Ions and Isotopes

Element Symbols

A properly-formatted element symbol looks like this:

Y

W

Z W is the atomic number X is the element's symbol (C for carbon, Ti for

titanium, etc) Y is the atomic mass (in units of amu—we'll

get to those later) Z is the charge

IMPORTANT: your periodic table has these numbers and symbols crammed wherever some graphic design intern found space for it. Use the legend and learn to tell which is the atomic number, and which is the atomic mass.

X

Atomic Number

This tells you which element it is.

Atomic number = # of protons

The number of protons is the thing that defines an element. If oxygen somehow stops having 8 protons...it's not oxygen anymore.

Y

8

ZX

Atomic Symbol

This ...also tells you what element it is.

You'll not that the number and the symbol are kind of redundant. For that reason, the number is frequently not written in.

It's assumed that you'll be able to figure out on your own that oxygen is element number 8.

Y

8

Z

Y Z

O

O

Atomic MassThis is how much the element weigh in atomic mass units (1 amu = 1.66x10-24 grams).

Each proton weighs 1 amu. Each neutron weighs 1 amu. Electrons are too light to matter. Therefore:

Atomic mass = # of protons + # of neutrons

Our oxygen must have 9 neutrons

IMPORTANT: the atomic mass is always a whole number. It DOES NOT come from the number labeled 'atomic mass' on the periodic table. Nor does it come from rounding that number. We will determine where that decimal comes from later on.

17

8

ZO

ChargeSomewhere along the way, you were likely taught that the number of electrons was the same as the number of protons. This is true...so long as the element is neutral (has no charge).

Charge comes from the balance of protons (positive) and electrons (negative).

This oxygen has a negative 2 charge, so it must have two more negatives (electrons) than positives (protons). Since oxygen has 8 protons (courtesy of the atomic number), this oxygen must have 10 electrons.

Notes: if there is no charge, you can write 0, or just leave it off. If there's just a + or -, the charge is plus or minus one. The convention is to put the number first, but writing '-2' is just fine.

17

8

2-O

Examples14C : 6 protons, 8 neutrons, 6 electrons

19F- : 9 protons, 10 neutrons, 10 electrons

23Na+ : 11 protons, 12 neutrons, 10 electrons

22Na+ : 11 protons, 11 neutrons, 10 electrons

104Pd4+ : 46 protons, 58 neutrons, 42 electrons

**Note that the cations did not gain protons (which would make them a different element); they are missing electrons**

Some VocabularyIon: anything with a charge.

Cation: a positive ion (pronounced 'cat-ion')Anion: a negative ion

Isotopes: the same element, but with a different mass (and therefore a different number of neutrons).

For example, most carbon in your body is 12C. About 1% is 13C. There's a tiny tiny trace of 14C. These are three isotopes of carbon.

Predictable IonsCertain elements, if they have a charge, will always have the same charge. For example, sodium (Na) will always end up as Na+

Others aren't predictable. Mn2+, Mn4+, and Mn7+

are all common.

Everything to the left of the 'steppy' bolded line is a metal (except hydrogen). If these elements have a charge, it will be positive.

Everything to the right (plus hydrogen) is a non-metal. These will typically be negative if they have a charge.

The purple/pink ones are metalloids—elements right on the border that sometimes behave like either metals or non-metals.

If you compare this color-coded periodic table to the one in your assignment book, you should find one very important element that is WRONG WRONG SUPER WRONG in your assignment book.

The alkali metals are always 1+(hydrogen is frequently 1+ as well, but it isn't an alkali metal)

The alkaline metals (or alkaline earth metals) are always 2+

(seems like a pattern)

The stuff in the middle (transition metals and rare earth metals) are unpredictable.

Aluminum and Gallium are 3+ (boron isn't a metal, and In/Tl have some weird stuff going on)

The nitrogen group is 3- (not bismuth; it's a metal!)

Let's fill out the rest of the pattern. Make note of the halogens (1-), and the noble gasses (they don't form charges, or tend to react)

Metals: always positive if they have a charge.Alkali Metals: first column, 1+Alkaline Earth Metals: second column, 2+Al, Ga: 3+Transition metals: everything in the middle. UnpredictableRare earth metals: bottom two rows. Unpredictable.Other metals: sometimes called 'poor metals'. Unpredictable.

Non-Metals: typically negative if they have a charge.N, P: 3-O, S, Se: 2-Halogens: second-to-last column, 1-Noble Gases: last column, no charge. Don't react.

Anything not mentioned: unpredictable or tends not to become an ion at all.

WHY?

Well, it has to do with patterns in electrons. Patterns that we will

discover...