Chapter 5 Models of the Atom by Christopher Hamaker Chapter 5 © 2011 Pearson Education, Inc.

Dalton Model of the Atom John Dalton proposed that all matter is made up of tiny particles. These particles are molecules or atoms. Molecules can be broken down into atoms by chemical processes. Atoms cannot be broken down by chemical or physical processes. © 2011 Pearson Education, Inc. Chapter 5

Subatomic Particles Three subatomic particles were discovered. Negatively charged electrons, e–. Positively charged protons, p+. neutrons, n0, with no charge An electron has a relative charge of -1, and a proton has a relative charge of +1. A neutron carries zero charge © 2011 Pearson Education, Inc. Chapter 5

Mass of Subatomic Particles An electron has a mass of 9.11 × 10-28 g. A proton has a mass of 1.67 × 10-24 g. © 2011 Pearson Education, Inc. Chapter 5

Types of Radiation There are three types of radiation: Alpha (a) Beta (b) Gamma (g) Alpha rays are composed of helium atoms stripped of their electrons (helium nuclei). Beta rays are composed of electrons. Gamma rays are high-energy electromagnetic radiation. © 2011 Pearson Education, Inc. Chapter 5

Atomic Notation Each element has a characteristic number of protons in the nucleus. This is the atomic number, Z. The total number of protons and neutrons in the nucleus of an atom is the mass number, A. We use atomic notation to display the number of protons and neutrons in the nucleus of an atom: Sy symbol of the element mass number (p+ and n0) atomic number (p+) A Z © 2011 Pearson Education, Inc. Chapter 5

Using Atomic Notation An example: Si The element is silicon (symbol Si). The atomic number is 14; silicon has 14 protons. The mass number is 29; the atom of silicon has 29 protons + neutrons. The number of neutrons is A – Z = 29 – 14 = 15 neutrons. 29 14 © 2011 Pearson Education, Inc. Chapter 5

Isotopes All atoms of the same element have the same number of protons. Most elements occur naturally with varying numbers of neutrons. Atoms of the same element that have a different number of neutrons in the nucleus are called isotopes. Isotopes have the same atomic number, but different mass numbers. © 2011 Pearson Education, Inc. Chapter 5

Isotopes, Continued Co C We often refer to an isotope by stating the name of the element followed by the mass number. Cobalt-60 is Carbon-14 is How many protons and neutrons does an atom of lead-206 have? The atomic number of Pb is 82, so it has 82 protons. Pb-206 has 206 – 82 = 124 neutrons. Co 60 37 C 14 6 © 2011 Pearson Education, Inc. Chapter 5

Isotopes Percent Abundance: Percent of atoms in a natural sample of a pure element that are a particular isotope of the element. Distribution of isotopes in any particular sample is generally constant. Is used to determine an atomic mass unit (amu) 23 elements have only one naturally occurring form. Most naturally occurring elements are mixtures of isotopes.

Simple and Weighted Averages A simple average assumes the same number of each object. A weighted average takes into account the fact that we do not have equal numbers of all the objects. A weighted average is calculated by multiplying the percentage of the object (as a decimal number) by its mass for each object and adding the numbers together. © 2011 Pearson Education, Inc. Chapter 5

Average Atomic Mass Since not all isotopes of an atom are present in equal proportions, we must use the weighted average. Copper has two isotopes: 63Cu, with a mass of 62.930 amu and 69.09% abundance. 65Cu, with a mass of 64.928 amu and 30.91% abundance. The average atomic mass of copper is: (62.930 amu)(0.6909) + (64.928 amu)(0.3091) = 63.55 amu © 2011 Pearson Education, Inc. Chapter 5

Periodic Table We can use the periodic table to obtain the atomic number and atomic mass of an element. The periodic table shows the atomic number, symbol, and atomic mass for each element. © 2011 Pearson Education, Inc. Chapter 5

Electrons Remember: Electrons are the smallest of the three subatomic particles we will deal with. They have very little mass. They reside in the electron cloud that surrounds the nucleus. Their movement rapidly about the nucleus defines the size of the atom

Electrons Electrons are the high energy part of the atom. The faster the electron moves the more energy it has and the farther from the nucleus it tends to travel. Electrons have spin.

Electrons Electron shell – a defined region of space about a nucleus that contains electrons with approximately the same energy. Shell number – ‘n’ used to identify the electron shell. These are numbered 1-7 Electrons in higher number shells have more energy.

Electrons How many electrons in an electron shell? Depends – Not all shells are equal. Lower shells have less electrons, less energy. Number of electrons in a shell follows the rule 2n2 where ‘n’ is the electron shell level. Lower level shells fill before higher ones. So how many electrons are in shell 3? 18

Electrons – Subshells and Orbitals Within a shell there are subshells and orbitals. A subshell ( or sublevel) is defined region of space within an electron shell that contain electrons of the same energy. These sublevels are given the designations s, p, d, and f. Number of subshells is equal to the number ‘n’ where ‘n’ is the shell number. Subshells are written with a number, ‘n’, and a letter. The superscript represents the number of electrons in the subshell. Ex: 1s2

Electron Shells and Subshells

Energy Levels and Sublevels, Continued The first energy level has one sublevel designated 1s. The second energy level has two sublevels designated 2s and 2p. The third energy level has three sublevels designated 3s, 3p, and 3d. © 2011 Pearson Education, Inc. Chapter 5

Electrons – Subshells and Orbitals Subshells are written with a number, ‘n’, and a letter, s, p, d, f (always in lower case). Number of electrons per subshell is defined and independent of shell number. s = 2 p = 6 d = 10 f = 14

Electron Occupancy in Sublevels The maximum number of electrons in each of the energy sublevels depends on the sublevel: The s sublevel holds a maximum of 2 electrons. The p sublevel holds a maximum of 6 electrons. The d sublevel holds a maximum of 10 electrons. The f sublevel holds a maximum of 14 electrons. The maximum electrons per level is obtained by adding the maximum number of electrons in each sublevel. © 2011 Pearson Education, Inc. Chapter 5

Electrons per Energy Level © 2011 Pearson Education, Inc. Chapter 5

Electrons – Subshells and Orbitals Electron orbital – a region of space within an electron subshell where an electron with a specific energy is most likely to be found. s = 1 p = 3 d = 5 f = 7 Each orbital can hold two electrons.

Quantum Mechanical Model An orbital is the region of space where there is a high probability of finding an atom. In the quantum mechanical atom, orbitals are arranged according to their size and shape. The higher the energy of an orbital, the larger its size. All s orbitals have spherical shapes. © 2011 Pearson Education, Inc. Chapter 5

Electron Configurations Electrons are arranged about the nucleus in a regular manner. The first electrons fill the energy sublevel closest to the nucleus. Electrons continue filling each sublevel until it is full, and then start filling the next closest sublevel. A partial list of sublevels in order of increasing energy is as follows: 1s < 2s < 2p < 3s < 3p < 4s < 3d < 4p < 5s < 4d … © 2011 Pearson Education, Inc. Chapter 5

Filling Electron Shells Electron shells fill according to the Aufbau Principle. Electrons normally occupy electron subshells in an atom in order of increasing subshell energy. Energy of subshells can overlap

Filling Diagram for Energy Sublevels The order does not strictly follow 1, 2, 3, etc. For now, use Figure 5.16 to predict the order of sublevel filling. © 2011 Pearson Education, Inc. Chapter 5

Electron Configurations The electron configuration of an atom is a shorthand method of writing the location of electrons by sublevel. The sublevel is written followed by a superscript with the number of electrons in the sublevel. For example, if the 2p sublevel contains two electrons, it is written 2p2. The electron sublevels are arranged according to increasing energy. © 2011 Pearson Education, Inc. Chapter 5

Writing Electron Configurations First, determine how many electrons are in the atom. Bromine has 35 electrons. Arrange the energy sublevels according to increasing energy: 1s 2s 2p 3s 3p 4s 3d … Fill each sublevel with electrons until you have used all the electrons in the atom: Fe: 1s2 2s2 2p6 3s2 3p6 4s2 3d 10 4p5 The sum of the superscripts equals the atomic number of bromine (35). © 2011 Pearson Education, Inc. Chapter 5

Writing Electron Configuration While this may seem tedious, you can learn a lot from the written electron configuration. This is a shorthand notation designating the subshells in an atom that are occupied by electrons. Start at the beginning and keep filling subshells until the correct number of electron is represented:

Remember the nuclear notation and ‘z’ is the number of protons which equals the number of electrons in a neutral atom: Element z Electron Configuration Hydrogen 1 1s1 Helium 2 1s2 Carbon 6 1s22s22p2 Neon 10 1s22s22p6 Sodium 11 1s22s22p63s1

Shapes of p Orbitals Recall that there are three different p sublevels. All p orbitals have dumbbell shapes. Each of the p orbitals has the same shape, but each is oriented along a different axis in space. © 2011 Pearson Education, Inc. Chapter 5

Shapes of d Orbitals Recall that there are five different d sublevels. Four of the d orbitals have a clover-leaf shape and one has a dumbbell and doughnut shape. © 2011 Pearson Education, Inc. Chapter 5

Chapter Summary Atoms are composed of protons, neutrons, and electrons. The protons and neutrons are located in the nucleus, and the electrons are outside the nucleus. Atoms are mostly empty space. The number of protons is referred to as the atomic number for the atom. © 2011 Pearson Education, Inc. Chapter 5

Chapter Summary, Continued All atoms of the same element have the same number of protons. Isotopes are atoms with the same number of protons, but differing numbers of neutrons. The mass number for an isotope is the total number of protons plus neutrons. © 2011 Pearson Education, Inc. Chapter 5

Chapter Summary, Continued The atomic mass of an element is the weighted average of the masses of all the naturally occurring isotopes. Electrons exist around the nucleus of atoms in discrete, quantized energy levels. Electrons fill energy sublevels, starting with the lowest energy sublevel and filling each successive level of higher energy. © 2011 Pearson Education, Inc. Chapter 5