Chapter 4 Atomic Structure

Before the atom…. Many cultures believed that all things were composed of the classical elements: Earth, Wind, Water, and Fire Much of their understanding came from things that they could see – they could not see atoms.

4.1 Atoms The discovery and study of atoms (the fundamental unit of all matter) has been done through indirect observations, logic, and scientific deduction. These particles can only be viewed with the most powerful telescopes.

4.1 Atoms Atoms were first proposed to exist by a Greek philosopher, Democritus, more than 2000 years ago. Democritus thought that atoms were invisible, indestructible, fundamental units of matter. His ideas agreed with later scientific theories, but lacked experimental support because scientific experiments were unknown in his time.

4.1 Atoms 2200 years after Democritus, John Dalton (1766-1844), proposed an atomic theory. Dalton, who studied chemistry very differently than Democritus, performed experiments to arrive at his atomic theory.

4.1 Daltons Atomic Theory 1. All elements are composed of submicroscopic, indivisible particles called atoms. 2. Atoms of the same element are identical. The atoms of any one element are different from those of any other element.

4.1 Dalton’s Atomic Theory 3. Atoms of different elements can physically mix together or can chemically combine with one another in simple whole-number ratios to form compounds. 4. Chemical reactions occur when atoms are separated, joined, or rearranged. However, atoms of one element are never changed into atoms of another element as a result of a chemical reaction.

4.1 Atoms atom – the smallest particle of an element that retains the properties of that element. Example: Pure copper “penny” would contain approximately 2.4 x 1022 copper atoms – that is 3.5 trillion times the number of people on this planet.

4.1 Atoms Even though atoms are so small, we can see images from them with the right technology. Scanning Tunneling Microscopes (STMs) and Atomic Force Microscopes (AFMs) can be used to visualize individual atoms. TEAM Microscope TEAM Published Article

4.1 Concept Practice 1. Democritus and Dalton both proposed that matter consists of atoms. Explain how their approaches to reaching the same conclusion differed? A: Democritus approached the subject logically, but without experimental evidence; Dalton based his ideas on an analysis of data obtained by experiments.

4.1 Concept Practice 2. Which of these statements would Dalton have agreed with (use Dalton’s atomic theory)? a. Atoms are the smallest particles of matter b. The mass of an iron atom is different from the mass of a copper atom c. Every atom of silver is identical to every other atom of silver d. A compound is composed of atoms of two or more different elements A: Dalton would have agreed with all of them.

4.2 Electrons, Protons, Neutrons Most of Dalton’s Atomic Theory is accepted today – one important revision is that atoms are not indivisible (they can be broken down) Protons, neutrons, and electrons are three subatomic particles that make up an atom. There are dozens of subatomic particles, but we will only be studying these three subatomic particles in chemistry.

4.2 Electrons The electron is the smallest subatomic particle with a mass 1/1840 the size of the proton. An electron has a negative charge (exactly 1-). English Physicist Sir J.J. Thompson discovered the electron in 1897 using the cathode ray tube. (see p. 86-87)

4.2 Protons The proton is a subatomic particle that has a positive charge (exactly 1+). A proton is a much more massive subatomic particle, when compared to the electron, but it is still very small. E. Goldstein discovered the proton in 1886 using a cathode ray tube and canal rays (a beam of protons.

4.2 Neutron The neutron is a subatomic particles that have no charge (neutral). Neutrons have a mass that is about the same as a proton. An English physicist, Sir James Chadwick, confirmed the existence of the neutron in 1932.

4.2 Electrons, Protons, Neutrons The fundamental building blocks of atoms are the electron, the proton, and the neutron. Table 4.1 Properties of Subatomic Particles Particle Symbol Relative Electrical Charge Approx. Relative Mass (amu*) Actual Mass (g) Electron e- 1- 1/1840 9.11x10-28 Proton p+ 1+ 1 1.67x10-24 Neutron n0 *1 amu = 1.66x10-24 g

4.2 Simple Rules about Matter and Electric Charges 1. Atoms have no electric charge – they are electrically neutral. 2. Electric charges are properties of particles. Electric charges are carried by particles of matter. 3. Electric charges exist in a single unit or multiples of a single unit. The are no fractions of charges. 4. Electric charges cancel when equal number of positively charged particles are balanced by negatively charged particles.

4.2 Concept Practice 3. Since all atoms have negatively charged electrons, shouldn’t every sample of matter have a negative charge? Explain. A: No. The (-) charge is cancelled by the (+) charge. 4. What experimental evidence did Thompson have for the following ideas? a. Electrons have a negative charge. b. Atoms of all elements contain electrons.

4.3 The Structure of the Nuclear Atom At first (before the discovery of the neutron) scientists thought that protons and neutrons were evenly distributed throughout the atom.

4.3 The Structure of the Nuclear Atom In 1911, Ernest Rutherford and his co-workers tested this theory of atomic structure by firing a beam of alpha particles at a thin gold sheet. From this experiment Rutherford proposed that almost all of the mass and all of the positive charge are concentrated at a small region at the center of an atom – he called this region the nucleus.

4.3 The Structure of the Nuclear Atom nucleus – the central core of an atom, composed of protons and neutrons. Almost all of mass in an atom is contained in a tiny nucleus which is extremely dense – if it was the size of a pea it would have a mass of 250 tons! The nucleus has a positive charge and occupies a very small volume of the atom – the rest of the atom is more or less empty space in which the negatively charged electrons are found.

4.3 Concept Practice 5. How did the results of Rutherford’s gold foil experiment differ from his expectations? A: He thought α particles would pass directly through, however some were deflected. 6. What is the charge, positive or negative, on the nucleus of every atom? A: Positive