Matter Is Made Up Of Atoms Chapter 4 Structure of the Atom An atom is the smallest particles of matter that retain the matter’s properties. Atoms have TWO PARTS: • A NUCLEUS • An ELECTRON CLOUD A Model of the Atom The Nucleus of the Atom The NUCLEUS of the atom is the SMALL, DENSE, SOLID central region of the atom. The NUCLEUS is made up of TWO smaller particles [sub-atomic particles] called: PROTONS and NEUTRONS PROTONS PROTONS are small sub-atomic particles that have a POSITIVE ELECTRICAL CHARGE. P+ Protons are responsible for the identity of an element. Represented by the symbol The ATOMIC NUMBER The number of PROTONS in an atom’s nucleus is called the ATOMIC NUMBER of the atom. Atoms can have from 1 to 109 protons, depending on which element is being referenced. The number of PROTONS determines which element an atom belongs to. NEUTRONS NEUTRONS are the other subatomic particle found in the atom’s nucleus. NEUTRONS are NEUTRAL; they have NO ELECTRICAL CHARGE. The symbol for a NEUTRON is: o N The ATOMIC MASS NUMBER The ATOMIC MASS NUMBER is the COMBINED NUMBER of PROTONS and NEUTRONS in the atom’s nucleus. Together, the protons and neutrons are responsible for 99.999% of the atom’s total mass. Look at a Periodic Table. Now look at the ATOMIC MASS NUMBERS for the elements. What do you notice about these numbers? THEY CONTAIN DECIMALS How atomic mass numbers get their decimals. We learned earlier that the AMN is the total of an atoms protons and neutrons combined. How can this give a number with a decimal? While ALL atoms of an element have the SAME NUMBER of PROTONS they don’t all have the same number of NEUTRONS…. The number of NEUTRONS can vary between atoms of the same element. ISOTOPES Atoms of an element that vary in AMN’s due to differing numbers of NEUTRONS are called ISOTOPES. The ATOMIC MASS NUMBER reported for an element on the PERIODIC TABLE is an AVERAGE of all the different ISOTOPES of that element. Writing Isotopes Isotopes can be written two ways: 1. Chemical Symbol and AMN -more simple Example C-14 2. Chemical Symbol with AN and AMN -More detailed Example 14 C 6 The Electron Cloud Unlike the NUCLEUS, the ELECTRON CLOUD: Is very large [about 10,000x the size of the nucleus]. Is NOT solid; it is an area of EMPTY SPACE surrounding the nucleus. Contains only one subatomic particle. The ELECTRON The ELECTRON is the smallest of the subatomic particles. It takes 1834 electrons to have about the same mass as ONE proton! e- The symbol for an electron is: Neutral atoms have as many electrons as they do protons! • The ATOMIC NUMBER is also the number of electrons in the Electron Cloud. Divisions of the Electron Cloud The Electron Cloud is divided into 7 main regions called PRINCIPLE ENERGY LEVELS. The PEL’s move outward away from the nucleus…1 is closest….7 is farthest away! Each PEL can hold a certain number of electrons. PEL 1 = 2 ePEL 2 = 8 ePEL 3 = 18 ePEL 4 = 32 ePEL 5 = 32 ePEL 6 = 32 ePEL 7 = 32 e- Sublevels Each PEL can further be divided into smaller divisions called SUBLEVELS. The number of sublevels in a PEL depends on it’s distance from the nucleus. PEL 1 has only 1 sublevel, called S PEL 2 has two sublevels: S and P PEL 3 has three sublevels: S, P and D PEL 4-7 has four sublevels: S, P, D and F S sublevels are spherical and hold a max of 2 e-. P sublevels are figure 8-like and hold a max of 6e-. D sublevels are “mushroom” shaped and lie along the X, Y, Z plane. D sublevels hold a max of 10 e-. F sublevels assume different shapes along the X, Y, Z plane and hold a max of 14 e-. Sublevel Filling Patterns Because the energy required to maintain the orbit of the more complex sublevels is more than that of more simple orbits, a sublevel will not completely fill before the next higher one begins receiving electron. AUFBAU Electron Distribution Chart 1s 2s 3s 4s 5s 6s 7s 2p(10) 3p(18)3d 4p(36)4d 5p(54)5d 6p(86) 6d 7p 7d [Ne =10] [Ar=18] 4f 5f 6f 7f [Kr=36] [Xe=54] [Rn=86] Orbital Notation Orbital Notation is used to illustrated the distribution of e- within the e- cloud. Number of e- in sublevels Example Mg – 1s2 2s2 2p6 3s2 Symbol Sublevels Practice Problems Write out the electron configuration of: 1. Oxygen 2. Calcium 3. Iodine Shorthand e- Configurations Locate the NOBLE GAS immediately before the element. Write the noble gas’ symbol in a bracket example [ Xe ] Subtract the AN of the noble gas from the element, then distribute the remaining electrons beginning at the next sublevel. Practice Write the shorthand configuration for Barium. nearest noble gas is Xe with 54 e-. Xe fills the sublevels through 5p. 56 – 54 = 2 e- to distribute beginning at 6s. Ba = 56 e- Ba [ Xe ] 6s2 Practice Write shorthand configurations for: • A. Arsenic • B. Iodine The most important PEL is the outermost one of the atom. The electrons on this level are involved in forming chemical bonds with other atoms. The electrons in the outermost PEL are known as VALENCE ELECTRONS, or Ve- for short.