Unit 1 – Atomic Structure

Atomic Structure Early Atomic Theory Democritus (400 BC) suggested that the world was made of 2 things: empty space and tiny, invisible particles called “atoms.”

Aristotle Aristotle believed matter was composed of 4 basic elements, water, fire, earth, and air. He was a well respected so everyone discounted Democritus’s atomic theory and believed Aristotle’s theory without question.

John Dalton (early 1800s) using the experimental observations of others, Lavoisier, and Proust proposed his own atomic theory.

Dalton’s Atomic Theory All elements are composed of tiny particles called atoms. Atoms of the same element are identical. The atoms of any one element are different from those of another element. *Isotopes of an element are not identical. 5

Atoms cannot be divided, created, or destroyed. *Atoms are divisible by a nuclear change. Atoms of different elements can combine with one another in simple, whole number ratios to form compounds. Chemical reactions occur when atoms are joined, separated, or rearranged. However, atoms of one element are not changed into atoms of another element.

Modern Atomic Theory All matter is composed of Atoms We also now know that atoms can be changed into isotopes(different elements) and be destroyed into smaller particles by nuclear division.;

Early Research on Atomic Particles Thomson (1867) used a cathode ray tube to prove the existence of negatively charged subatomic particles called electrons.

Robert Millikan (1909) used his oil drop experiment to prove that the charge on of an electron is -1.

Rutherford Rutherford (1913) - his gold foil experiment, used a device to prove that the atom is made up of empty space with, a small dense, positively charged nucleus.

Thomson – used a modified cathode ray tube to prove the existence of a positively charged subatomic particle called a proton. It has equal but opposite charge to the electron (+1) and a mass 1,840 times heavier than an electron.

Chadwick Chadwick (1932) - used a device to prove that the nucleus contained neutral particles of the same mass as the proton called neutron.

Warm-up 9/19/13 1. Compare the different views Aristotle and Democritus had about what matter was made of. 2. Which Greek philosopher was correct ? 3. Which later scientist proved his view? 14

Atomic number - is the number of protons in an atom Atomic number - is the number of protons in an atom. This number is unique for all elements and the atomic number thus identifies the element. Since atoms are electrically neutral, THE NUMBER OF PROTONS EQUALS THE NUMBER OF ELECTRONS.

Isotopes are atoms of the same element that differ in the number of neutrons in the nucleus. Isotopes of the same element have the different chemical properties.

Isotopes = Neutron Difference The number of neutrons determines the particular isotope of the element. Mass Number – the total number of protons and neutrons in an atom. Mass # = protons + neutrons Neutrons = Mass # - protons

Practice Problem: How many protons, neutrons and electrons in Oxygen-16? How many protons, neutrons and electrons in Oxygen-17?

Subatomic Particle Chart Charge Relative mass Location Electron -1 1/1840 Outside Nucleus Proton +1 1 Neutron

Calculating Atomic Mass Atomic Mass is the weighted average of the masses of the isotopes of that element. A weighted average takes into consideration both the mass and the abundance of each isotope. The correct unit for atomic mass is amu.

Relative atomic mass of the element To calculate relative atomic mass, multiply the mass number of each isotope by its percent abundance changed to a decimal and total. (move decimal 2 places to the left) (Mass #)(isotope’s relative abundance) + (Mass #)(another isotopes Rel. abund) = Relative atomic mass of the element

EXAMPLE: 95.00% Symbol Abundance Calculation Average Atomic Mass 32S 0.76% 34S 4.22% 36S 0.014%

Example 2: Neon has 2 isotopes, Ne-20 with an abundance of 90% and Ne-22 with an abundance of 10%. Calculate the average atomic mass of neon.

Warm-up #1 Draw the picture to the left and answer the question. Then, write and answer these questions: What is the full name of this element? If I added a neutron to the nucleus of this atom, what would its name be? If I added a proton to this atom, what would its name be?