Holt McDougal Chapter 4 Section 2

Explore the scientific theory of atoms (also known as atomic theory) by describing the structure of atoms in terms of protons, neutrons and electrons, and differentiate among these particles in terms of their mass, electrical charges and locations within the atom

What scientists helped to develop atomic theory?

What part of atoms did Thomson discover?

What part of atoms did Rutherford discover?

Democritus

Greek philosopher in 4th century BCE

Tiny, indivisible units and void

Atomos unable to be cut or divided

No evidence; all conjecture and theory

1808, British schoolteacher John Dalton performed experiments

Atoms DO exist!

Atoms could form compounds

Law of Definite Proportions

How was Dalton’s theory different from that of Democritus?

Carbon makes up 27% of the mass of CO2. How many grams of C are there in 88g of CO2?

1897, British scientist J.J. Thomson working with cathode rays

Discovered electrons, negatively charged particles

Plum Pudding by another

Thomson called Lord Calvin

What was the Greek perspective of atoms?

How did Dalton support and detract from the Greek perspective of atoms?

How did JJ Thomson discover electrons?

Was a student of Thomson

Tested the idea that the mass of an atom was spread evenly over its volume

Gold Foil experiment

Bounce back indicatesa concentrated center

Rutherford concluded that atom’s positive charge is concentrated at the center

Nucleus dense positively charged core of atom

So tiny that like a marble in a football stadium

Negative electrons orbit the nucleus

•No specific orbits

•Only established the existence of the positive nucleus

Scientist Contribution

First proposed that the universe is made up of tiny, indivisible units called atoms

Carried out scientific experiments that showed that atoms exist

Showed that atoms contain smaller particles called electrons

Showed that the positive charge in an atom is concentrated in a nucleus at its center

Explore the scientific theory of atoms (also known as atomic theory) by describing the structure of atoms in terms of protons, neutrons and electrons, and differentiate among these particles in terms of their ass, electrical charges and locations within the atom

What do atoms of the same element have in common?

What are isotopes?

How is an element’s average atomic mass calculated?

Scientist Contribution

First proposed that the universe is made up of tiny, indivisible units called atoms

Carried out scientific experiments that showed that atoms exist

Showed that atoms contain smaller particles called electrons

Showed that the positive charge in an atom is concentrated in a nucleus at its center

Democritus

Dalton

Thomson

Rutherford

1913, Danish physicist Neils Bohr

Fixed orbits for the electrons

James Chadwick, Bohr’s student

Found the neutron in the nucleus

Led to discovery of fission and the atomic bomb

Established by 1926

Area where electrons are most likely to be found

100,000 times larger than the diameter of the nucleus

Impossible to accurately describe path and location at any given time

Orbital a region in which an electron is most likely to be

Particle Charge Mass (kg) Location

Proton +1 1.67 x 10 -27 Inside the nucleus

Neutron 0 1.67 x 10 -27 Inside the nucleus

Electron -1 9.11 x 10 -31 Outside the nucleus

The number of protons is the same!!!

# of protons identify the atom and element

Protons positive Electrons negative

1 proton + 1 electron = neutral charge

Gain/lose electron = ion (ionized atom)

Ions are NOT neutral

Opposites attract

Protons and electrons attract each other

Farther apart = less attraction

Atoms of same element = same # protons

Atomic # = # of protons

Neutral atoms means the atomic # = protons = electrons (i.e., net charge = zero)

Find the atomic number and name for the following elements:

1.H2.Sc3.Ta4.Bh5.F6.Sn7.U

Hydrogen 1 Scandium 21 Tatalum 73 Bohrium 107 Flourine 9 Tin 50 Uranium 92

How many electrons does each of the listed elements have?

Most atoms have protons and neutrons

Atomic Mass # = protons + neutrons

Atomic Mass = Atomic Number + neutrons

How can atoms of the same element have different mass #’s when they all MUST have the same atomic #?

Which two types of subatomic particles are located in the nucleus?

How does the mass of an electron compare with the mass of a proton?

Why are atoms neutral?

What force holds atoms together?

Atoms of an element with different mass #’s

Remember, the AN (# of protons) is ALWAYS the same

# of neutrons is different from one isotope to another

Protium

Deuterium

Tritium

What is the atomic number, atomic mass, and electrical charge for each?

Some more than others (e.g., O has AM of 16 usually; about 0.2% have mass of 18)

Some isotopes are unstable and decay

How can decayed isotopes form new/different isotopes of the same element (even though they are breaking apart)?

3517Cl

•Chemical symbol represents the element

•AMN is written on top

•AN is written on bottom

•How can you compute the number of neutrons?

Too tiny 1 atom of F has mass (kg) of less than one trillionth of one billionth of one gram

1 1 1

1,000,000,000,000,000

X 1,000,000,000,000

X 1

What do you think that number looks like? Write it down and compare with your neighbor

Describes the masses of atoms

SI standard of measure for atoms

A AMU = 1/12 mass of C-12 atom

Mass number = protons + neutrons (i.e., whole number)

Atomic mass = mass of single atom of element (i.e., usually a decimal)

What is the AN of most atoms of O?

What is the AMN of most atoms of O?

How many protons are in the nucleus of an atom of Cl?

What unit do scientist use to measure the masses of atoms?