ATOM

Early Thoughts

Greeks

matter is made up of

particles--4 elements

4 elements --air--fire--water-

-- earth

Aristotle-- Continuous theory

Democritus --Discontinuous theory

Matter can be divided indefinitely

Matter can not be divided indefinitely Atoms-indivisible

Rich -Paradis

ATOMIC THEORIES Dalton’s Atomic Theory

• All elements are composed of individual atoms. •All atoms of a given element are identical • Atoms of different elements are different. •Compounds are formed by the combination of atoms of different elements (Law of Definite Proportions) Reactions are rearrangements of atoms

Fire ball

Sphere Uniform density

high v oltage

Cathode

Tube

Negative

electrode Positive

electrode

Cathode ray

-cathode rays are a form of radiation that

contain an electrical charge

-rays move from negative to positive

-rays are negative (electrons)

JJ Thomson’s Model used the cathode ray tube to show one of the smaller units (particle)

that make up the atom Thomson theorized that an atom contains small negatively charged

particles -------electrons

His model is the “Plum Pudding”

Still uniform density

Bulk of the atom is positive

Negative electrons float around

Rutherford’s Gold Foil Experiment

Hypothesis Alpha emitter

(positvie charge) screen

Gold foil

Because the atom has uniform density, the alpha

particles will pass straight through with little to no

deflection.

Like a bullet through a stick of butter.

Rutherford’s Gold Foil Experiment

Results Conclusion

The atom does not have uniform density.

Because the alpha particles passed through, there must

be empty space.

Because the alpha particles were bounced back, they must

have hit something dense.

Because the alpha particles were deflected, the dense part

must have a positive charge.

Rutherford’s model

Dense positive

center

Mostly empty or unoccupied space

Electrons float around randomly in the empty space

Chadwicks

-discovered the neutron

-when doing studies, he found that the atom often weighed twice as much as he

anticipated with the number of protons

-he determined that there must be some other particle with the same mass as a

proton but had no charge

-neutrons are located in the dense positive center (nucleus)

Particle Symbol Charge Mass Location

proton p + 1 1 amu nucleus

neutron n 0 1 amu nucleus

electron e - 1 1/1836

mass of a

proton

shell

nucleons

Nuclear charge

Mass number = # of protons + #of neutrons

Atomic number =#of protons X A

Z

Protons

And

neutrons

# of Protons

K 39

19 Se 79

34

I 131

53 P-

N-

E-

19

20

19

P-

N-

E-

34

45

34

P-

N-

E-

53

78

53

I-131

When an atom is neutral (no charge),

the number of electrons will equal the protons.

Calculate the number of protons, neutrons, and electrons.

Mass # Rewrite:

Ion -has a charge (pos or neg)

Gain electrons = negative ion

Lose electrons = positive ion

P-

17Cl - 35

N-

E-

20Ca2+ 40

P-

N-

E-

17

18

18

20

20

18

Isotope -Atoms with the same atomic number, but different mass number -same bottom, different top -differ in the number of neutrons

C 12

6

C 14

6

Atomic Mass

1. Write percent as decimal. 2. Multiply decimal by the mass #. 3. Add them up. 4. Units will be atomic mass units (amu)

75% Cl-35 25% Cl-37

.75 x 35 = 26.25

.25 x 37 = 9.25

35.50 amu

-is the weighted average of all naturally occurring isotopes.

-based on the percent abundance and mass

Bohr

Model

•proposed a model of the atom showing a dense nucleus with electrons found in surrounding orbits. •Bohr model shown shows a nucleus surrounded by electrons in circular orbits. •Each electron must posses just the right amount of energy to keep it in place around the nucleus.

energy levels principal energy levels shells

Electron configuration

the distribution of the electrons in an atom

2(n2) Energy level

2-8-8

1st

level

2nd

level

3rd

level

Energy level Max # of e’s

1 2(12) 2 2 2(22) 8 3 2(32) 18 4 2(42) 32 5 2(52) 50 6 2(62) 72 7 2(72) 98

In theory

We will assume

only 32 per level

quantum-- a discrete amount of energy -- each energy level represents a specific quanta -- to move from one level to another, you must use that much energy

Ground state all electrons are in the lowest possible level will follow the pattern: 2-8-18 Pattern on periodic table

Excited state when an electron absorbs energy and jumps to a higher level can’t stay there pattern skips 2-7-1 NOT on periodic table WAS: 2-8 (moved 1 e- to the next level)

Atoms can not stay excited, when electrons drop to a lower level, energy is released in the form of light(photon). Each element gives off a specific color and spectrum.

kernel valence electrons

represents atom from the nucleus to the valance shell

are electrons in the outer energy level of the atom

continuous

spectrum bright line spectra

R O Y G B I V

•white light through a prism or a

diffraction gradient

•shows all the colors is the visible spectrum

•diffraction gradient separates the light

•each element gives off their own bright line spectrum •each line represents an electron dropping to a lower level •fingerprint of an element

Modern Theory -- Wave (Quantum) Mechanical Model

electron cloud electrons don’t travel in fixed paths but are located in regions of space

called orbitals we don’t know the exact location of the electron but can guess where it is most likely located based on probability