Chapter 3 Atomic Structure

1. Chapter 3 • Atomic Structure

2. Chapter 3 • Section 2 • Atomic Structure

3. Objectives • Describe the experiments of Thomson, Millikan, Rutherford and Bohr and explain how they contributed to our present understanding of atomic structure. • Describe the mass and charge differences among electrons, protons and neutrons. • Define atomic number, mass number and isotopes in terms of the number of electrons, protons and neutrons. • Determine the atomic mass of an element from the percent abundance of its isotopes, or the abundance from its atomic mass.

14. The Modern Atom • Today we define the atom as the smallest particle of an element that retains the chemical properties of that element. • All atoms consist of two regions: • The nucleus is a very small region located near the center of an atom and contains positively charged particles called protons.

15. Surrounding the nucleus is a region occupied by negatively charged particles called electrons. • This region is very large compared to the size of the nucleus. • Protons, electrons and neutrons are called subatomic particles.

16. The Electron

24. Robert Millikan Robert Millikan was an American physicist. In 1909 conducted experiments to prove the electron existed. Oil drop experiment. Determined the charge of a single electron. (1.592 × 10−19coulomb)

25. Robert Millikan • The electron does carry a negative charge. • The mass of an electron is very small. • (9.1 x 10-31 Kg) • Because atoms are electrically neutral, they must contain a positive charge to balance the negative charge. • Because of the low mass of electrons, atoms must contain other particles that account for most of the mass.

26. Robert Millikan

42. Chapter 3 • Section 3 • Atomic Structure

43. Atomic Nucleus • Except for the hydrogen atom, all atomic nuclei are made up of two kinds of particles, protons and neutrons. • A proton has a positive charge equal in magnitude to the negative charge of an electron. Atoms are electrically neutral. • A neutron which has no charge.

44. Atomic Number Atomic Number (Z) – represents the number of protons in the nucleus of each atom of that element. Atoms of the same element have the same number of protons. Atoms of different elements have different number of protons.

45. Atomic Number On the periodic table, the elements are placed in order of increasing atomic number. At the top left of the table is hydrogen, H, which has an atomic number of 1. All atoms of the element hydrogen have one proton in the nucleus.

46. Next is helium, which has two protons, and therefore an atomic number of two. • This is followed by lithium with an atomic number of three – three proton in the nucleus. • The atomic number for each element is listed on the periodic table above the elements symbol.

47. The atomic number identifies an element. Because all atoms are neutral (no charge), we know the atomic number is equal to the number of electrons. # of protons = # of electrons Example: Silver (Ag) has an atomic number of 47. Therefore, silver has 47 protons and 47 electrons in each silver atom.

48. Isotopes The simplest atoms are those of hydrogen. All hydrogen atoms contain one proton. However, hydrogen atoms can contain different number of neutrons. Three types of hydrogen atoms are known with a neutron count of 0, 1 or 2.

49. Isotopes The most common type of hydrogen atom is called protium. It accounts for 99.985% of the hydrogen atoms found on earth. The nucleus of protium consist of one proton and zero neutrons.

50. Isotopes Another type of hydrogen atom is deuterium. This accounts for 0.015% of hydrogen atoms. The nucleus of deuterium consist of one proton and one neutron.