Los Atomicos

Atoms, Molecules and Ions

Dalton’s Atomic Theory (1808)
1. Elements are composed of extremely small particles called atoms. All atoms of a given element are identical, having the same size, mass and chemical properties. The atoms of one element are different from the atoms of all other elements.

2. Compounds are composed of atoms of more than one element. The relative number of atoms ofeach element in a given compound is always the same.
3. Chemical reactions only involve the rearrangement of atoms. Atoms are not created or destroyed in chemical reactions.

2

16 X

+

8Y

8 X2Y

J.J. Thomson, measured mass/charge of e(1906 Nobel Prize in Physics)

Measured mass of e(1923 Nobel Prize in Physics)

e- charge = -1.60 x 10-19 C Thomson’s charge/mass of e- = -1.76x 108 C/g e- mass = 9.10 x 10-28 g

(Uranium compound)

(1908 Nobel Prize in Chemistry)

 particle velocity ~ 1.4 x 107 m/s (~5% speed of light)

1. atoms positive charge is concentrated in the nucleus 2. proton (p) has opposite (+) charge of electron (-) 3. mass of p is 1840 x mass of e- (1.67 x 10-24 g)

Rutherford’s Model of the Atom

atomic radius ~ 100 pm = 1 x 10-10 mnuclear radius ~ 5 x 10-3 pm = 5 x 10-15 m

Chadwick’s Experiment (1932)
H atoms - 1 p; He atoms - 2 p mass He/mass H should = 2 measured mass He/mass H = 4

 + 9Be
neutron (n) is neutral (charge = 0)
n mass ~ p mass = 1.67 x 10-24 g

1n

+ 12C + energy

Subatomic Particles (Table 2.1)
Particle
-

Mass (g)
-28

Charge (Coulombs) -1.6 x 10
-19

Charge (units) -1 +1 0Electron (e ) 9.1 x 10 Proton (p+)

1.67 x 10-24 +1.6 x 10-19 0

Neutron (n) 1.67 x 10-24

mass p = mass n = 1840 x mass e-

Atomic number (Z) = number of protons in nucleus
Mass number (A) = number of protons + number of neutrons = atomic number (Z) + number of neutrons

Isotopes are atoms of the same element (X) with different numbers of neutrons in their nuclei
Mass Number Atomic NumberA ZX 2 1H

Element Symbol

1 1H 235 92

(D)
238 92

3 1H

(T)

U

U

Do You Understand Isotopes?
How many protons, neutrons, and electrons are in
6 protons, 8 (14 - 6) neutrons, 6 electrons

14 6

C

?

How many protons, neutrons, and electrons are in
6 protons, 5 (11 - 6) neutrons, 6 electrons

11 6

C?

Noble Gas Halogen

Group

Alkali Earth MetalAlkali Metal

Period

A molecule is an aggregate of two or more atoms in a definite arrangement held together by chemical bonds

H2

H2O

NH3

CH4

A diatomic molecule contains only two atoms
H2, N2, O2, Br2, HCl, CO

A polyatomic molecule contains more than two atoms
O3, H2O, NH3, CH4

An ion is an atom, or group of atoms, that has a net positive or negative charge.

cation –ion with a positive charge If a neutral atom loses one or more electrons it becomes a cation.

Na

11 protons 11 electrons

Na+

11 protons 10 electrons

anion – ion with a negative charge If a neutral atom gains one or more electrons it becomes an anion.

Cl

17 protons 17 electrons

Cl-

17 protons 18 electrons

A monatomic ion contains only one atom

Na+, Cl-, Ca2+, O2-,Al3+, N3-

A polyatomic ion contains more than one atom

OH-, CN-, NH4+, NO3-

Do You Understand Ions?
How many protons and electrons are in
13 protons, 10 (13 – 3) electrons

27 3+ ? 13 Al

How many protons and electrons are in
34 protons, 36 (34 + 2) electrons

78 Se 234

?

A molecular formula shows the exact number of atoms of each element in the smallest unit of asubstance

An empirical formula shows the simplest whole-number ratio of the atoms in a substance

molecular H2O C6H12O6 O3

empirical H2O CH2O O

N2H4

NH2

ionic compounds consist of a combination of cations and an anions

• the formula is always the same as the empirical formula
• the sum of the charges on the cation(s) and anion(s) in each formula unit must equal zero

The ionic...