanation of 4 Observations Three subatomic particles were proposed to explain the four observations made by Thomson and Rutherford: protons, electrons, and neutrons.

The Electron The first to be discovered, electrons have a negative charge and almost no mass (compared to protons and neutrons). Electrons account for the volume of an atom.

The Proton Protons are positively charged, neutralizing the charge of electrons. They have mass, and are located in the nucleus.

The Neutron Because of the discovery of isotopes, scientists predicted that a third particle would be discovered that exists in the nucleus, has a mass equal to that of a proton, but has no charge. The existence of this neutral particle, called a neutron, was confirmed in the early 1930s.

Subatomic Particles Name Electron e- -1 1/2000 Proton p+ +1 1 Neutron Relative mass Relative mass Name Symbol Symbol Charge Charge Electron e- -1 1/2000 Proton p+ +1 1 Neutron n0 1

Modern View of the Atom The atom has two regions and is 3- dimensional. The nucleus is at the center and contains the protons and neutrons.

Modern View of the Atom The electron cloud is the region where you might find an electron and most of the volume of an atom.

Atomic Number The atomic number of an element is the number of protons in the nucleus of an atom of that element. The number of protons determines identity of an element, as well as many of its chemical and physical properties.

Atomic Number Because neutral atoms have no overall electrical charge, a neutral atom must have as many electrons as there are protons in its nucleus. Therefore, the atomic number of an element also tells the number of electrons in a neutral atom of that element.

Masses The mass of a neutron is almost the same as the mass of a proton. The sum of the protons and neutrons in the nucleus is the mass number of that particular atom. Isotopes of an element have different mass numbers because they have different numbers of neutrons, but they all have the same atomic number.

Masses Isotopes of an element have different mass numbers because they have different numbers of neutrons, but they all have the same atomic number. Isotopes are always identified by their mass number, and can be represented in two ways: Two equivalent representations of the carbon – 12 isotope: C-12 12C

Symbols Elements can be represented by using the symbol of the element, the mass number and the atomic number. X Mass number Atomic number The mass number is the number of protons + the number of neutrons.

X A Z Symbols Mass number is represented by the letter A. Mass number Atomic number Z Atomic number is represented by the letter Z.

Symbols Example Determine the following for the fluorine atom depicted below. 19 F (9) number of protons 9 number of neutrons (10) number of electrons (9) atomic number (9) e) mass number (19)

Symbols Problem Determine the following for the bromine atom depicted below. 80 Br (35) number of protons 35 number of neutrons (45) number of electrons (35) atomic number (35) e) mass number (80)

Se or Se-78 Symbols Problem If a neutral element has an atomic number of 34 and a mass number of 78 what is the (34) number of protons number of neutrons (44) number of electrons (34) complete symbol (2 ways) 78 Se or Se-78 34

Pa or Pa-231 Symbols Problem If a neutral element has 91 protons and 140 neutrons what is the (91) atomic number mass number (231) number of electrons (91) complete symbol (2 ways) Pa or Pa-231 231 91

Pt or Pt-195 Symbols Problem If a neutral element has 78 electrons and 117 neutrons what is the (78) atomic number mass number (195) number of protons (78) complete symbol (2 ways) Pt or Pt-195 195 78

Information in the Periodic Table The average atomic mass is the weighted average mass of all the naturally occurring isotopes of that element. The unit is the Atomic Mass Unit (amu).

Calculating Atomic Mass

Calculating Atomic Mass Copper exists as a mixture of two isotopes. The lighter isotope (Cu-63), with 29 protons and 34 neutrons, makes up 69.17% of copper atoms. The heavier isotope (Cu-65), with 29 protons and 36 neutrons, constitutes the remaining 30.83% of copper atoms.

Calculating Atomic Mass To determine the average atomic mass, first calculate the contribution of each isotope to the average atomic mass, being sure to convert each percent to a fractional abundance. Will the average atomic mass of copper be closer to 63 amu or 65 amu?

Calculating Atomic Mass Mass contribution = mass of isotope x abundance of isotope For Cu-63: Mass contribution = 62.930 amu x 0.6917 = 43.529 amu For Cu-65: Mass contribution = 64.928 amu x 0.3083 = 20.017 amu

Calculating Atomic Mass The average atomic mass of the element is the sum of the mass contributions of each isotope. Atomic mass Cu = mass contribution Cu-63 + mass contribution Cu-65 Atomic mass Cu = 43.529 + 20.017 = 63.546 amu

Ions While changing the number of neutrons in an atom changes the mass and creates an isotope, changing the number of electrons changes the charge of an atom and creates an ion.

Ions The charge is the sum of the charges contributed by the protons and electrons. Charge = (p+) – (e-) The charge of an atom is written as a superscript on the right side of an element’s symbol: Ex. Fluorine has 9 protons with 10 electrons: F1- or simply F- Sodium has 11 protons with 10 electrons: Na1+or simply Na+

Ions O2- Be2+ Zr3+ Br- Write the symbol for the following ions: oxygen (O) with 10 electrons beryllium (Be) with 2 electrons zirconium (Zr) with 37 electrons bromine (Br) with 36 electrons O2- Be2+ Zr3+ Br-

Summary of Subatomic Particles mass charge location changing the number creates… proton, p+ 1 +1 nucleus a different element neutron, n0 an isotope electron, e- ~0 -1 electron cloud an ion

End of Day 1