The Structure of the Atom Chapter 4.2

Bell Ringer The ancient Greek philosopher Democritus coined what word for a tiny piece of matter that cannot be divided? Element Electron Atom Molecule Dalton’s theory (~1800; based on behavior of gasses) included all but one of the following points. Which is not from Dalton? All elements are composed of atoms. Most of an atom’s mass is in its nucleus. Compounds contain atoms of more than one element. In a specific compound, atoms of different elements always combine in the same way. J. J. Thomson’s gas tube electrical experiments (~1897) provided first evidence of: Atoms Nucleus Subatomic particles Elements Rutherford’s gold foil experiment (~1914) provided evidence for one of these: Negative and positive charges are spread evenly throughout an atom. Alpha particles have a positive charge. Gold is not as dense as previously thought. There is a dense, positively charged mass in the center of an atom.

Properties of Subatomic Particles J.J. Thomson was the first to detect subatomic particles (later named electrons) Electron: Negatively charged subatomic particle. Found in the space outside the nucleus. Very light mass (~1/2000 of Hydrogen atom) Charge of 1-.

Thomson’s Model of the Atom Thomson’s Experiments In his experiments, J. J. Thomson used a sealed tube containing a very small amount of gas. Sealed tube filled with gas at low pressure Glowing beam Positive plate Metal disk Negative plate Metal disk Metal disk Source of electric current Source of electric current 4

Properties of Subatomic Particles Rutherford found positive charge varies among the elements. Based on experiments with gold and other elements. Proton: Positively charged subatomic particle Found in the nucleus of an atom. Charge of 1+. Each nucleus contains at least one proton.

Rutherford’s Atomic Theory The Gold Foil Experiment Alpha particles Undeflected particle Deflected particle Gold atoms Slit Beam of alpha particles Alpha particles Screen Source of alpha particles Nucleus 6

Properties of Subatomic Particles James Chadwick 1932 - the English physicist carried out an experiment to show that neutrons exist. Radioactive decay produced particles about the mass of a proton but without any charge. Proved the particles produced were neutral because a charged object did not deflect their paths. Neutron: Neutral (no charge) subatomic particle Found in the nucleus of an atom. Mass ~ equal to that of a proton.

Comparing Subatomic Particles Everything we know about subatomic particles is based on how the particles behave in experiments. Scientists still do not have an instrument that can show the inside of an atom.

Comparing Subatomic Particles Mass Protons and neutrons have almost the same mass. Mass of ~2000 electrons = the mass of one proton. Charge An electron charge (-1) is equal in size but the opposite sign of a proton charge (+1). Neutrons have no charge. Location in the atom Nucleus - Protons and neutrons Outside the nucleus - electrons

Subatomic Particles - Protons Atoms of different elements have different numbers of protons. Atomic Number = Number of protons in an atom of that element. For any given element: All atoms have same atomic number. Hydrogen has 1 proton; atomic number = 1. Helium has 2 two protons; atomic number = 2 Etc. Each element has a unique atomic number.

Atomic Number and Mass Number Each element has a different atomic number. A The atomic number of sulfur (S) is 16. B The atomic number of iron (Fe) is 26. C The atomic number of silver (Ag) is 47.

Atomic Number and Mass Number Atoms are neutral when: Each positive charge in an atom is balanced by a negative charge. i.e. # of protons = # of electrons Each proton has an electron to offset its charge Examples: Hydrogen atomic number = 1 Each neutral hydrogen atom has 1 proton & 1 electron. Sulfur atomic number = 16 Each neutral sulfur atom has 16 protons & 16 electrons.

Atomic Number and Mass Number Mass Number = Sum of protons + neutrons. To find # of neutrons (when mass # & atomic # are known): # of neutrons = mass # - atomic # Example: Aluminum with mass # = 27 Mass # = 27 Atomic # = 13. 27 - 13 protons = 14 neutrons

Atomic Number and Mass Number

Charged Atoms An atom becomes charged when it looses or gains an electron (e-). Positive Charge = loss of e-’s H becomes H+1 Negative Charge = gaining of e-’s H becomes H-1 Electrons are the only subatomic particle that can be gained or lost.

Isotopes Isotopes = same element; different # neutrons Isotopes of an element: Same # of protons i.e. same atomic number Different # of neutrons Different atomic mass because of the neutrons

Isotopes Isotopes are referred to by mass number For example, oxygen has 3 isotopes: “oxygen 16”, “oxygen 17”, and “oxygen 18” All three oxygen isotopes have same reactions Examples: All combine with hydrogen to form water All combine with iron to form rust.

Isotopes Most elements: Physical and chemical properties of all isotopes are very similar. Hydrogen is an exception. Hydrogen-1 no neutrons (most H is hydrogen-1) Hydrogen-2 has one neutron Hydrogen-3 has two neutrons. Because a hydrogen-1 atom has only one proton, adding a neutron doubles its mass.

Isotopes Heavy Water - hydrogen-2 atoms & oxygen-16 Hydrogen-2 atoms twice the mass of Hydrogen-1 Properties of heavy water are different from the properties of ordinary water.

Assessment Questions Of the subatomic particles that form the atom, the one with the smallest mass is: Neutron. Electron Proton Nucleus 20

Assessment Questions Of the subatomic particles that form the atom, the one with the positive charge is: Neutron. Electron Proton Nucleus

Assessment Questions Of the subatomic particles that form the atom, the one with the neutral charge is: Neutron. Electron Proton Nucleus 22

Assessment Questions Of the subatomic particles that form the atom, which ones are found in the nucleus? Neutron. Electron Proton None 23

Assessment Questions In which way do isotopes of an element differ? number of electrons in the atom number of protons in the atom number of neutrons in the atom net charge of the atom