Structure of the Atom (Early Studies) - Chemistry...

Structure of the Atom (Early Studies)

The ancient Greeks were the first to postulate that matter consists of indivisible constituents Later it was realized that the atom consisted of charged entities. Early 1900s CRT (Cathode Ray Tubes) JJ Thompson

1. Rays are same no matter what material is used to generate them

2. Rays are a stream of negatively charged

particles. Charge of electron determined by Milliken

Structure of the Atom (Studies of Radioactivity)

Matter is composed of negative, positive, and neutral material

Structure of the Atom (Plum Pudding Model)

J. J. Thompson postulated that the atom

consisted of a uniform positive sphere of matter in which electrons were embedded.

Structure of the Atom (Scattering of α Particles)

Plum Pudding Model not consistent with experimental observations

Structure of the Atom (Rutherford’s Nuclear Model)

Rutherford posits that the atom is mainly empty

and contains a small but extremely dense region of positive charge

The Nucleus!

Modern View of Atom

Atom is almost entirely empty space

1 nm = 10-9m = 10 Å Å = angstrom = 10-10 m Atom Diameter = 0.1-0.5 nm Nuclear Diameter = 10-5 nm Ratio of nuclear size to atomic radius: 10-4 Carbon Atom Diameter = 0.154

Modern View of the Atom (Components of the Atoms)

mass charge (a.m.u.) (a.u.)

proton (p) 1.0073 +1

electron (e) 5.486 x 10-4 –1

neutron (n) 1.0087 0

Isotopes

Number of protons = atomic number

Number of protons DEFINES identity of element

For a given element, number of protons is fixed, but number of neutrons can vary

Cl35

17Cl

37

17

Isotopes: atoms of a given element with a different number of neutrons

H1

1H

2

1H

3

1

F19

9

NEUTRAL

ATOM

Na23

11Cl

35

17

ION ION

The Atomic Mass Scale

• We define: Mass of 12C = exactly 12 amu 1 amu = 1.66054 x 10-24 g 1 g = 6.02214 x 1023 amu

• Using atomic mass units: 1H weighs 1.635 x 10-24 g 16O weighs 2.6560 x 10-23 g

Average Atomic Masses

Relative atomic mass: average masses of isotopes: Natural abundance carbon (98.892% 12C + 1.108% 13C) Can also find 14C in trace amounts (radioactive) Calculate the Average Mass of Natural Abundance Carbon (0.98892)(12 amu) + (0.0108)(13.00335) = 12.011 amu

Atomic weight (AW) is also known as average atomic mass Atomic weights are listed in the periodic table

The Mole and Avogadro’s Number

The mole is a connection between microscopic (what we don’t see, i.e. atoms) and macroscopic (what we can see). It’s equivalent to using the term dozen to refer to eggs for example. However, a mole is much larger; A mole of eggs would be 6.02 x 1023 eggs!

1 mole of anything (atoms, molecules, gree peas) = Avogadro’s number of them

Experimentally, 1 mole of 12C has a mass of 12 g.

Introduction to Thermochemistry

Two types of Energy Kinetic Potential First Law of Thermodynamics ΔE = q + w

Enthalpy = ΔH

Chapter 5.1-5.3

Energy Energy can be converted from one form to

another . . . Kinetic Energy energy of motion Potential Energy stored energy Electronic Energy The potential energy associated with the electrons in atoms and molecules Chemical Energy The potential energy associated with bonding

Three bonds Four bonds H H H H O O O

H HO

H H

Energy Units What is the SI Unit of Energy?

a) Cal b) Kcal c) Erg d) Joules e) BTUs

Energy . . . but total energy remains constant. First Law of Thermodynamics:

Law of Conservation of Energy Energy lost by the System Is gained by the Surroundings

System: what you are interested in. An atom, molecule, or a chemical reaction Surroundings: Everything else

Changes in Energy

E = internal energy: the capacity to do work or transfer heat ΔE = Efinal –Einitial = q + w

w = work = action of force through a distance (often seen as pressure x volume change (PΔV)

Work done to the system (+) q = heat (thermal energy) = energy transferred due to a difference in temperature. Heat added to the system (+) Energy Exothermic System Surroundings ΔE = –

Energy Endothermic Surroundings System ΔE = +

State Function A function whose value does not depend on the pathway used to get to the present state

State Function: only depends on the current state (composition, T,P): does not depend on past history

State Functions

State functions are written as uppercase letters (E, H, P, O, V, T, S . . .)

q and w are not state functions but ΔE (=q + w) is a state function

Changes in state functions are path-independent

Enthalpy (ΔH) Energy transferred when P is constant

(=qp) = ΔH

Usually run chemical reactions at constant pressure (atmospheric)

What You Need to Know • Difference between kinetic and potential energy (can

you make up your own examples?)

• Know units of energy and order of magnitude of different kinds of energies (photon energy, etc.)

• Watch for electronic energy, i.e., the energy of an electron

• What is the First Law of Thermodynamics?

• Watch for how the potential energy of electrons gets converted to kinetic energy (a photon)

• What is meant by the system? The surroundings?

• What does the sign of energy mean:

o + means energy into the system (endothermic)

o –means energy out of the system (exothermic)

• Notice State Functions (like P, T, V) as we encounter them and keep the definition of state function in mind. Why is it useful to know that these are state functions?

• Remember that Enthalpy is another word for energy: specifically it is an energy measured when the system is at constant pressure; but for now, just know that it is a form of energy (heat)