basic-chemistry

This is a page about basic chemistry. Here are some helpful hints about studying this material:

Realize that we are just talking about adding and subtracting, when there is math on this page.

You will focus on several simple rules.

We don't have to cover all of chemistry here, just the basics.

There aren't really that many elements you have to learn about in introductory geology, and you'll understand a great deal.

Protons, Neutrons, and Electrons

The atom is the basic structure of chemical elements. Atoms are made of:

protons, with a + charge

neutrons, neutral in charge

electrons, with a - charge

Protons and neutrons, which are about the same size, are each much larger than electrons, which are tiny .

Protons and neutrons form the nucleus of an atom, in a dense clump.

Electrons are located around the nucleus in a structured cloud.

Most of the volume in an atom is empty space within the outer area where the electrons are located. Atoms have a "size" formed by the outside boundary of electron positions.

Carbon Atom

In the illustration above, electrons are shown in what may be called "orbits" around the nucleus. They really don't move that way, but they do move. At any instant, if you could say "stop action!," the electrons would be occupying specific regions around the nucleus, leading to the description that the electrons form a kind of structured cloud around the nucleus. We will return to the topic of the electrons when we talk about bonding in the next section, but in the following discussion we focus on the nucleus.

Protons and Element Names

The number of protons in an atom determines its name. For example, for some geologically important elements:

# protons	element	# protons	element
`1`	Hydrogen (H)	...
`2`	Helium (He)	`22`	Titanium (Ti)
`3`	Lithium (Li)	...
`4`	Beryllium (Be)	`26`	Iron (Fe)
`5`	Boron (B)	...
`6`	Carbon (C)	`28`	Nickel (Ni)
`7`	Nitrogen (N)	`29`	Copper (Cu)
`8`	Oxygen (O)	`30`	Zinc (Zn)
`9`	Fluorine (F)	...
`10`	Neon (Ne)	`37`	Rubidium (Rb)
`11`	Sodium (Na)	`38`	Strontium (Sr)
`12`	Magnesium (Mg)	...
`13`	Aluminum (Al)	`80`	Mercury (Hg)
`14`	Silicon (Si)	...
`15`	Phosphorus (P)	`82`	Lead (Pb)
`16`	Sulfur (S)	...
`17`	Chlorine (Cl)	`92`	Uranium (U)
`18`	Argon (Ar)	...
`19`	Potassium (K)
`20`	Calcium (Ca)

Periodic Table of the Elements

Periodic Table

The periodic table lists the elements in order of proton count, starting at upper left with hydrogen, going across to helium, then row by row, increasing toward lower right. The proton count is also called the atomic number, or Z. We'll talk about the periodic table later in more detail, but for now, simply learn that it is the proton count that is the key to an atom's identity.

Symbols

When you see symbols such as ³ He, ⁶ Li, ... ¹² C, ¹⁴ N, ¹⁶ O..., the number to the upper left refers to the mass number. The mass number is the sum of the count of protons and neutrons in the nucleus. These could also have been written as helium-3, lithium-6, ... carbon-12, nitrogen-14, oxygen-16, etc. Note: The numbers in the periodic table above are for the atomic number, which is the number of protons. You will note that the numbers in the periodic table are simply posted above the element symbol within the given square.

So, don't confuse this:

atomic number = number of protons
mass number = sum of the number of protons and neutrons

Take carbon, for example. You see in the table above and on the periodic table that carbon has 6 protons. When you see ¹² C or carbon-12, you know the 12 refers to the mass number, the sum of the number of protons and neutrons. So, how many neutrons does carbon-12 have? 12 (mass number) - 6 (# protons, or atomic number) = 6 (# neutrons).

And for oxygen, we see above that oxygen has 8 protons. So, when you see oxygen-16 or ¹⁶ O, you can get the number of neutrons: 16 (mass number) - 8 (# protons, or atomic number) = 8 (# neutrons).

Isotopes

There can be a different number of neutrons in atoms of an element, leading to unique combinations of proton and neutron counts. These unique atomic configurations of an element are called its isotopes.

Proton and neutron counts for the first 10 elements, in the most commonly occuring forms, are shown in the following table:

element:	hydrogen	helium	lithium	beryllium	boron	carbon	nitrogen	oxygen	fluorine	neon
# protons:	`1`	`2`	`3`	`4`	`5`	`6`	`7`	`8`	`9`	`10`
# neutrons:	¹ H `0`	³ He `1`	⁶ Li `3`	⁹ Be `5`	¹⁰ B `5`	¹² C `6`	¹⁴ N `7`	¹⁶ O `8`	¹⁹ F `10`	²⁰ Ne `10`
	² H `1`	⁴ He `2`	⁷ Li `4`	¹⁰ Be `6`	¹¹ B `6`	¹³ C `7`	¹⁵ N `8`	¹⁷ O `9`		²¹ Ne `11`
	³ H `2`					¹⁴ C `8`		¹⁸ O `10`		²² Ne `12`

And for the next 10 elements:

element:	sodium	magnesium	aluminum	silicon	phosphorus	sulfur	clorine	argon	potassium	calcium
# protons:	`11`	`12`	`13`	`14`	`15`	`16`	`17`	`18`	`19`	`20`
# neutrons:	²³ Na `12`	²⁴ Mg `14`	²⁷ Al `14`	²⁸ Si `14`	³¹ P `16`	³² S `16`	³⁵ Cl `18`	³⁶ Ar `18`	³⁹ K `20`	⁴⁰ Ca `20`
		²⁵ Mg `13`		²⁹ Si `15`		³³ S `17`	³⁶ Cl `19`	³⁸ Ar `20`	⁴⁰ K `21`	⁴² Ca `22`
		²⁶ Mg `14`		³⁰ Si `16`		³⁴ S `18`	³⁷ Cl `20`	⁴⁰ Ar `22`	⁴¹ K `22`	⁴³ Ca `23`
						³⁶ S `20`				⁴⁴ Ca `24`
										⁴⁶ Ca `26`
										⁴⁸ Ca `28`

So, from this you appreciate that, given a single atom with a given number of protons and neutrons, you can do this:

Look at the number of protons and identify the element.
Look at the sum of the number of protons and neutrons (which is the mass number) and identify the particular isotope of the element.
Then look up the characteristics of the isotope, including whether or not it is radioactive. There are several very nice websites that let you look up this information easily.

Radioactivity

Some isotopes of a given element may be radioactive, or unstable. This means that some change in the proton and/or neutron count can happen as a result of several types of decay, and because the proton and/or neutron count changes, the atom "transforms" into another isotope. In the tables below for examples, you will see phrases like "decays to x in y yrs", and you will see the term half-life. These are ways of describing the changes that happen over time to unstable isotopes; some decay very, very rapidly, on the order of seconds, while others decay at slower rates of a few to thousands of years, while some decay at very, very slow rates on the order of millions or billions of years. We will return to the topic of radioactivity when we discuss age dating, but for now just appreciate that some isotopes are unstable, while others are stable.

Some Examples

Here is the kind of isotopic information you can easily find on the web:

Hydrogen

Hydrogen has one proton. It exists in three isotopes, as follows:

# protons	# neutrons	isotope	symbol	abundance	radioactivity
`1`	`0`	hydrogen-1*	¹ H	`99.985%`	stable
`1`	`1`	hydrogen-2*	² H	`0.0115%`	stable
`1`	`2`	hydrogen-3*	³ H	trace	decays to ³ He in 12.32 yrs

hydrogen-1 is normal hydrogen. hydrogen-2 is called deuterium. hydrogen-3 is called tritium. Note the very short half-life of tritium. Tritium can be used to date bottles of wine, that are perhaps (reportedly) 100 years old or so, just to make sure the bottle of wine isn't a fake.

Carbon

Carbon has 6 protons. Carbon exists in two common isotopes, carbon-12 and carbon-13, and one rare isotope, carbon-14, as follows:

# protons	# neutrons	isotope	symbol	abundance	radioactivity
`6`	`6`	carbon-12	¹² C	`98.9%`	stable
`6`	`7`	carbon-13	¹³ C	`1.1%`	stable
`6`	`8`	carbon-14	¹⁴ C	`0.0000000001%*`	decays to ¹⁴ N in 5730+-40 yrs

* that's about one part in a trillion; Note that 98.9% ( ¹² C) + 1.1% ( ¹³ C) = 100%, such that any other isotope of carbon, like ¹⁴ C, is going to have very low abundance.

Carbon-14 is used in carbon dating, which works to back about 50,000 - 70,000 years. For fossils or other carbon-containing materials older than that, the carbon-14 has decayed away.

Experimentally, other isotopes of carbon can be created: ⁹ C, ¹⁰ C, ... ¹⁵ C, ¹⁶ C, etc. These other isotopes are unstable (radioactive) and have very rapid decay rates of just a few seconds or so.

Magnesium

Magnesium has 12 protons. Magnesium exists in three stable isotopes, as follows:

# protons	# neutrons	isotope	symbol	abundance	radioactivity
`12`	`12`	magnesium-24	²⁴ Mg	`78.99%`	stable
`12`	`13`	magnesium-25	²⁵ Mg	`10%`	stable
`12`	`14`	magnesium-26	²⁶ Mg	`11.01%`	stable

Chlorine

Chlorine has 17 protons. At least nine isotopes have been recognized, ranging from ³² Cl to ⁴⁰ Cl. Most of these are synthetic, having been observed through lab work only, and these are very unstable. We know that each chlorine isotope has 17 protons, and doing the math, we see that ³² Cl to ⁴⁰ Cl would have 15 to 23 neutrons, as follows:

# protons	# neutrons	isotope	symbol	abundance	radioactivity
`17`	`15`	chlorine-32	³² Cl	`*`	very rapid decay
`17`	`16`	chlorine-33	³³ Cl	`*`	very rapid decay
`17`	`17`	chlorine-34	³⁴ Cl	`*`	very rapid decay
`17`	`18`	chlorine-35	³⁵ Cl	`75.8%`	stable
`17`	`19`	chlorine-36	³⁶ Cl	`*`	decays to ³⁶ S and ³⁶ Ar in 308,000 yrs
`17`	`20`	chlorine-37	³⁷ Cl	`24.2%`	stable
`17`	`21`	chlorine-38	³⁸ Cl	`*`	very rapid decay
`17`	`22`	chlorine-39	³⁹ Cl	`*`	very rapid decay
`17`	`23`	chlorine-40	⁴⁰ Cl	`*`	very rapid decay

* These other isotopes are extremely rare or synthetic (lab-created only). ³⁶ Cl can be used in age dating in the range of 60,000 to 1 million years.

Potassium

Potassium has 19 protons. Potassium exists in two stable isotopes and one very long-lived isotope, as follows:

# protons	# neutrons	isotope	symbol	abundance	radioactivity
`19`	`20`	potassium-39	³⁹ K	`93.26%`	stable
`19`	`21`	potassium-40	⁴⁰ K	`0.012%`	decays to ⁴⁰ Ar in 1,277,000,000 yrs'*'
`19`	`22`	potassium-41	⁴¹ K	`6.73%`	stable

* Potassium-40 is very important in age dating. That half-life is 1.277 billion years, a very, very slow decay rate. The significance of the slow rate is that even for very old rocks and minerals, there is still enough original potassium-40 left over to measure with our atom-counting instruments, so we can do age dating.

Web Resources

The photograph in the banner image is from a nice box of minerals I used in teaching.