Atoms

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3

A small number of elements make up >99% of the solid Earth.

O = oxygen

Na = sodium

Mg = magnesium

Al = aluminum

Si = silicon

S = sulfur

Ca = calcium

Fe = iron

Ni = nickel

Slide 15

The Periodic Table: the Crust

The crust is a little more elementally interesting (again, as a result of differentiation), but it is still mainly made of a small number of elements.

C = carbon

P = phosphorus

K = potassium

Ti = titanium

Mn = manganese

Slide 16

Atomic Weight: It’s all in the Nucleus

Since electrons weigh virtually nothing, the mass of an atom is concentrated in its nucleus.

Each atom can be described by its atomic weight (or mass),

which is the sum of the protons and neutrons.

lithium:

atomic number = 3

3 protons

4 neutrons

atomic weight = 3 + 4 = 7

BUT . although each element has a defined number of protons,

the number of neutrons is not fixed.

Atoms with the same atomic number but variable numbers of neutrons are called isotopes.

Slide 17

Isotopes

Carbon (atomic # 6) has three natural isotopes

with atomic weights of 12, 13 and 14.

isotope #p #n

====== == ==

C-12 6 6

C-13 6 7

C-14 6 8

Tin (Sn, atomic # 50) has ten natural isotopes with atomic masses of 112, 114, 115, 116, 117, 118, 119, 120, 122 and 124. How many protons and neutrons do these isotopes have?

Slide 18

Radioactive or Stable?

Radioactivity is a nuclear phenomenon: it comes as a result of a particular structure in a nucleus.

A radioactive atom is considered unstable. All unstable atoms emit radioactivity (usually by ejecting nuclear particles) in order to reach a stable configuration.

This is the process of radioactive decay

(about which we will talk on 6/9).

So, not all atoms will be radioactive, just a small proportion of isotopes with unstable nuclei.

The bulk of isotopes are stable, or non-radioactive.

Slide 19

Stable and Radioactive Isotopes

Carbon (atomic # 6) has three natural isotopes

with atomic weights of 12, 13 and 14.

isotope #p #n

====== == ==

C-12 6 6

C-13 6 7

C-14 is a radioactive isotope; C-12 and C-13 are stable.

Over time the proportion of C-12/C-14 and C-13/C-14

will increase until there is no C-14.

(unless some process makes new C-14 .)

C-14 6 8

Slide 20

Radioactivity Inside You