Atomic Theory

Atoms Matter is made of atoms Atoms are the basic building block of matter

History of the Atom 400 BC Democritus THE ATOM IS AN INDIVISIBLE PARTICLE the atom is the smallest particle of matter 1808 Dalton THE SOLID SPHERE MODEL atoms are solid , indestructible spheres (like billiard balls) 1897 J.J. Thompson THE RAISIN BUN MODEL atoms are solid spheres made up of solid positive mass with tiny negative particles embedded in the positive

History of the Atom 1911 Rutherford THE PLANETARY MODEL the atom is composed of a positive nucleus and negative electrons which surround the nucleus atom is mostly empty space Gold foil experiment

The Gold Foil Experiment by Rutherford Prediction In accordance to the J.J. Thomson model of an atom, the alpha particles should have passed directly through the gold foil for all instances. Observation most of the alpha particles passed straight through the foil, a small percentage of them were deflected at an angles and some were even backscattered. Conclusion two conclusions: one, an atom is mostly empty space and scattered electrons and two, an atom must have a positively charged centre that contains most of its mass

History of the Atom 1913 Niels Bohr electrons orbit the nucleus in different energy levels electrons can move from one level to another by gaining or losing packets of energy electrons are more stable when they are closer to the nucleus 1932 Sir James Chadwick the nucleus contains particles called neutrons and positively charge protons

History of the Atom

Structure of the Atom Consists of sub-atomic particles. protons neutrons electrons Protons and neutrons are located in the nucleus Electrons orbit the nucleus

Determining the number of subatomic particles Each element has an atomic number and a mass number Atomic number = no. of protons = no. of electrons Mass number = no. of protons + no. of neutrons Therefore, no. of neutrons = mass number – atomic number

Example: How many protons, neutrons and electrons does carbon have? Atomic number = 6 Mass number = 12 Number of protons: 6 Number of neutrons: 12-6 = 6 Number of electrons: 6

Atomic Symbol An international recognized system that allows anyone to commuicate information about the atom mass number A E chemical symbol Z atomic number

Isotopes and ions Isotope is an atom of an element with a different number of neutrons and hence a different mass number. An Ion is an atom that has lost or gained electron/s.

The Bohr Model Electrons move around the nucleus in fixed energy levels called shells. Shells close to the nucleus are lower in energy while shells farther away are high in energy. Shells are number outwards from the nucleus (1,2,3..) and are also lettered (K,L,M,N...)

Number of electrons in shells Different shells hold different numbers of electrons. The maximum number of electrons that can fit into a shell is 2n2 (n = shell number) Shell number (n) Representative letter Maximum number of electrons in this shell (2n2) 1 K 2 X 1 2 = 2 2 L 2 X 2 2 = 8 3 M 2 X 3 2 = 18 4 N 2 X 4 2 = 32

Drawing Atoms Bohr diagram number of protons and neutrons written in the center electrons drawn in circular orbits around the nucleus 10 P 10 N

Example: Chlorine Atomic number is 17. Mass number is 35. 17 protons and electrons 17 P 18 N Mass number is 35. 35 - 17 = 18 neutrons

Subshells Subshells are energy levels found within shells. There are four different types of subshells Named- s, p, d, f . Subshell energy: s<p<d<f subshell Number of electrons s 2 p 6 d 10 f 14

Shells and subshells

Electron configuration The way in which electrons are arranged around the nucleus of an atom is called the electron configuration of the atom.

Aufbau Principle Electrons occupy orbitals of lower energy first.

Order of electron filling Electrons fill shells and subshells of lowest energy first.

Write the full electronic configuration of: Aluminum – 13 electrons Calcium – 20 electrons Iron – 26 electrons Bromine – 35 electrons

Answers Aluminum – 13 electrons Calcium – 20 electrons 1s22s22p63s23p1 Calcium – 20 electrons 1s22s22p63s23p64s2 Iron – 26 electrons 1s22s22p63s23p64s23d6 Bromine – 35 electrons 1s22s22p63s23p64s23d104p5

ShellSubshell Orbital Orbital is the 3 dimensional space around a nucleus in which electrons are most likely to be found Each orbital can hold up to 2 electrons. Timberlake LecturePLUS 2000

Orbitals s subshell: spherical 1 orbital-2 electrons z p subshell: pair of lobes 3 orbital- 6electrons x y Shape represents electron density (not a path the electron follows)

d subshell: double dumpbells 5 orbital-10 electrons x y xy yz xz z2 x2-y2

Ground state and excited state At ground state the electron is at its lowest possible energy level (electrons are as close to the nucleus as possible). Example: Sodium (Z=11) Ground state electron configuration: 2, 8, 1

Evidence for the Bohr Model If atoms are given large amounts of energy (heat, light or electricity), the electrons can jump to an orbit further away from the nucleus. The atom is then in an excited state. Fluorine Ground state F: 1s2 2s2 2p5 Exited state F*: 1s2 2s1 2p5 3s1

Evidence for the Bohr Model When the electrons return to the ground state, they release this energy in the form of light. The light released has a specific wavelength corresponding to the difference between the energy levels.

Frequency A n = 3 Frequency B n = 2 Frequency C n = 1

Electromagnetic Energy

Evidence for the Bohr Model Emission Spectra are emissions of light from atoms that have been heated or provided with energy.

Evidence for the Bohr Model Emission Spectra are different for each material. Fe They can be used to determine the identity of an unknown sample of an element.

Hydrogen Helium Practise 1) Draw 2 Bohr-Rutherford diagrams for lithium; one in the ground state and one in the excited state. Make sure to include the energy put into the system, and energy released. 2) Explain why different elements have different emission line spectrums.

Practise Identify each of the unknown gases based on the emission line spectrums provided. The 4 possibilities are oxygen, hydrogen, helium and neon. A B C D