Atomic Theory & Periodic Table Unit 3 Part 1 (Ch. 4 & 5) Atomic Theory Introduction, Isotopes

Substances Are Made of Atoms Atomic Theory – atoms are the building blocks of all matter Atom – smallest part of an element that retains its identify Current atomic theory evolved from the atomic models of ◦ Thomson--Dalton ◦ Rutherford--Bohr

Laws Support Existence of Atoms The Law of Conservation of Mass – mass cannot be created or destroyed in ordinary chemical or physical changes The Law of Multiple Proportions – when two elements combine to form 2 different compounds, the mass of one element that combines with a given mass of the other is in the ratio of small whole numbers

Dalton’s Atomic Theory John Dalton, 1808 Dalton combined the previous laws with the Greek concept of the atom Dalton believed that a few kinds of atoms made up all matter

5 Principles of Dalton’s Theory 1. All matter composed of atoms which cannot be subdivided, created or destroyed 2. Atoms of given element are identical in physical and chemical properties 3. Atoms of different elements differ in physical and chemical properties 4. Atoms of different elements combine in simple, whole-number ratios to form compounds 5. In chemical reactions, atoms combine, separate or rearrange, but are never created, destroyed, or changed.

New Data’s Effect on Dalton’s Theory First two principles are not true in all cases Today, scientists can divide atoms into smaller particles and can destroy and create atoms Dalton did not predict that some atoms combine with like atoms – oxygen, O 2

Structure of the Atom Subatomic Particles – smaller particles that make up the atom NameSymbolChargeMass (kg) Electrone9.109x Protonp x Neutronn01.675x10 -27

Electrons Discovered by using cathode ray tubes ◦ By J. J. Thomson

Thomson’s Model Plum-pudding model Thomson proposed that negatively charge electrons were embedded in a positively charged ball of matter

Rutherford and the Nucleus Nucleus – an atom’s central region Gold foil experiment – disproved Thomson’s model of the atom

Rutherford’s Atomic Model Atom has a concentrated positive charge The positive-charge region has a large mass relative to the alpha particles Radius of atom is 10,000 times larger than radius of the nucleus

Protons and Neutrons Proton: ◦ Positive charge ◦ Found in nucleus ◦ Number of protons = atomic number (which determines identity of the atom) Neutron: ◦ No charge ◦ Found in nucleus ◦ Help stabilize nucleus

Atomic Number Atomic number ◦ Number of protons in the nucleus of an atom ◦ Same for all atoms of an element ◦ Number of electrons in neutral atoms 6 C Carbon

Mass Number Mass number – ◦ sum of the number of protons and neutrons in the nucleus of an atom ◦ Can vary among atoms of the same element ◦ Example – atomic number for oxygen is 8, but can have mass numbers of 16, 17 or 18 (8, 9 or 10 neutrons) ◦ Note – atomic number identifies an element, but the mass number does not

Atomic Structures - Symbols Each element has a symbol of one or two letters Atomic number is always on the lower left side of the symbol Mass number is written on the upper left side of the symbol atomic # mass # entire notation 1H1H 1H1H 1 1 H

Isotopes Isotope ◦ an atom that has the same number of protons as other atoms of the element, but a different number of neutrons ◦ Can be written as name, hyphen, mass number like helium-3 or as seen below ◦ Different isotopes have different masses 3 2 He 4 2 He

Atomic Mass Atomic Mass – mass of an atom expressed in atomic mass units (amu) Periodic table shows the average of the atomic masses of the naturally occurring isotopes of the element (in amu) AMU – defined as one-twelfth of the atomic mass of the carbon-12 isotope