Atoms and the Periodic Table Chapter Three Subatomic Particles Atoms are composed of subatomic particles Particle Symbol Mass (g) Mass (amu) Charge Proton p 1.672622 x 10-24 1.007276 +1 Neutron n 1.674927 x 10-24 1.008665 0 Electron e 9.109328 x 10-28 5.485799 x 10-4 -1
The Structure of an Atom Attractive and Repulsive Forces in Atomic Structure
Structure of the Atom Protons and neutrons reside in a small, dense nucleus. Electrons are spread out in space outside the nucleus. Most of the atom is empty space. Atoms are neutral overall, i.e., no net charge Atomic Number (Z) - The number of protons in the nucleus of an atom. Determines the type of atom (which element it is) Also number of electrons in a neutral atom Mass Number (A) - The sum of the protons plus neutrons in the nucleus of an atom. Therefore, # of neutrons = A - Z Example Problem 3.2 If Phosphorus has a mass number of A = 31, how many protons, neutrons and electrons are there?
Atoms and Ions An atom is electrically neutral. #e = #p #e = Z Ion - An atom or other chemical species that carries an overall positive or negative charge. Monatomic ion - a single atom with a positive or negative charge Cation - a positive ion (#e < #p) Anion - a negative ion (#e > #p) Isotopes Isotopes- Atoms of the same element that have different mass numbers - Same number of protons - Different number of neutrons
How to Indicate Isotopes Nuclide - A nucleus with a particular mass number Symbols of Nuclides Write the symbol (Sy) of the element. Put the atomic number (Z) at the lower left. Put the mass number (A) at the upper left. A Z Sy Sample Isotope Problem 12 C 13 C 14 C protons neutrons electrons amu
Atomic Weight (Mass) Atomic Weight (Mass) - The average mass of all of the isotopes of an element that occur in nature. Carbon-12 is assigned a mass of exactly 12 amu. Standard At. Wt. = fract. abund 1 x mass 1 + fract. abund 2 x mass 2 + fract. abund 3 x mass 3 +. Atomic Weight of Magnesium Isotope Approximate Mass (amu) Abundance 24 Mg 23.99 78.99% 25 Mg 24.99 10.00% 26 Mg 25.93 11.01%
The Periodic Table Periodic Table - A chart in which elements are arranged in order of increasing atomic number such that elements having similar chemical properties lie beneath one another. The atomic number (Z), the symbol, and the atomic mass of each element is given. The Periodic Table Groups(Families) - The columns in the table Periods - The rows in the table Groups can be designated by numbers or names Representative Elements (Main Group Elements) Elements of groups 1,2 and 13-18 (old 1A, 2A and 3A - 8A) Alkali metals (1A), Alkaline earth metals (2A), Halogens (7A), Noble gases (8A) Transition metals Elements of groups 3-12 (old 3B - 8B and 1B & 2B)
The Periodic Table The Periodic Table Atomic Radius versus Atomic Number
Periodic Properties Atomic Size Decreases in going across a period Increases in going down a group Ionization Energy Energy required to remove an electron from an atom Increases in going across a period Decreases in going down a group Electron Affinity Energy released when an electron is added to an atom Increases in going across a period Generally decreases in going down a group (some exceptions) Properties of Electrons All moving objects have properties of both particles and waves - If the particle is large, e.g., a soccer ball, the associated wave is not a measurable property of the object Electrons are sufficiently small that they can be described as both particles and waves - wave-particle duality Current theories of the electronic structure of atoms are based on the wave properties of the electron
Quantum Mechanical Model of Atomic Structure Electrons are not perfectly free to move about in an atom The energy of the electron is quantized Quantized- only certain specific values are permitted - Steps versus ramp analogy These values of energy are called energy levels - Electrons prefer to reside in the lowest possible energy level Electrons in an atom that are in the same energy level are at about the same average distance from the nucleus - Electrons that are closer to the nucleus have lower energies because they are attracted mores strongly by the nucleus Location of An Electron Shells- like layers of an onion, as you progress from the center the larger it is, thus it can hold more electrons Subshells- each energy shell is further sub-divided into energy sublevels where the number of sublevels is equal to n which is the shell number - subshells (energy sublevels) are designated by letters - subshells increase in energy in the order of: s < p < d < f Oribtals- regions in space within an atom where specific electrons are most likely to be found
Orbitals The number of orbitals in a subshell increases - s subshells are composed of 1 orbital - p subshells are composed of 3 orbitals - d subshells are composed of 5 orbitals - f subshells are composed of 7 orbitals Each orbital can contain a maximum of 2 electrons of opposite spin - s subshells contain 2 electrons - p subshells contain 6 electrons - d subshells contain 10 electrons - f subshells contain 14 electrons Shapes of s and p Orbitals
Summary of Electron Distribution in Atoms Energy Perspective of Electron Distribution in Atoms
Electron Configurations Electron configurations- the exact arrangement of electrons in the shells and subshells of atoms 3 Rules apply for placement of the electrons - Rule 1: Electrons occupy the lowest-energy orbitals available and are filled in the order shown in Fig 3.5 - Rule 2: Each orbital can hold only 2 electrons which must be of opposite spin - Rule 3: Two or more orbitals of the same energy (i.e., p,d,f subshells) are each half filled by 1 electron with the same spin before any one oribtal is filled by the second electron of opposite spin Order of Filling Electron Orbitals Note that above the 3p level there is a crossover of energy levels, thus 4s orbital must filled before 3d orbitals, 5s orbital before 4d orbitals
Written Graphical Representation of Electron Configurations Electron Configuration and the Periodic Table Relationship between rows (periods, shells) and columns (groups, subshells) Main group elements (s or p) Transition group elements (d or f)
Valence Shell Electrons Valence electrons - electrons found in the outermost shell (valence shell) Elements in the same group (column) of the periodic table have similar electron configurations in their valence shell - Group 1 (1A) : ns 1 - Group 2 (2a): ns 2 - Group 13 (3A): ns 2 np 1 - Group 17 (7A):ns2 np 5 - Group 18 (8A): ns 2 np 6 Where n is the shell number