Chapter 3 https://youtu.be/thnDxFdkzZs?list=PL8dPuuaLjX

tPHzzYuWy6fYEaX9mQQ8oGr

The smallest particle of an element that retains the chemical properties of that element.

Regions: ◦ Nucleus: very small region located at the center of atom ◦ Outside the nucleus: electrons most in the “electron cloud”

Subatomic Particles: ◦ Neutron, neutral found in nucleus (mass number subtracted

by number of protons indicates neutrons) ◦ Proton, positive found in nucleus (atomic number indicates

number of protons in the atom) ◦ Electron, negative found surrounding the nucleus (equal

number of protons and electrons)

Dalton’s atomic theory explains all three of these laws. ◦ All elements were composed of atoms and that only whole

numbers of atoms can combine to form compounds. The following statements sum up his theory: All matter is composed of extremely small particles called atoms. Atoms of an element are identical in size, mass, and other

properties. 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.

Several changes have been made to Dalton’s theory… ◦ Dalton said…Atoms of an element are identical in size,

mass, and other properties. ◦ Modern Theory states: Atoms of an element have the

same number of protons (atomic number) but can have different masses (because of the different number of neutrons)

◦ Dalton said…Atoms cannot be subdivided, created, or

destroyed. ◦ Modern Theory states: Atoms cannot be subdivided,

created, or destroyed in ordinary chemical reactions. However, these changes CAN occur in nuclear reactions!

Late 1800s, JJ Thomson, used an cathode ray tube to determine the existence of negative electrons by passing an electric current passed through low pressure gases. ◦ Cathode-ray tube experiment Cathode (- charged metal disk side of the tube) When electricity was passed through the tube, the

directly opposite side of the cathode would glow. The assumed the glow was due to the particle stream

and called this glow a “cathode ray.” Particles that compose cathode rays are negatively

charged (since they were attracted to the + anode end) Ultimately, this is how an electron was found!

Ernest Rutherford’s Gold Foil Experiment ◦ Fired helium nuclei (alpha particles) at a thin sheet

of gold foil ◦ A screen record where they hit

Most of the particles passed right through A few particles were deflected

• Only about 1 in 8000 alpha particles bounced off the gold foil.

VERY FEW were greatly deflected ◦ Conclusion: The nucleus is small The nucleus is dense The nucleus is positively charged

Protons Positive charge equal in magnitude to the negative charge of

an electron. 1.673 x 10-27 kg

Neutrons

Electrically neutral charge. 1.675 x 10-27 kg Neutrons are slightly larger in mass than protons

The nuclei of atoms of different elements differ

in their number of protons, thus the number of protons determines the atom’s identity.

Generally, particles that have the same charge repel one another.

More than one proton in the nucleus you would expect to be unstable. However, it’s the opposite. When two or more protons are extremely close to each other, they have a strong attraction.

Nuclear force is known as short-range proton-neutron, proton-proton, and neutron-neutron forces hold the nuclear particles together.

Developed the planetary model and proposed that the electrons orbit around the nucleus in a circular part at a constant speed. ◦ Incorrect!!

Quantum Mechanical Model ◦ The model we use today!! ◦ Also called the electron cloud model. ◦ The electrons don’t have a distinctive orbit, they

have a probability region where they can be found. Think about a fly buzzing around the room…very

similar to how an electron would act!

Charge Mass Location

Proton

+

1 amu

Nucleus

Neutron

None

1 amu

Nucleus

Electron

-

Almost nothing…

Outside Nucleus (in motion)

For any element:

# of Protons = Atomic #

# of Electrons = # of Protons = Atomic #

# of Neutrons = Mass # - Atomic #

Atoms of different elements have different number of protons.

The atomic number of an element is the number of protons of each atom of that element.

Hydrogen, H, has atomic number 1. ◦ One proton in each atom of Hydrogen.

Atoms of the same element that have different masses. Remember, protons and neutrons make up the

majority of the mass of an atom. All isotopes of hydrogen have the same number of

protons and electrons but vary in neutrons.

Most elements consist of isotope mixtures.

Tin has 10 stable isotopes, the most of any element.

Just because every atom of Hydrogen has one proton…it doesn’t mean they have the same number of neutrons.

Actually, three types of Hydrogen atoms exist!

Protium ◦ Accounts for 99.9885% of

hydrogen atoms on Earth ◦ No neutrons, 1 proton ◦ Total mass of 1

Deuterium ◦ Accounts for 0.0115% of

hydrogen atoms on Earth ◦ 1 neutron, 1 proton ◦ Total mass of 2

Tritium ◦ Radioactive and is rarely

found on Earth only in trace amounts.

◦ 2 neutrons, 1 proton ◦ Total mass of 3

Two Methods: ◦ Hyphen-Notation – where the mass number appears after

the element with a hyphen.

◦ Nuclear-Symbol – the composition that shows the mass number and atomic number Nuclide – is a general term for a specific isotope of an element

Mass is neither created nor destroyed during ordinary chemical reactions or physical changes.

A chemical compound contains the same elements in exactly the same proportions by mass regardless of the size of the sample or source of the compound.

If two or more different compounds are composed of the same two elements, then the ratio of the masses of the second element combined with a certain mass of the first element is always a ratio of small whole numbers.

Scientists use the relative atomic masses because they are most convenient.

Standard for this measure is the carbon-12.

The weighted average of the atomic masses of the naturally occurring isotopes of an element.

Depends on mass and relative abundance

Calculated by multiplying the atomic mass of each isotope by its relative abundance and adding the results.

Suppose you have a box containing two sizes of marbles. If

25% of the marbles have masses of 2.00 g each and 75% have masses of 3.00 g each, how is the weighted average calculated? Suppose you have 100 marbles.

25% of 100 = 25 marbles 75% of 100 = 75 marbles 25 marbles x 2.00 g = 50 g 75 marbles x 3.00 g = 225 g 50 g + 225 g = 275 g ÷ 100 marbles = 2.75 g per marble

What chemical laws can be explained by Dalton’s theory?

Three compounds containing potassium and oxygen are compared. Analysis shows that for each 1.00 g of O, the compounds have 1.22 g. 2.44 g, and 4.89 g of K, respectively. Show how these data support the law of multiple proportions.