Atoms and the Periodic Table

Atom Nucleus located in center of atom is small, dense and positively charged. Contains protons and neutrons Region outside the nucleus contains electrons

Sub-Atomic Particles ParticleMass Charge Location (amu) Proton Nucleus Electron 1/ Outside nucleus Neutron 1 0 Nucleus

Atomic number (Z) is the number of protons in an atom The number of protons determines the identity of the element Mass number (A) is the total number of protons and neutrons in an atom

Atom X Mass numberCharge AC X Z Atomic number X is atomic symbol

An atom has 47 protons, 61 neutrons and 47 electrons. What is its atomic number? What is its mass number? What is its charge? What element is it? What is its atomic mass?

Ag Mass numberCharge 47p + 61n p e - Ag 47 p 47 Atomic number Ag atomic symbol

31 X 15 atomic number mass number number of protons number of electrons number of neutrons element symbol

31 X 15 atomic number15 mass number31 number of protons15 number of electrons15 number of neutrons31-15 = 16 element symbolP

Isotopes Isotopes are atoms of the same element that have different numbers of neutrons Isotopes have different masses Isotopes of an element have the same chemical properties.

Isotope Protons Neutrons Mass # Name 12 C6 612 Carbon C Carbon C Carbon-14

C-14 is radioactive. Radioactive isotopes have the same chemical properties as other isotopes of the same element

Atomic Mass of an Atom Each proton and neutron has a mass of approximately 1 amu For C-13, mass would be: 6 protons x 1amu/proton=6 amu 7 neutrons x 1 amu/neutron=7 amu Mass of C-13 atom= 13 amu

Atomic Mass Unit (amu) Atomic Mass Unit is defined as 1/12 the mass of the 12 C (C-12) isotope of carbon

Atomic Mass of Element Often called atomic weight Atomic mass of an element is weighted average of the masses of each isotope Need to know abundance of each isotope to calculate atomic mass

Chlorine occurs as two isotopes Cl-35 and Cl % Cl-35 (or ) 24.23% Cl-37 (or ) x 35 amu= amu x 37 amu= amu Cl Atomic mass = amu

Calculate the atomic mass of Imaginarium 90 % occurs as Im % occurs as Im-210

Calculate the atomic mass of Imaginarium 90 % occurs as Im % occurs as Im x 200 amu = 180 amu 0.10 x 210 amu= 21 amu mass of “average” 201 amu atom

Atom Neutral atom has equal number of protons and electrons + charges = - charges Overall charge of zero (neutral)

Ions Atom can gain or lose electrons to form ion Gain electrons → negative ion (anion) + charges < - charges → - ion Lose electrons → positive ion (cation) + charges > - charges → + ion

Atoms are so small that we cannot easily measure distances Can measure energy (in form of light): Spectroscopy Can think of entire electromagnetic spectrum as if it were light

Electromagnetic Radiation Travels at speed of light: 3.0 x 10 8 m/s Travels as waves --- Wavelength (λ) and Energy inversely related Small λ -- High energy Long λ -- Low energy Spectrum can be divided into regions based on wavelength and energy

The Wave Nature of Light All waves have a characteristic wavelength,, and amplitude, A. The frequency,, of a wave is the number of cycles which pass a point in one second. The speed of a wave, v, is given by its frequency multiplied by its wavelength: v = For light, speed = c.

Fig. 2.10

Emission Spectrum Pass electricity through vacuum tube that has gas phase atoms Specific wavelengths of light are emitted Each element emits characteristic wavelengths

Visible light spectrum is continuous Emission spectrum is discontinuous Only certain bands seen (allowed energy amounts – quanta)

Fig. 2.11

Bohr Atom Electrons are found in “orbits” around the nucleus Each “orbit” is called a principal energy level (n) --- or shell The higher the energy level, the farther out from the nucleus it is The higher the energy level, the greater the energy of the electrons in it

Bohr Atom Electrons can only have certain allowed amounts of energy Quantum: a discrete amount of energy Photon: a packet of energy (light)

Ground state: Electrons in lowest possible energy level Excited state: Electron has absorbed energy and been promoted to a higher energy level (farther from nucleus) Relaxation occurs when electron emits energy and goes into a lower energy level (closer to nucleus) Only specific amounts of energy can be absorbed or emitted (quantum of energy)

If energy emitted corresponds to a visible wavelength, we see it as a colored line in the emission spectrum The pattern is unique for each element Bohr’s model worked for Hydrogen, but not for more complicated atoms (more e - )

Fig. 2.11

Nered Arblue Hgblue Nayellow

Principal Energy Level Element# e H11 He22 Li32 1 Be42 2 C62 4 F92 7 Ne102 8 Na Mg Cl Ar K