1. To Understand The Atom Is To Understand Chemistry

2. Learning Objectives Describe the three particles in the atom Describe the three particles in the atom Define atomic number and mass number Define atomic number and mass number Describe isotopes Describe isotopes Write symbols for elements Write symbols for elements Determine the numbers of particles in any atom from the element symbol Determine the numbers of particles in any atom from the element symbol Determine average atomic mass from isotope distribution Determine average atomic mass from isotope distribution

3. Atoms are not the smallest thing Growing evidence for the divisibility of the indivisible

4. Electrostatics and electricity Static electricity was observed by Thales (300 BC). Some “charged” objects repel and others attract Static electricity was observed by Thales (300 BC). Some “charged” objects repel and others attract The voltaic cell (Volta, 18 th century) generated electrical current from chemical reactions The voltaic cell (Volta, 18 th century) generated electrical current from chemical reactions Mechanical electrical generation was achieved in 1825 Mechanical electrical generation was achieved in 1825 The point: Atoms are neutral. If indivisible, where do electrical charges come from? The point: Atoms are neutral. If indivisible, where do electrical charges come from?

5. Faraday’s prescience “Although we know nothing of what an atom is, we cannot resist forming some idea of a small particle; and though we are in equal ignorance of electricity, there is an immensity of facts which justify us in believing that the atoms of matter are associated with electrical powers to which they owe their most striking qualities, and amongst them their chemical affinity.” “Although we know nothing of what an atom is, we cannot resist forming some idea of a small particle; and though we are in equal ignorance of electricity, there is an immensity of facts which justify us in believing that the atoms of matter are associated with electrical powers to which they owe their most striking qualities, and amongst them their chemical affinity.”

6. Ray of hope 1858 1858 Discovery of cathode rays by Julius Plucker Discovery of cathode rays by Julius Plucker Application of a large voltage across an evacuated tube causes a current to flow. The current flow is accompanied by radiation from the excited gas molecules Application of a large voltage across an evacuated tube causes a current to flow. The current flow is accompanied by radiation from the excited gas molecules How does the neutral and indivisible atom create a charge? How does the neutral and indivisible atom create a charge?

7. Cathode rays are negatively charged particles 1897.J. Thomson demonstrates that cathode rays consist of negatively charged particles. The first sighting of the electron: a particle much smaller than an atom.

8. The Thomson model of the atom "I regard the atom as containing a large number of smaller bodies which I will call corpuscles, these corpuscles are equal to each other.... In the normal atom, this assemblage of corpuscles forms a system which is electrically neutral. Though the individual corpuscles behave like negative ions, yet when they are like negative ions, yet when they are assembled in a neutral atom the negative assembled in a neutral atom the negative effect is balanced by something which effect is balanced by something which causes the space through which the corpuscles are spread to act as if it had a charge of positive electricity equal in amount to the sum of the negative charges of the corpuscles…”

9. X-rays and atoms’ invisible rays 1895 1895 Wilhelm Conrad Roentgen discovers X-rays while doing experiments with cathode rays Wilhelm Conrad Roentgen discovers X-rays while doing experiments with cathode rays

10. Radioactivity: the search for invisible rays 1896 1896 Henri Becquerel discovered radioactivity, which suggested that some atoms were capable of decomposing to give smaller particles. Henri Becquerel discovered radioactivity, which suggested that some atoms were capable of decomposing to give smaller particles. 1903 1903 Frederick Soddy and William Ramsey demonstrated that uranium decayed to give helium. Direct proof that atoms were divisible. Frederick Soddy and William Ramsey demonstrated that uranium decayed to give helium. Direct proof that atoms were divisible.

11. 1909 Ernest Rutherford’s gold foil experiment The atom’s inner secrets exposed by its own offspring The atom’s inner secrets exposed by its own offspring

12. Hierarchy of discovery The path to knowledge occurs in steps, each depending on a previous advance The path to knowledge occurs in steps, each depending on a previous advance The battery and electricity: Volta 1799 Cathode rays: Plucker 1858 X-rays: Roentgen 1895 Radioactivity: Becquerel 1896 The nucleus: Rutherford 1909

13. The nucleus Tiny Tiny Incredibly dense – contains all the mass of the atom Incredibly dense – contains all the mass of the atom Positively charged Positively charged Contains protons (charged) and neutrons (neutral) – not discovered until much later Contains protons (charged) and neutrons (neutral) – not discovered until much later

14. Summary of Atom Pictures Dalton: Indivisible atom Dalton: Indivisible atom Thomson: Electrons Thomson: Electrons Rutherford: Nucleus Rutherford: Nucleus

15. Comparison of subatomic particles

16. Atoms are neutral: # electrons = # protons The number of protons in the nucleus of an atom determines the atomic number (Z) and indicates the element's identity. For a neutral atom, the atomic number also describes the number of electrons around the nucleus. The number of protons in the nucleus of an atom determines the atomic number (Z) and indicates the element's identity. For a neutral atom, the atomic number also describes the number of electrons around the nucleus.

17. Variations on the number of neutrons in the nucleus give rise to different isotopes of the same element.

18. Element notation: Atomic number and mass number Mass number: number of protons + neutrons Atomic number: number of protons Element symbol Counting particles: Counting particles: Number of electrons = number of protons = 6 Number of electrons = number of protons = 6 Number of neutrons = mass number – atomic number (13 – 6 = 7) Number of neutrons = mass number – atomic number (13 – 6 = 7)

19. Atomic mass Atoms are very small and so normal units of mass are inconvenient for describing atomic mass Atoms are very small and so normal units of mass are inconvenient for describing atomic mass Atomic Mass Unit Atomic Mass Unit Mass of one atom of carbon-12 = 12 amu Mass of one atom of carbon-12 = 12 amu 1 amu = 1.660 x 10 -24 g 1 amu = 1.660 x 10 -24 g

20. Similarity of atomic mass and mass number Almost all mass is concentrated in the nucleons Almost all mass is concentrated in the nucleons Means mass number (integer) and atomic mass are very similar Means mass number (integer) and atomic mass are very similar

21. Isotopes cause disparity Carbon has two isotopes: C-12 and C-13 Carbon has two isotopes: C-12 and C-13 Atomic mass C-12 = 12 amu; C-13 = 13.0034 amu Atomic mass C-12 = 12 amu; C-13 = 13.0034 amu Atomic mass C is weighted average of the masses of the two isotopes (98.89 % C-12 + 1.11 % C-13 Atomic mass C is weighted average of the masses of the two isotopes (98.89 % C-12 + 1.11 % C-13 = 12.011 amu

22. What of the electrons? We now understand the atom to contain a tiny positively charged massive nucleus surrounded by a comparatively vast empty space containing the electrons We now understand the atom to contain a tiny positively charged massive nucleus surrounded by a comparatively vast empty space containing the electrons When atoms combine the electrons must interact When atoms combine the electrons must interact We need to understand the arrangement of electrons in the atom We need to understand the arrangement of electrons in the atom

23. Rudiments of elements All of matter is made from elements, in combination with other elements or alone. All of matter is made from elements, in combination with other elements or alone. There are some ninety naturally occurring – have “always” been. There are some ninety naturally occurring – have “always” been. More than twenty have been created artificially More than twenty have been created artificially

24. Elements organized – a triumph of chemistry The periodic table of the elements is organized into 18 groups and 7 periods. Elements are represented by one- or two-letter symbols and are arranged according to atomic number. The periodic table of the elements is organized into 18 groups and 7 periods. Elements are represented by one- or two-letter symbols and are arranged according to atomic number.

25. The Periodic Table: Groups and Periods Groups: columns of elements Periods: rows of elements

26. Elements in groups have similar properties One of the eight main groups of elements, Group 1A is known as the alkali metals group. Elements in this group are highly reactive and form alkaline (basic) solutions with water. One of the eight main groups of elements, Group 1A is known as the alkali metals group. Elements in this group are highly reactive and form alkaline (basic) solutions with water.

27. All groups have characteristic properties One of the eight main groups of elements, Group 8A is known as the noble gas group. Elements in this family have such a low reactivity that they were formerly known as the inert gases. One of the eight main groups of elements, Group 8A is known as the noble gas group. Elements in this family have such a low reactivity that they were formerly known as the inert gases. Although their chemical properties are very uninteresting, their importance to understanding chemistry cannot be understated Although their chemical properties are very uninteresting, their importance to understanding chemistry cannot be understated

28. Group 7A – the halogens Reactive nonmetals Reactive nonmetals Fluorine and chlorine are gases Fluorine and chlorine are gases Bromine liquid Bromine liquid Iodine solid Iodine solid