Atoms – The Basics

Inside the Atom

Atoms are made up of smaller particles

These particles are found in different regions of the atom

Atomic Number & Mass Number

Atomic number = # p+ For atoms atomic number also =

number of e- Mass number = # p+ + # n0

Mass number is a whole number Number on the periodic table is average

atomic mass (not a whole number) How can you solve for numbers of

neutrons?

How Atoms Differ: Ions Different elements have different numbers

of p+

Ions have charges the number of e- ≠ number of p+ charge = # p+ - # e-

Ions are the SAME element, but DIFFER because they have either a positive or negative charge.

If an atom GAINS electrons, its overall charge becomes more negative.If it LOSES electrons, its charge becomes more positive

Isotopes

Atoms with the same number of p+ but different number of n0 are isotopes.

Mass number of isotopes of the same element changes

Isotopes are the SAME element, but DIFFER because they have different masses.

EX:

Symbols of Ions and Isotopes

For ions chemists use the following notation:

Or just:

MassChargeNumber

AtomicNumber

X57 226Fe

2Fe

For isotopes chemists use the following notation:

Name of element – mass number Examples:

carbon-12carbon-14uranium-236

Determine the Numbers of p+, n0, and e- for the following symbols given:

17 28O

204Hg

37Cl

calcium - 46

nickel - 60

Complete the following table

Protons Neutrons Electrons

Na+

Bromine- 84

O2- with an atomic mass of 13amu

Average Atomic Mass

The masses of p+ and no are the same and they are actually tiny (1.67 x 10-24 g). We round it to 1 amu.

The mass of an e- is even smaller 1/1840 of a proton or neutron so we say it is approximately 0 amu.

We can do this because scientists measure the mass of an atom relative to a standard mass and that is carbon-12.

Scientists agreed that carbon-12 has a mass of EXACTLY 12 atomic mass units (amu).

So, why do the elements on the Periodic Table

have masses with decimals???

They are Average Atomic Masses

Average Atomic Masses

This is the weighted average mass of all the isotopes of an element.

Examples of weighted averages: Semester and yearly grades Taxes Budget

Calculating the Weighted Average Atomic Mass

1. Multiply the mass of each isotope by the % of the isotope

2. Add the products

Example

Calculate the atomic mass of magnesium. The three magnesium isotopes have atomic masses and relative abundances of 23.985 amu (78.99%), 24.986 amu (10.00%) and 25.982 amu (11.01%).

24.31 amu

You can also estimate which isotope is the most abundant.

Ex: Fluorine has an atomic mass of 18.99840 amu. It has several naturally occuring isotopes Fluorine-14, Fluorine-16, Fluorine-19, Fluorine-21, Fluorine-22.

Guess which one is the most abundant? Helium has two naturally occurring

isotopes: helium-3 and helium-4. The atomic mass of helium is 4.003amu. Which isotope is more abundant in nature and why?