Atomic Structure

Chemistry Timeline #1B.C.

400 B.C. Demokritos and Leucippos use the term "atomos”

1500's George Bauer: systematic metallurgy Paracelsus: medicinal application of minerals

1600'sRobert Boyle:The Skeptical Chemist. Quantitative experimentation, identification of elements

1700s'Georg Stahl: Phlogiston TheoryJoseph Priestly: Discovery of oxygen Antoine Lavoisier: The role of oxygen in combustion, law of conservation

of mass, first modern chemistry textbook

2000 years of Alchemy

Chemistry Timeline #2

1800's Joseph Proust: The law of definite proportion (composition) John Dalton: The Atomic Theory, The law of multiple proportions Joseph Gay-Lussac: Combining volumes of gases, existence of diatomic

molecules Amadeo Avogadro: Molar volumes of gases Jons Jakob Berzelius: Relative atomic masses, modern symbols for the elements Dmitri Mendeleev: The periodic table J.J. Thomson: discovery of the electron Henri Becquerel: Discovery of radioactivity

1900's Robert Millikan: Charge and mass of the electron Ernest Rutherford: Existence of the nucleus, and its relative size Henry Moseley: Organizes Periodic Table by Atomic Number Chadwick: Discovery of Neutron Meitner & Fermi: Sustained nuclear fission Ernest Lawrence: The cyclotron and trans-uranium elements

Dalton’s Atomic Theory (1808)

Atoms cannot be subdivided, created, or destroyed

Atoms of different elements combine in simple whole-number ratios to form chemical compounds

In chemical reactions, atoms are combined, separated, or rearranged

All matter is composed of extremely small particles called atoms

Atoms of a given element are identical in size, mass, and other properties; atoms of different elements differ in size, mass, and other properties

John Dalton

Modern Atomic TheorySeveral changes have been made to Dalton’s theory.

Dalton said:Atoms of a given element are identical in size, mass, and other properties; atoms of different elements differ in size, mass, and other properties

Modern theory states:Atoms of an element have a

characteristic average mass which is unique to that element.

Modern Atomic Theory #2

Dalton said:

Modern theory states:

Atoms cannot be subdivided, created, or destroyed

Atoms cannot be subdivided, created, or destroyed in ordinary chemical reactions. However, these changes CAN occur in nuclear reactions

Discovery of the ElectronIn 1897, J.J. Thomson used a cathode ray tube to deduce the presence of a negatively charged particle.

Cathode ray tubes pass electricity through a gas that is contained at a very low pressure.

Thomson’s Atomic Model

Thomson believed that the electrons were like plums embedded in a positively charged “pudding,” thus it was called the “plum pudding” model.

J.J. Thomson

Mass of the Electron1909 – Robert Millikan determines the mass of the electron.

The oil drop apparatus

Mass of the electron is 9.109 x 10-31 kg

Conclusions from the Study of the Electron

Cathode rays have identical properties regardless of the element used to produce them. All elements must contain identically charged electrons.

Atoms are neutral, so there must be positive particles in the atom to balance the negative charge of the electrons

Electrons have so little mass that atoms must contain other particles that account for most of the mass

Rutherford’s Gold Foil Experiment

Alpha particles are helium nuclei Particles were fired at a thin sheet of gold foil

Particle hits on the detecting screen (film) are recorded

Rutherford’s Findings

The nucleus is small The nucleus is dense The nucleus is positively charged

Most of the particles passed right through

A few particles were deflected VERY FEW were greatly deflected

“Like howitzer shells bouncing off of tissue paper!”

Conclusions:

Atomic Particles

Particle

Charge

Mass (kg) Location

Electron

-1 9.109 x 10-31 Electron cloud

Proton +1 1.673 x 10-27 Nucleus

Neutron

0 1.675 x 10-27 Nucleus

The Atomic Scale

Most of the mass of the atom is in the nucleus (protons and neutrons)

Electrons are found outside of the nucleus (the electron cloud)

Most of the volume of the atom is empty space

Helium-4

Image: User Yzmo Wikimedia

Commons.

Period

Group

Alkali M

etal

Noble G

as

Halogen

Alkali E

arth Metal

IsotopesIsotopes are atoms of the same element having different masses due to varying numbers of neutrons.Isotope Proto

nsElectron

sNeutron

sNucleus

Hydrogen–1

(protium)

1 1 0

Hydrogen-2

(deuterium)

1 1 1

Hydrogen-3

(tritium)

1 1 2

Atomic Masses

Isotope Symbol Composition of the nucleus

% in nature

Carbon-12

12C 6 protons6 neutrons

98.89%

Carbon-13

13C 6 protons7 neutrons

1.11%

Carbon-14

14C 6 protons8 neutrons

<0.01%

Atomic mass is the average of all the naturally isotopes of that element.

Carbon = 12.011

Atomic NumberAtomic number (Z) of an element is the number of protons in the nucleus of each atom of that element.

Element # of protons

Atomic # (Z)

Carbon 6 6

Phosphorus

15 15

Gold 79 79

Mass NumberMass number (A) is the number of protons and neutrons in the nucleus of an isotope.

Mass # = p+ + n0

Nuclide p+ n0 e- Mass #

Oxygen - 10

- 33 42

- 31 15

8 8 1818

Arsenic 75 33 75

Phosphorus 15 3116

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

XAZ

H11 H (D)2

1 H (T)31

U23592 U238

92

Mass Number

Atomic NumberElement Symbol

How many protons, neutrons, and electrons are in C146 ?

How many protons, neutrons, and electrons are in C116 ?

6 protons, 8 (14 - 6) neutrons, 6 electrons

6 protons, 5 (11 - 6) neutrons, 6 electrons

Do You Understand Isotopes?

Ions

Not only can atoms have a different number of neutrons but they can also have a different number of electrons.

When they have more or less electrons they are called ions.

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

cation – ion with a positive chargeIf a neutral atom loses one or more electronsit becomes a cation.

anion – ion with a negative chargeIf a neutral atom gains one or more electronsit becomes an anion.

Na 11 protons11 electrons Na+ 11 protons

10 electrons

Cl 17 protons17 electrons Cl-

17 protons18 electrons

Determining the Electrons

In a neutral atom the number of electrons is the same as the number of protons.

When an atom becomes an ion to calculate the number of electrons you can use the following equation:

Ion charge = p+ - e-

Determining the Electrons (Cont.)

So for the following ion:N-3

the charge is -3 which means…

Ion charge = p+ - e-

-3 = 7 – (# electrons)

… or there are 10 electrons.

13 protons, 10 (13 – 3) electrons

34 protons, 36 (34 + 2) electrons

Do You Understand Ions?

How many protons, and electrons are in Al2713 ?3+

How many protons and electrons are in Se7834

2- ?