4.1 Studying Atom Early evidence used to develop models of atoms.
Democritus said that all matter consisted of extremely small particles that could NOT be divided called these particles atoms from the Greek word atomos, which means uncut or indivisible.
Aristotle thought that all substances were made of only four elements earth, air, fire, and water. Aristotle s views on the structure of matter were accepted for many centuries. By the 1800s, scientists had enough experimental data to support an atomic model.
Dalton s Atomic Theory Dalton proposed the theory that all matter is made up of individual particles called atoms, which cannot be divided.
Dalton s Theory All elements are composed of atoms. All atoms of the same element have the same mass, and atoms of different elements have different masses.
Dalton s Theory Compounds contain atoms of more than one element. In a particular compound, atoms of different elements always combine in the same way.
Thomson s Model of the Atom Thomson s experiments provided the first evidence that atoms are made of even smaller particles (subatomic particles).
Thomson s Model Thomson revised Dalton s model to account for these subatomic particles. The atom has neither a positive nor a negative charge, but there must always be some positive charge in the atom. The atom is filled with a positively charged mass of matter that has negative charges evenly scattered throughout it.
Rutherford s Atomic Theory According to Rutherford s model, all of an atom s positive charge is concentrated in its nucleus.
Rutherford s Model Rutherford proposed a new model. The positive charge of an atom is not evenly spread throughout the atom. Positive charge is concentrated in a very small, central area. The nucleus of the atom is a dense, positively charged mass located in the center of the atom.
4.3: Modern Atomic Theory
Bohr s Model of the Atom Focuses on electrons and their arrangement. Bohr stated that electrons move with constant speed in fixed orbits around the nucleus, like planets around a sun. Bohr correctly assigned energy levels to electrons, but electrons do not move in fixed orbits around the nucleus.
What are Energy Levels? The possible energies that electrons in an atom can have are called energy levels. When an atom gains or loses energy, the energy of an electron can change. An electron in an atom can move from one energy level to another when the atom gains or loses energy. An electron cannot exist between energy levels.
Bohr s Model of the Atom Electron Nucleus
Electron Cloud Model The electron cloud model replaced Bohr's vision of electrons moving in predictable paths. An electron cloud is a visual model of the most likely locations for electrons in an atom. probability of finding an electron is higher in the denser regions of the cloud.
Electron Cloud Model An electron cloud is a good approximation of how electrons behave in their orbitals. An orbital is a region of space around the nucleus where an electron is likely to be found. The electron cloud represents all the orbitals in an atom.
Electron Cloud Model
Electron Cloud Model The level in which an electron has the least energy the lowest energy level has only one orbital. Higher energy levels have more than one orbital.
Electron Configuration An electron configuration is the arrangement of electrons in the orbitals of an atom. The most stable electron configuration is the one in which the electrons are in orbitals with the lowest possible energies.
We will draw Bohr Models to illustrate the number of electrons for an element.
Before drawing a Bohr Model Horizontal rows on the Periodic Table are called periods. The row number tells you how many energy levels, or rings, the element has. Elements in the same row have the same number of rings. The number of rings increases as you move DOWN the Periodic Table. Example: Sodium (Na) and Magnesium (Mg) are in the third row so they both will have three rings in their Bohr Model.
Horizontal Rows=Period=Number of Rings 1 2 Will not draw Bohr Models for Transition Metals 3 4 5 6
Before drawing a Bohr Model Vertical columns on the Periodic Table are called groups. Group numbers tell you how many valence electrons each element in that column has. Valence electrons are electrons that are located on the LAST energy level. The number of valence electrons increases as you move ACROSS the Periodic Table. Example: Sodium and Potassium are both in the 1 st column, so they both will have one valence electron in their last energy level. If the group number is higher than nine, so 10-18, drop the one to determine the number of valence electrons.
Vertical Columns=Groups=Number of Valence Electrons 1 2 3 4 5 6 7 2,8 Will not draw Bohr Models for Transition Metals
Drawing Bohr Models 1. Determine the number of rings, or energy levels. (Look at the period, or row, number.) 2. Determine the number of protons, neutrons, and electrons. 3. Determine the number of valence electrons. (Look at the group, or column, number.) 4. Draw the correct number of rings. 5. Draw the correct number of electrons on each ring, or energy level. (Use Energy Levels, Orbitals, and Electrons table.)