Atomic Structure and the Periodic Table

Ch. 5

Atoms

5-1

Early Models of the Atom

• Democritus – 4th century BC – world made up of empty space and tiny particles called atoms (atomos) ‘indivisible’– Hypothesized without

using experiments

Dalton’s Atomic Theory

1. All matter is made of atoms2. Atoms are indestructible and cannot be divided

into smaller particles (later proved wrong)3. All atoms of one element are exactly alike

(later proved wrong), but they are different from atoms of other elements

4. Atoms combine in simple whole number ratios to form compounds

5. In chemical reactions, atoms combine, separate, or are rearranged.

Done in the 1800’s through experiments!

Structure of the Nuclear Atom

5-2

Atomic Structure1. Electron: (e-)

– Discovered by Thompson’s cathode-ray tube (1897)

• Cathode Ray = positive and negative electrodes connected, beam travels from cathode (-) to anode (+)

– Found electrons have negative charge• Beam in cathode ray attracted to anode (+), must

be negatively charged.

– Relatively no mass (experiment by Millikan 1916)

Thompson’s Cathode-Ray Experiment

2. Proton – has positive charge (p+), greater mass than an electron

- Discovered by Goldstein in 1886

3. Neutron – has neutral charge (no), equal in mass to proton

-Discovered by Chadwick in 1932

4. Nucleus = small, positively charged central core, made up of protons and neutrons-Discovered by Rutherford’s gold foil experiment in 1911-Fired stream of positive particles at gold foil, most passed right through (atom mostly empty space) while a few bounced off (very small positive nucleus at center)

Play movie

Development of Modern Atomic Theory

• Law of conservation of matter (mass) = in a chemical reaction matter is neither created nor destroyed.– All elements are recycled/rearranged!!

• Law of definite proportions = elements within a compound are always in certain proportions by mass– Ex: NaCl (table salt) is always

39% Na and 61% Cl.

Distinguishing between Atoms

5-3

Atomic Number

• Atomic # = Number of protons in the nucleus of an atom

atomic # = p+

• Determines the identity of the element• Periodic table is organized by atomic number• Ex: What is the atomic number of…

He ______ C ______ N ______ O ______• Also! # of protons = # of electrons

p+ = e-

– Ex: How many electrons does Cl have? _______

Mass Number• Mass # = Sum of the protons and neutrons

mass # = p+ + no

Why do we not include electrons???

neutron = mass # - atomic #

• Periodic Table symbols:

12 C 6

Mass # (rounded)

Atomic #

Element

Let’s Practice!• How many?

Protons ____Neutrons ____Electrons ____

• Write it:? C?

• How many?Protons ______Neutrons ______Electrons ______

• Write it:? S?

Remember!!!

• Mass # = p+ + n

• Atomic # = p+

• e- = p+

• n = mass # - atomic #

Atomic Mass + Isotopes• Atomic Mass =

weighted average mass of isotopes

• Isotopes = one element with same proton # but different neutron #– Carbon has 3

possible isotopes:14 13 12 C C C 6 6 6

Carbon-14, Carbon-13, Carbon-12

Try it!

• Ex: Isotope of Chlorine:37 Cl17

How many?Protons ______Neutrons ______Electrons ______

Atomic Mass Unit

• Atomic Mass Unit (amu) = 1/12 the mass of a carbon-12 atom.

• Used to compare element’s masses to a standard.

• Units for measuring atom’s masses

The Periodic Table:Organizing the Elements

5-4

Development of the Periodic Table

• In 1869, Mendeleev created Periodic Table of Elements – Organized it by increasing atomic mass

and elements with similar properties in same columns.

– Able to predict future elements properties

• In 1913, Moseley organized it by atomic number.

Modern Periodic Table

• Organized by atomic number

• Periodic Law: grouped according to physical and chemical properties

• Period = horizontal row of periodic table; numbered down the side, 7 periods

• Group = vertical column, also called a family; 18 groups

• Representative elements = Group 1,2,13-18

– Group 1 – Alkali metals

– Group 2 – Alkaline earth metals

– Group 17 – Halogens

– Group 18 – Noble gases

• Transition metals = groups 3-12

Group Designations

Physical States of Elements

• Periodic Table shows the states of the elements at room temperature and normal pressure– Most are solids– Only two are liquids– Gaseous elements are in the top right hand

corner except for Hydrogen– The rest are made synthetically (man-

made)through nuclear reactions (43, 61, 85, 87, and > 93)

Classifying Elements

• Metals - majority of elements, occupy left side and center

• Nonmetals - occupy top right hand corner, (includes H)

• Metalloids - along the boundary

Metals• Have luster, many colors, conduct heat and

electricity, usually bend w/o breaking (malleable)

• Almost all are solids at room temp. and have extremely high melting points

Nonmetals

• Are abundant in nature– Oxygen and Nitrogen: 99% atmosphere– Carbon in more compounds than all

others combined!• Poor conductor of heat and electricity,

brittle when solid• Many gases at room temp, low melting

point

Metalloids

• Have properties of both metals and nonmetals

• 7 Metalloids: B, Si, Ge, As, Sb, Te, Po (touch staircase, no Al + At)

• Some are semiconductors = element that does not conduct electricity as well as metal, but does conduct slightly better than nonmetal– Ex: Silicon – used in computer industry