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

Particle Mass Charge Location

(amu)

Proton 1 + 1 Nucleus

Electron 1/1835 - 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 number Charge A C

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 number Charge47p + 61n 108 0 47p+ + 47 e-

Ag47 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 number 15

mass number 31

number of protons 15

number of electrons 15

number of neutrons 31-15 = 16

element symbol P

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

12C 6 6 12 Carbon-12

13C 6 7 13 Carbon-13

14C 6 8 14 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 amu7 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 12C (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-37

75.77% Cl-35 (or 0.7577)

24.23% Cl-37 (or 0.2423)

0.7577 x 35 amu = 26.52 amu

0.2423 x 37 amu = 8.965 amu

Cl Atomic mass = 35.49 amu

Calculate the atomic mass of Imaginarium

90 % occurs as Im-200

10 % occurs as Im-210

Calculate the atomic mass of Imaginarium

90 % occurs as Im-200

10 % occurs as Im-210

0.90 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 108 m/s

Travels as waves --- Wavelength (λ) and Energy inversely

relatedSmall λ -- High energyLong 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

http://astro.u-strasbg.fr/~koppen/discharge/

• Ne red

• Ar blue

• Hg blue

• Na yellow

Principal Energy Level Element # e- 1 2 3 4

H 1 1He 2 2Li 3 2 1Be 4 2 2C 6 2 4F 9 2 7Ne 10 2 8Na 11 2 8 1Mg 12 2 8 2Cl 17 2 8 7Ar 18 2 8 8K 19 2 8 8 1