chemistry chapter 4 notes section 4.3: how atoms differ

Chemistry Chapter 4 Notes

Section 4.3: How Atoms Differ

Particle Symbol Location Charge Mass

Proton

Neutron

Electron


Proton p+

Neutron

Electron


Proton p+ Inside Nucleus

Neutron

Electron



+1

Neutron

Electron



+1 1 (a.m.u.)

Neutron

Electron



+1 1 (a.m.u.)

Neutron N0

Electron



+1 1 (a.m.u.)

Neutron N0 Inside Nucleus

Electron



+1 1 (a.m.u.)


0

Electron



+1 1 (a.m.u.)


0 1 (a.m.u.)

Electron



+1 1 (a.m.u.)


0 1 (a.m.u.)

Electron e-



+1 1 (a.m.u.)


0 1 (a.m.u.)

Electron e- Outside Nucleus



+1 1 (a.m.u.)


0 1 (a.m.u.)


-1



+1 1 (a.m.u.)


0 1 (a.m.u.)


-1 0 (a.m.u.)

1) Once the particles that make up the atom were discovered, Henry Moseley (1887-1915) discovered that the number of protons (positive charge) in each atom was different.

This meant each atom could be identified by the number of protons it had, and so this number was 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

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 was also discovered that atoms of the same element (having the same atomic number) sometimes had different masses. These 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 - 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 it usually doesn’t matter which isotope we have.

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

ii) The average atomic mass is a weighted average of the mass of the isotope, and the percent abundance in nature. Average Atomic Mass = (Mass of Isotope 1 x % abundance) + (Mass of Isotope 2 x % abundance) + …

What is the average atomic mass of element J?

Isotope Mass Abundance63J 62.93 69.17 %65J 64.93 30.83 %

63J = 62.93 × .6917 =65J = 64.93 × .3083 =

63J = 62.93 × .6917 = 43.53

65J = 64.93 × .3083 = 20.12

63J = 62.93 × .6917 = 43.53

65J = 64.93 × .3083 = 20.12

63.55

Element J’s average atomic mass is 63.55

Section 4.4: Unstable Nuclei and Radioactive Decay

1) When referring to nuclear reactions people commonly think of nuclear fission (the splitting of large atoms into smaller pieces) and nuclear fusion (the combining of small atoms into one large one), but on earth these reactions do not occur naturally.

2) Naturally occurring nuclear reactions result from the unusual number of neutrons of an isotope which makes 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) These reactions are called nuclear reactions, as they involve changes in the nucleus. Most common nuclear reactions involve fission, a large atom breaking apart into smaller pieces.

A)These reactions 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.

C) Sometimes an unstable nucleus will change into many different elements as it tries to become more stable. This is called radioactive decay.

3) 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 to be discovered was called alpha radiation and came from alpha particles.

i) Because the mass numbers must be equal, 226 = 222 + x. So the mass of the alpha particle must be 4.

ii) Because the atomic numbers must be equal, 88 = 86 + x. So the atomic number of the alpha particle must be 2.

iii) 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 He

B) The second type of radiation to be discovered was called beta radiation and came from beta particles.

i) Because the mass numbers must be equal, 14 = 14 + x. So the mass of the beta is zero.

ii) Because the atomic numbers must be equal, 6 = 7 + x. So the atomic number must be -1.

iii) 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 to be discovered was called gamma radiation and came from gamma particles.

i) Because the mass numbers must be equal, 238 = 234 + 4 + x. So the mass of the gamma particle must be zero.

ii) Because the atomic numbers must be equal, 92 = 90 + 2 + x. So the atomic number must also be zero.

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

00γ

Geiger Counter

Alpha, Beta, and GammaRadiation

chemistry chapter 4 notes section 4.3: how atoms differ

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