Electrons in Atoms

Electrons in Atoms: Additional Concepts Bohr Model of the Atom Why are elements’ atomic emission spectra discontinuous rather than continuous? Niels Bohr, a young Danish physicist working in Rutherford’s laboratory in 1913, proposed a quantum model for the hydrogen atom that seemed to answer this question. Topic 9 Topic 9

Energy states of hydrogen Bohr proposed that the hydrogen atom has only certain allowable energy states. Electrons in Atoms: Additional Concepts Topic 9 Topic 9 The lowest allowable energy state of an atom is called its ground state.

Energy states of hydrogen When an atom gains energy, it is said to be in an excited state. Electrons in Atoms: Additional Concepts Topic 9 Topic 9 And although a hydrogen atom contains only a single electron, it is capable of having many different excited states.

Energy states of hydrogen Bohr went even further with his atomic model by relating the hydrogen atom’s energy states to the motion of the electron within the atom. Bohr suggested that the single electron in a hydrogen atom moves around the nucleus in only certain allowed circular orbits. Electrons in Atoms: Additional Concepts Topic 9 Topic 9

Energy states of hydrogen The smaller the electron’s orbit, the lower the atom’s energy state, or energy level. Conversely, the larger the electron’s orbit, the higher the atom’s energy state, or energy level. Electrons in Atoms: Additional Concepts Topic 9 Topic 9

Energy states of hydrogen Bohr assigned a quantum number, n, to each orbit Electrons in Atoms: Additional Concepts Topic 9 Topic 9

Quantum Mechanical Model of the Atom Atomic Orbital 3-dimensional region around the nucleus of an atom that describes an electron’s probable location Electrons in Atoms: Additional Concepts Topic 9 Topic 9

Hydrogen’s Atomic Orbitals Recall that the Bohr atomic model assigns quantum numbers to electron orbits. In a similar manner, the quantum mechanical model assigns principal quantum numbers (n) that indicate the relative sizes and energies of atomic orbitals. Electrons in Atoms Topic 9 Topic 9

Hydrogen’s Atomic Orbitals That is, as n increases, the orbital becomes larger, the electron spends more time farther from the nucleus, and the atom’s energy level increases. Therefore, n specifies the atom’s major energy levels, called principal energy levels. Electrons in Atoms: Additional Concepts Topic 9 Topic 9

Hydrogen’s Atomic Orbitals An atom’s lowest principal energy level is assigned a principal quantum number of one. When the atom’s electron(s) occupy an orbital with n = 1, the atom is in its ground state. Electrons in Atoms: Additional Concepts Topic 9 Topic 9

Hydrogen’s Atomic Orbitals Principal energy levels contain energy sublevels. Principal energy level 1 consists of a single sublevel, principal energy level 2 consists of two sublevels, principal energy level 3 consists of three sublevels, and so on. Electrons in Atoms: Additional Concepts Topic 9 Topic 9

Hydrogen’s Atomic Orbitals Electrons in Atoms: Additional Concepts Topic 9 Topic 9 To better understand the relationship between the atom’s energy levels and sublevels, picture the seats in a wedge- shaped section of a theater.

Hydrogen’s Atomic Orbitals As you move away from the stage, the rows become higher and contain more seats. Similarly, the number of energy sublevels in a principal energy level increases as n increases. Electrons in Atoms: Additional Concepts Topic 9 Topic 9

Hydrogen’s Atomic Orbitals Sublevels are labeled s, p, d, or f according to the shapes of the atom’s orbitals. All s orbitals are spherical and all p orbitals are dumbbell shaped; however, not all d or f orbitals have the same shape. Electrons in Atoms: Additional Concepts Topic 9 Topic 9

Hydrogen’s Atomic Orbitals Each orbital may contain at most two electrons. The single sublevel in principal energy level 1 consists of a spherical orbital called the 1s orbital. Electrons in Atoms: Additional Concepts Topic 9 Topic 9

Hydrogen’s Atomic Orbitals The two sublevels in principal energy level 2 are designated 2s and 2p. The 2s sublevel consists of the 2s orbital, which is spherical like the 1s orbital but larger in size. Electrons in Atoms: Additional Concepts Topic 9 Topic 9

Hydrogen’s Atomic Orbitals The 2p sublevel consists of three dumbbell- shaped p orbitals of equal energy designated 2p, 2p, and 2p. Electrons in Atoms: Additional Concepts Topic 9 Topic 9

Hydrogen’s Atomic Orbitals Principal energy level 3 consists of three sublevels designated 3s, 3p, and 3d. Each d sublevel consists of five orbitals of equal energy. Four d orbitals have identical shapes but different orientations. However, the fifth, d orbital is shaped and oriented differently from the other four.

Periodic Sublevels

Sublevels

Atomic Orbitals 4 orbitals –s orbital sphere shaped- 2 electrons –p orbital dumbbell shaped, 2 electrons each – 6 electrons total –d orbital 2 electrons each – 10 total –f orbital 2 electrons each-14 electrons

Sublevel Order Pg. 138 Figure 5-19 Shows the order for filling orbitals Which is filled first, 4s or 3d? Which is filled first, 5f or 6d?

Electron Configuration The arrangement of electrons in an atom Write the electron configuration for: –Li –Al –Ag

Electrons in Atoms Continued Day 2

Electron Diagrams Principles and Rules

Aufbau Principle States that each electron occupies the lowest energy orbital available Describes the sequence in which orbitals are filled with electrons

Pauli Exclusion Principle States that a maximum of 2 electrons may occupy a single atomic orbital, but only if the electrons have opposite spins –Which is why you have to draw one arrow up and one arrow down symbolizing the opposite spins of the electrons

Hund’s Rule States that single electrons with the same spin must occupy each equal-energy orbital before additional electrons with opposite spins can occupy the same orbitals –Ex. Nitrogen and Chromium

Aufbau Diagrams Pg. 135

Aufbau Diagrams Shows the energy of each sublevel All orbitals related to an energy sublevel are equal energy –Ex. All 2p orbitals are of equal energy In a multi-electron atom, the energy of the sublevels within a principal energy level have different energies. –Ex. 2p orbitals are of higher energy than the 2s orbitals

Aufbau Diagrams In order of increasing energy, the sequence of energy sublevels within a principal energy level is s, p, d, and f. Orbitals related to energy sublevels within one principal energy level can overlap orbitals related to energy sublevels within another principal level. –Ex. 4s orbital has lower energy sublevel than the 5 orbitals related to the 3d sublevel

Aufbau Diagrams Name the following elements: –Li –Al –Ag Using the atomic number, find the number of electrons for the elements above: Place the electrons in the aufbau diagram

Electrons in Atoms Continued Day 3

Electron Configuration The arrangement of electrons in an atom Write the electron configuration for O on your white boards

Noble Gas Electron Configuration A method of representing electron configuration of noble gases using bracketed symbols –Ex. [Ne] represents the electron configuration for the noble gas Neon Electron configuration for an element can be represented using the noble-gas notation for the noble gas in the previous period and the electron configuration for the energy level being filled.

Electron Configuration Written in the order that the orbitals were filled What is the electron configuration for Se? –1s 2 2s 2 2p 6 3s 2 3p 6 4s 2 3d 10 4p 4 What is the noble gas configuration for Se? –[Ar]4s 2 3d 10 4p 4

Noble Gas Electron Configuration Write the noble gas electron configuration for: –Li –Al –Ag

Valence Electrons The electrons located in an atoms outermost orbitals or highest principal energy level Determine the chemical properties of an element How many valence electrons do the following elements have: –Se –Sr –Te –Ne

Electron Dot Structures Because valence electrons are involved in forming chemical bonds, chemist often represent them visually using a simple shorthand notation called electron-dot structures Devised by American chemist G. N. Lewis

Electron Dot Structures Consists of the element’s symbol surrounded by dots representing the atom’s valence electrons –The element’s symbol represents the atomic nucleus and inner-level electrons Dots representing the valence electrons are placed one at a time on the four sides of the element’s symbol and then paired up until you have accounted for all valence electrons

Electron Dot Structures Draw the electron dot structures for the following elements: –Se –Sr –Te –Ne

Electrons in Atoms Day 4

Element Foldable Element,Symbol, # of Electrons, # of Valence Electrons Aufbau Diagram, Electron Configuration, & Noble Gas Configuration Noble Gas Configuration Electron Dot Structure – Bromine –Strontium –Antimony –Rhenium –Terbium –Titanium –Silicon