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13/11/11 The nature of the atom 2 © Zanichelli editore 2016 2 2

13/11/11 A model of the atom The atom contains a very dense nucleus, formed by protons and neutrons, surrounded by moving electrons. Quantum mechanics and the atomic orbital model are used to describe the behavior of electrons in atoms. p+ n e– 3 © Zanichelli editore 2016 3 3

The uncertainty principle 13/11/11 The uncertainty principle According to the uncertainty principle, it is impossible to precisely measure the simultaneous position and speed of a small particle like an electron. In fact, it is impossible to study the electron without giving energy, thus altering its speed or its position. 4 © Zanichelli editore 2016 4 4

13/11/11 Energy levels Electrons can have only specific energy states, called energy levels. They can never have intermediate energy states between two levels. Excited states Ground state 5 © Zanichelli editore 2016 5 5

The atomic orbital model /1 13/11/11 The atomic orbital model /1 The atomic orbital represents a possible energy state of the electron and refers to the physical region where there is a 90% probability of locating the electron. Each atomic orbital is characterized by three quantum numbers. The principal quantum number (n) defines the energy level of the orbital. It can only be an integer, positive number: 1, 2, 3… All electrons with the same value of n form the energy level. 6 © Zanichelli editore 2016 6 6

The atomic orbital model /2 13/11/11 The atomic orbital model /2 The secondary quantum number (l) describes the shape of the atomic orbital and its energy sublevel. It can only be an integer, positive number, from 0 to (n–1). Value of l Letter Shape s sphere 1 p dumbbell shape 2 d double dumbbell shape Numero Immagine 176273831 7 © Zanichelli editore 2016 7 7

The atomic orbital model /3 13/11/11 The atomic orbital model /3 The magnetic quantum number (m) describes the different spacial orientations of orbitals that have the same l value. It can be an integer number from –l to +l (including 0). Value of n Value of l Value of m Sublevel Number of orbitals 1 1 s 2 –1, 0, 1 2 s 2 p 3 –2, –1, 0, 1, 2 3 s 3 p 3 d 5 8 © Zanichelli editore 2016 8 8

The orbital energy diagram 13/11/11 The orbital energy diagram In this diagram, orbitals are displaced according to an increase in energy. 4 p 3 d 4 s Energy 3 p 3 s 2 p 2 s 1 s 9 © Zanichelli editore 2016 9 9

The spin quantum number 13/11/11 The spin quantum number Each electron is associated with a a spin quantum number that can have a value of +½ and –½. According to the Pauli exclusion principle, it is impossible to have electrons with the same quantum numbers in an atom. Each orbital can therefore have a maximum of two electrons with antiparallel spin. 10 © Zanichelli editore 2016 10 10

Electron configurations 13/11/11 Electron configurations In order to write the electron configuration of an atom, electrons are displaced in the orbital energy diagram starting from the orbital with lower energy. C 1s2 2s2 2p2 2p 2s 1s 11 © Zanichelli editore 2016 11 11

13/11/11 Valence electrons The electrons in the outermost orbital are called valence electrons. All the other electrons represent the core. The distribution of electrons in the most external orbital is called the external electronic configuration. The elements that belong to a chemical group have the same external electronic configuration. valence electron 12 © Zanichelli editore 2016 12 12

13/11/11 The size of atoms The atomic radius is the average distance between a nucleus and the outermost electrons. It is half the distance between two equal atoms linked together. 2 r In the periodic table, the atomic radius usually decreases in periods from left to right and increases in groups from top to bottom. 13 © Zanichelli editore 2016 13 13

13/11/11 Ionization energy The ionization energy is the energy required to remove an electron from a neutral gaseous atom. It is expressed in kJ/mol. In the periodic table, these values increase along each period and decrease along each group. 14 © Zanichelli editore 2016 14 14

13/11/11 The electron affinity The electron affinity is the energy that is emitted or absorbed when an electron is added to a neutral, gaseous atom, that becomes a negative ion. As the same as ionization energy, in the periodic table these values increase along each period and decrease along each group. 15 © Zanichelli editore 2016 15 15

13/11/11 From atoms to ions The electron configuration of atoms of noble gases is particularly stable, making them inert with respect to chemical reactivity. Metals easily form cations with the electron configuration of the nobel gas preceding them in the periodic table. Non-metals easily form anions with the electron configuration of the noble gas proceding them in the periodic table. 16 © Zanichelli editore 2016 16 16