Chapter 2

Atoms, molecules, and ions

1

The Atomic TheoryHistory

The Structure of the AtomProton, Neutron, Electron

Atomic Number, Mass NumberIsotopesThe Periodic Table

DAB/CAS/CSU/2010

Matter

2

Brownian Motion

3

Atomic theory

4

Atoms are incredibly small.What we know about them is based on

indirect evidence.

History of atomic theory

5

500 BC Democritus proposed the idea of atom but had no experimental data to supportBelieved that all matter consists of very

small, indivisible particles, which he named atomos (meaning uncuttable or indivisible).

1800 Dalton proposed an atomic theory supported by an experimental data

Dalton’s atomic theory: Support

6

Support for his theory came when he began looking at the masses of elements that combined with each other

He found that elements combine by simple ratio by mass

Dalton’s atomic theory:

7

1. All matter is composed of tiny indivisible particles called atoms.

2. All atoms of the same element are identical (having same size, mass, and chemical properties). The atoms of one element are different from the atoms of all other elements.

3. Atoms of two or more elements can combine in small whole number ratios to form compounds.

4. A chemical reaction involves only the separation, combination, or rearrangement of atoms; it does not result in their creation or destruction.

Dalton’s atomic theory: Support

8

Law of Multiple Proportions• If two elements can combine to form more than

one compound, the masses of one element that combine with a fixed mass of the other element are in ratios of small whole numbers.

Example: CO & CO2

Law of Definite Proportion• Different samples of the same compound

always contain its constituent elements in the same proportion

Dalton’s atomic theory: Support

9

Law of Conservation of MassMatter can neither be created nor destroyedThe mass of atoms before and after reaction

is conserved

Dalton’s atomic theory: Support

10

Law of Conservation of Mass

Dalton’s atomic theory:

11

1. Atoms of the same element have different masses

2. Atoms have subatomic particles (protons, electrons, neutrons.)

Problems with Daltons Theory

So, Dalton’s theory is not completely correct!

The structure of the atom

12

Subatomic Particles

13

Particle Symbol Charge RelativeMass

(amu)

Electron e- 1- 0

Proton p+ + 1

Neutron n 0 1

Subatomic Particles: Discovery

14

Electron was discovered by JJ Thomson in ～ 1900 using his cathode ray tube.

Electron charge was found by American Robert Millikan ～ 1910 by an oil-drop experiment.

Proton was found by the adaptation of cathode ray tube ～ 1900.

Neutron was discovered by Chadwick in 1932.

The Electron Discovery

15

1890’s discovery of radiation lead to discovery of electron

Radiation – the emission and transmission of energy through space in the form of waves

Used cathode ray tubeCathode – negatively charged plated, emits

an invisible rayAnode – positively charged tube

16

Thomson’s plum pudding model of the atom

Subatomic Particles: Discovery

17

Plum pudding model: atom has a weak positive charge.

Rutherford tested the idea by positively charged particles at gold foil.

He expected particles to pass through.His results surprised him.

Rutherford’s Gold Foil Experiment:

Movie on Gold Foil Experiment

18

Subatomic Particles: Discovery

19

Atoms is mostly empty space.Protons are found in the dense nucleus in

the center of the atom.

Rutherford’s Gold Foil Experiment: Results

Location of Subatomic Particles

20

10-13 cm electrons

protons

neutrons

10-8 cm

nucleus

Rutherford’s Model:

21

Problem:Could not explain why electrons would not

spiral into the nucleus.

Implications of Rutherford’s Exp’t.

22

Nucleus – at the center of the atomDense central core within the atom

Contains positively charged particle (protons)

Protons carry the same charge as that of electron

Mass of protons is 1840 times that of electron

Neutrons – neutral particles within the nucleus having a mass slightly greater than that of protons

23

Particle Mass (g) Coulumb Charge unit

Electron 9.10939 x 10 -

28

-1.6022 x 10-

19

-1

Proton 1.67262 x 10 -

24

+1.6022 x 10-

19

+1

Neutron 1.67493 x 10 -

24

0 0

24

ATOMS TO ELEMENTS

25

The number of protons, electrons, and neutrons in them.

Example:Gold and Mercury

What makes elements different from one another?

Atomic Number, Z

26

Counts the number of

protonsin an atom

Atomic Number on the Periodic Table

27

11Na

Atomic Number

Symbol

All atoms of an element have the same number of protons

28

11Na

11 protons

Sodium

Learning Check

29

State the number of protons for atoms of each of the following:A. Nitrogen 1) 5 protons 2) 7 protons 3) 14 protons

B. Sulfur 1) 32 protons 2) 16 protons 3) 6 protons

C. Barium1) 137 protons 2) 81 protons 3) 56 protons

Mass Number, A

30

Counts the number of

protons and neutronsin an atom

Mass Number, A = p + n

n = A - p

Atomic Symbols

31

Show the mass number and atomic

number

Give the symbol of the element

mass number

23 Na sodium-23

atomic number 11

Number of Electrons

32

An atom is neutral The net charge is zero

Number of protons = Number of electrons

Atomic number = Number of electrons

Atomic Number =Number of protons

+ 2 + (-2) = 0

Subatomic Particles in Some Atoms

33

16 31 65 O P Zn 8 15 30

8 p+

8 n8 e-

15 p+

16 n15 e-

30 p+

35 n30 e-

Subatomic Particles in Some Atoms

34

16 31 65 O P Zn 8 15 30

8 p+

8 n8 e-

15 p+

16 n15 e-

30 p+

35 n30 e-

What happens when atoms losses electrons?

35

2311Na

When Na losses 1 electron?

11 p+

11 e-

12 n

2311Na+

11 p+

10 e-

12 n

11 p+

11 e-

+11

-10 1

What happens when atoms gains electrons?

36

3517Cl

When Cl gains 1 electron?

17 p+

17 e-

18 n

17 p+

18 e-

18 n

17 p+

17 e-

+17

- 18 - 1

3517Cl-

What happens when atoms gains electrons?

37

168O

When O gains 2 electron?

8 p+

8 e-

8 n

8 p+

10 e-

8 n

8 p+

8 e-

+ 8

- 10 - 2

168O

Isotopes

38

Atoms with the same number of

protons, but different numbers of

neutrons. Atoms of the same element (same

atomic number) with different mass numbers

Isotopes of chlorine

35Cl 37Cl17 17

chlorine - 35 chlorine - 37

17 p+

18 n17 e-

17 p+

20 n17 e-

686

676

666

Learning Check

39

Naturally occurring carbon consists of three isotopes, 12C, 13C, and 14C. State the number of protons, neutrons, and electrons in each of these carbon atoms.

12C 13C 14C 6 6 6

#p _______ _______ _______

#n _______ _______ _______

#e _______ _______ _______

Learning Check

40

An atom of zinc has a mass number of 65.

A. Number of protons in the zinc atom 1) 30 2) 35 3) 65

B. Number of neutrons in the zinc atom 1) 30 2) 35 3) 65

C. What is the mass number of a zinc isotope

with 37 neutrons? 1) 37 2) 65 3) 67

Learning Check

41

Write the atomic symbols for atoms with the following:

A. 8 p+, 8 n, 8 e- ___________

B. 17p+, 20n, 17e- ___________

C. 47p+, 60 n, 47 e- ___________

16O8

37Cl17

107Ag 47

Atomic Mass on the Periodic Table

42

11Na

22.99

Atomic Number

Symbol

Atomic Mass

Atomic Mass

43

Atomic mass is the weighted average mass of all the atomic masses of the isotopes of that atom.

Example of an Average Atomic Mass

44

Cl-35 is about 75.5 % and Cl-37 about 24.5% of natural chlorine. 35 x 75.5 = 26.4 100

35.5 37 x 24.5 = 9.07

100