Atomic Structure

Output - processing

Input (Directed Content)

Self-study chapter History of the model of the atom All matter is made up of atoms. Up until the mid-1800s, most scientists thought that atoms were solid through and through. However, over the last 100 years, it has been discovered that atoms contain even sma­­ller particles inside! Bohr's model Proton -   1 positive electric charge (+1) -   Relative mass of 1 -   Symbol – p Neutron -   No electric charge -   Relative mass of 1 -   Symbol – n Electron -   1 negative electric charge (-1) -   Relative mass of 1/1840 (negligible) – not zero! -   Symbol – n Atoms… ü  are electrically neutral ü  have an equal number of positively charged protons & negatively charged electrons ü  have positive & electric charges that cancel out exactly                    --> Proton Number a.k.a. atomic number (number of protons in the atom)  o   Represented by Z o   Proton number = number of electrons o   *** ATOMS OF DIFFERENT ELEMENTS HAVE DIFFERENT NUMBER OF PROTONS. Each element has a unique proton number. --> Nucleon Number a.k.a. mass number (total number of protons & neutrons in an atom)  o   Represented by A o   The mass of an atom depends on the number of protons & neutrons in the atom’s nucleus. John Dalton –   o   was colour blind   o   first person to use the word “atom” after Democritus   o   his ideas on nature of atoms help to answer:      à Do atoms of an element have the same mass?          à Can atoms be created or destroyed?      à Do atoms of different elements have different masses?                                                                       Isotopes What are isotopes? Isotopes are atoms of the same element with the same number of protons but different number of neutrons. ·     The chemical properties of isotopes are similar because chemical reactions involve only the electrons and NOT the protons and neutrons. ·     Nucleon number of isotopes differ à relative masses of isotopes differ (physical properties differ), e.g. boiling point/density/melting point. Uses of Isotopes: Isotopes that emit high-energy radiation à radioisotopes à classified as radioactive substances à damage living cells & cause cancer [dangerous] à can be safely used & can have important applications if handled properly Electronic Configuration  Ø Atom’s nucleus is positively charged  Ø Surrounded by negatively charged electrons  Ø Nucleus is extremely tiny compared to the size of the atom  Ø Electron arrangement determines the chemical properties of the atom HOW ARE THE ELECTRONS ARRANGED? Ø Electrons move within regions known as electron shells FIRST SHELL: Ø Closest to nucleus Ø Maximum of 2 electrons Ø Filled first SECOND, THIRD, SUBSEQUENT SHELLS: Ø Maximum of 8 electrons Ø Filled in order VALENCE ELECTRONS: Ø Shell furthest from the nucleus is outer/valence shell (electrons in this shell à valence electrons) Ø Diagram of atom’s outer electronic structure only shows the valence electrons in the valence shell Ø Chemical properties of an atom depend on the number of valence electrons Periodic Table Ø Elements are arranged in increasing proton number Ø 7 horizontal rows of elements à periods Ø 8 vertical columns à groups

History of the model of the atom: 1. Democritus was the first one to establish that all matter was made of “atoms”. 2. Aristotle disagreed with Democritus on the idea that atoms existed. He did not have any atomic model since he thought that atoms did not even exist. 3. John Dalton’s ideas and findings contributed to a more advanced model of the atom. He came us with a theory that all matter is made of atoms, atoms cannot be created or destroyed, and atoms of different elements combine in a fixed ration to form chemical compounds. 4. J.J. Thompson discovered the negatively charged electrons in the atoms and said that the atom was mostly positive. 5. Ernest Rutherford felt that J.J. Thompson’s theory was wrong and hence, came up with a new theory that there was a nucleus in atom that was positive instead of the whole atom. - Rutherford’s gold foil experiment (assisted by Geiger and Marsden) proved that a small part of the atom was made of a positively charged nucleus.  - Groups must be presented, as Group I and Group II (not Group 1 and Group 2), in Roman numerals.