2 Review of Atoms and Atomic Theory So far we have described the atom as a nucleus of protons and neutrons surrounded by electrons – this works well for a simple explanation, but does not explain certain properties of elements.For example, why do metals give off a characteristic color when heated in a flame?Answer: Because the electrons are giving off light energy as they return to a lower energy level. More about this later!!!
3 Dalton’s Atomic Theory, Review: All elements are composed of tiny indivisible particles called atomsAtoms of the same element are identical. The atoms of one element are different from atoms of a different elementAtoms of different elements can physically mix together, or chemically combinein simple whole-number ratios toform compoundsChemical reactions occur whenatoms are separated, joined orrearranged.
4 JJ ThompsonJJ Thompson (who discovered the electron) realized that Dalton’s model did not take into account electrons – so he proposed a revised model often called the plum-pudding modelThis had a central positively charged mass onto which electrons were just stuck to the outside.It did not describe the number of protons and electrons or how they were arranged.
5 Ernest RutherfordAfter discovering the nucleus, Ernest Rutherford proposed the nuclear atom, in which electrons surrounded a dense nucleus – leaving lots of empty spaceLater experiments showed a nucleus of + charged protons and neutrally charged neutrons, surrounded by – charged electrons.So why didn’t the – charged electrons fall into the + charged protons of the nucleus?
6 Bohr Model of Electrons In the early 1900’s, Niels Bohr, who was a student of Rutherford, came up with a new atomic theoryBohr proposed that electrons were in circular orbits around the nucleus, much like planets orbit the sun (thus it was called the planetary model)Bohr suggested that electrons in a particular orbit have a fixed energy, and that electrons do not lose energy and cannot fall into the nucleus.
7 Energy Levels of Electrons The energy level of an electron is the region around the nucleus where the electron is likely to be moving. These are also called electron shells.These energy levels are like the rungs on a ladder – just like you can notstand between rungs on aladder, electrons cannot bebetween energy levels.
8 Quantums of EnergyA quantum of energy is the amount of energy needed to move an electron from its present energy level to the next high level.The term quantum leap comes from describing this sudden change fromone level to the next.The higher the energylevel, the farther theelectron typically isfrom the nucleus.
9 The Quantum Mechanical Model (The Cloud Model) Finally, in 1926 Erwin Schrödinger took the atom to its current shapeThe modern description of the electrons in an atom is called the quantum mechanical model and is based on the mathematical equations from Schrödinger.
10 The Cloud ModelThe Quantum Mechanical Model (also called the Cloud Model) does not describe electrons in an orbit like the planets.The probability of finding an electron within a certain volume of space surrounding the nucleus is represented as fuzzy cloud, which represents where an electron would be found 90% of the time.
11 Atomic OrbitalsIn the Bohr model we had orbits, but since the quantum mechanical model does not have orbits, but rather regions of probability, the regions are called atomic orbitals. (They may also be calledelectron clouds.)
12 Principle Quantum Number-the main Energy Level occupied by an electron The main energy level of an electron (or shell) is designated by means of a principle quantum number (n)This is assigned in order of increasing energy levels: (shells) n =1,2,3,4,5,. . .Within each shell,there are sublevels. Thenumber of sublevels foundwill be the same as thevalue of n.
13 Where will the electrons be found? In each shell, electrons will be found in regions of probability called atomic orbitals.Each orbital is given a letter symbol: S, P, D, or F: primarily based on shapeEach orbital can holdtwo electrons.
14 S, P, D, or F: Shapes of Orbitals S orbitals are sphericalP orbitals are shaped like dumbbells (or peanuts): there are 3 of these: x, y, and zD orbitals are like two sets of dumbbells at right angles to each otherF orbitals are too complex to visualize
15 Sublevel 1: 1sFor the first shell (n =1), there is one sublevel called 1s.In the s atomic orbital, there is an equal probability of finding anelectron in any directionfrom the nucleusS=spherical
16 Sublevel 2: 2s and 2pFor the second shell (n =2), there are two sublevels: 2s and 2pThe 2s orbital is spherical, while the 2p orbitals are dumbbell shapeThe 2p orbital is of higher energy level than the 2s orbitalThe 2p orbital consists of three p orbitals, each aligned on the x, y & z axisThus the second energy level has four orbitals 2s, 2z, 2y, 2z
17 Sublevel 3: 3s, 3p, and 3dFor the third shell (n=3), there are 3 sublevels: 3s, 3p and 3dAs before, there is one 3s orbital and three 3p orbitals – but 3d has 5 orbitals, for a total of 9 orbitalsHow many electronscan Sublevel 3 hold?
18 Sublevel 4: 4s, 4p, 4d, and 4fFor the fourth shell (n=4), there are 4 sublevels: 4s, 4p, 4d and 4fThe 4th principle energy level has 16 orbitals: One 4s, three 4 p, five 4d and seven 4f orbitalsHow many electronscan the 4th sublevelhold?
19 Electron Configuration The arrangement of electrons in an atom is usually shown by writing an electron configuration.Example: Hydrogen (AtomicNumber 1) is 1s1Helium (Atomic Number 2) is 1s2Neon (Atomic Number 10) is 1s2 2s2 2p6
20 Rule 1: Aufbau Principle Aufbau Principle:Electrons always fill orbitals of lower energy first. 1s is filled before 2s, and 2s before 2p.Electrons are placed in increasing energy:1s<2s<2p<3s<3p<4s<3d<4p<5s<4d<5p<6s<4f<5d<6p<7s<5f<6d<7p
22 Rule 2: Pauli Exclusion Principle This discovery was made in 1925 by German chemist Wolfgang Pauli →No two electrons within a particular atom can have identical quantum numbers. If two electrons do occupy the same orbital, they must have opposite spin.
23 Rule 3: Hund’s RuleWhen an electron joins an atom and has to choose between two or more orbitals of the same energy, the electron will prefer to enter an empty orbital rather than one already occupied.As more electrons are addedto the atom, electrons willhalf-fill orbitals of the sameenergy before pairing withexisting electrons to fill orbitals.
24 Practice:Writing out Electron Configuration of Silicon Step AStep BStep CStep DStep E1s22s2p63s23p2
25 Now, You Practice!Write out the electron configuration of Potassium: