Chemistry Chapter 4 Notes

What you need to know from 4.1-4.2

• Mendeleev organized the periodic table first and put the elements in order of increasing atomic mass

• Moseley reorganized it by atomic number• Thompson discovered electrons by bending

them in a cathode ray tube towards a charged plate.

• Rutherford shot “alpha particles” at gold foil and most went through and concluded the atom is mostly empty space with a nucleus in the center

Section 4.3: How Atoms Differ

1) The number [#] of protons in each atom is different.

[This means each atom can be identified by the number of protons it has,] and so the # of protons is called the “atomic number”.

A) The atomic number is the new way that we organize the periodic table [elements go from 1 to 118].

B) [The atomic number is the whole number often listed near the top of the boxes of the periodic table]

*draw picture

C) [Because single atoms are totally neutral in charge, we must have the same number of electrons as we do protons. This means] the atomic number = number of protons = number of electrons.

2) [It has also been discovered that atoms of the same element (having the same atomic number) sometimes have different masses.] Atoms of the same element but with different masses were called isotopes.

A) Isotopes have a different number of neutrons in their nucleus than usual. This could make the element lighter or heavier, but it is still the same element.

B) The mass of an atom is the sum of the protons and neutrons, and is called the mass number. [Thus the mass number of isotopes can change as the number of neutrons changes.]

C) The number of neutrons = mass number(round if not whole) - atomic number (number of protons)

*What is the number of neutrons of carbon with a mass number of 12, 13, and 14?

What is the number of neutrons of carbon with a mass number of 12, 13, and 14?

12 - 6 = 613 - 6 = 714 - 6 = 8

D) To symbolize a specific isotope, for example carbon with mass of 14, we can say carbon-14 or

14

C6

E) [In nature, the isotopes are mixed in with the normal atoms, and they usually behave similarly ]

i) [For use in usual chemistry experiments, we use the average atomic mass. This is usually the decimal number listed in the bottom of the boxes of the periodic table.] Average

Atomic Mass* Draw

Picture

ii) [The average atomic mass is not whole because it is the avg. of masses. There are more of one isotope than others, so its not even.]

iii) The whole number closest to the average atomic mass is the isotope that has the largest abundance.

*What is the mass of the isotope of oxygen that is in the largest abundance?

The average atomic mass is 15.9994, and some of the isotopes of oxygen are oxygen-15, oxygen-16, and oxygen-18.

What is the mass of the isotope of oxygen that is in the largest abundance?

The average atomic mass is 15.9994, and some of the isotopes of oxygen are oxygen-15, oxygen-16, and oxygen-18.

Oxygen-16 is in greatest abundance

Section 4.4: Unstable Nuclei and Radioactive Decay

1) The unusual number of neutrons in an isotope can make it unstable [(unusually high in energy). This often results in the isotope changing from one element into another element in an attempt to become more stable (lower in energy).]

A) Reactions where atoms change into other atoms are called nuclear reactions, [as they involve changes in the nucleus.]

B) During these nuclear reactions, rays and particles are given off, which is called radiation, or radioactive decay

2) [When radioactive decay occurs, there are three different types of radiation that can be given off. Each type has a different mass, and sometimes a charge.]

A)The first type of radiation-alpha radiation.

i) [The element Helium has a mass of 4 and an atomic number of 2, so] the alpha particle is just like a helium atom without any electrons;

4

2 He4

2 α

B) The second type of radiation - beta radiation.

i) The electron has a mass of zero and a charge of -1, so the beta particle is just like an electron;

0

-1 β

C) The last type of radiation - gamma radiation.

i) The gamma particle was the last to be found because it has no mass and no charge;

0 γ 0