The Modern Model

The modern atom is composed of two regions:

1. Electron cloud- Most of the volume of an atom and is the region where the electron can be found (extranuclear)

2. The nucleus – with protons and neutrons so that it has a Positive charge and almost all the mass

Major Subatomic Particles

• Atoms are measured in picometers, 10-12 meters• Hydrogen atom, 32 pm radius

• Nucleus is tiny compared to atom• If the atom were a stadium, the nucleus would be a marble

• Radius of the nucleus is on the order of 10-15 m• Density within the atom is near 1014 g/cm3

Name Symbol Charge Relative Mass (amu)

Actual Mass (g)

Electron e- -1 1/1840 9.11x10-28

Proton p+ +1 1 1.67x10-24

Neutron no 0 1 1.67x10-24

Nuclear Symbols• Every element is given a corresponding symbol which is composed of 1 or 2 letters (first letter upper case, second lower), as well as the mass number and atomic number

E A

Z

elemental symbolelemental symbol

mass numbermass number

atomic numberatomic number

Elemental Classification• Atomic Number (Z) = number of protons (p+) in the nucleus

• Determines the type of atom• Li atoms always have 3 protons in the nucleus, Hg always 80

• Mass Number (A) = number of protons + neutrons [Sum of p+ and nº]

• Electrons have a negligible contribution to overall mass

• In a neutral atom there is the same number of electrons (e-) and protons (atomic number)

• Find the Find the • number of protonsnumber of protons• number of neutronsnumber of neutrons• number of electronsnumber of electrons• atomic numberatomic number• mass numbermass number

W184 74F

199 Br

80 35

• If an element has an atomic number of 34 and a If an element has an atomic number of 34 and a mass number of 78 what is the:mass number of 78 what is the:

• number of protons in the atom?number of protons in the atom?• number of neutrons in the atom?number of neutrons in the atom?• number of electrons in the atom?number of electrons in the atom?• complete symbol of the atom?complete symbol of the atom?

• If an element has 91 protons and 140 neutrons If an element has 91 protons and 140 neutrons what is the:what is the:

• atomic number?atomic number?• mass number?mass number?• number of electrons?number of electrons?• complete symbol?complete symbol?

Isotopes• Atoms of the same element can have different numbers of neutrons and therefore have different mass numbers

• The atoms of the same element that differ in the number of neutrons are called isotopes of that element

H11

Protium

H21Deuterium

H31

Tritium

Naming Isotopes

• When naming, write the mass number after the name of the element

• Examples:

• carbon- 12

• carbon -14

• uranium-235

Review:

1. Write the nuclear symbols ( ) and name including mass for three isotopes of oxygen in which there are 8, 9, and 10 neutrons respectively.

C126

Ions

• When atoms gain or lose electrons they form something called IONS. Energy is released when this happens.

• Ions are electrically charged atoms. (positive or negative)

Cations (+)

• CATIONS: are positively charged ions. Cations are formed when an atom loses electrons.

• Metals and Hydrogen form cations

• Ex: Na lose 1 electron

Na1+

Anions (-)

• ANIONS: are negatively charged ions. Anions are formed when atoms gain electrons.

• Most non-metals form anions.

• Ex: Cl gains one electron

Cl1-

Charges of Ions

Group 1 loses 1 electron = +1Group 2 loses 2 electrons = +2Group 13 loses 3 electrons = +3Group 15 gains 3 electrons = -3Group 16 gains 2 electrons = -2Group 17 gains 1 electron = -1

Group18 has a full outer shell = very stable

Noble Gases

• Group 8A (He, Ne, Ar, etc.)

• Have a full valence shell (complete octet)

• They neither want to gain or lose electrons

• Therefore, DO NOT form IONS

(+) charge = subtract electrons from # protons

(-) charge = add electrons from # protons

Only the number of electrons can change:

How many protons and electrons do each of the following have?

a) O-2

b) Br-1

c) Ca+2

d) Se-2

e) Mg+2

f) Au3+

g) P-3

h) Mn+4

Average Atomic Mass

• Did you ever notice that the mass number on the periodic table is not a whole number?

• There are the decimal numbers on the periodic table.

**This is because it is an average mass for all the isotopes of each atom

How heavy is an atom of oxygen?•There are different kinds of oxygen atoms (different isotopes)

• 16O, 17O, 18O

• We are more concerned with average atomic masses, rather than exact ones

• Based on abundance of each isotope found in nature

• We can’t use grams as the unit of measure because the numbers would be too small

• Instead we use Atomic Mass Units (amu)• Standard amu is 1/12 the mass of a carbon-12 atom

• Each isotope has its own atomic mass

Calculating AveragesYou have five rocks, four with a mass of 50 g, and one with a mass of 60 g. What is the average mass of the rocks?

Total mass = 4 x 50 g + 1 x 60 g = 260 g

Average mass = (4 x 50 g + 1 x 60 g)/5 = (260 g)/5

= (4 x 50 g)/5 + (1 x 60 g)/5 = 260 g/5

Average mass = 0.8 x 50 + 0.2 x 60

80% of the rocks were 50 grams

20% of the rocks were 60 grams

Average = (% as decimal) x (mass1) + (% as decimal) x (mass2) +

(% as decimal) x (mass3) + …

Calculating Averages

• Calculate the av atomic mass of copper if copper has two isotopes

• 69.1% has a mass of 62.93 amu• The rest (30.9%) has a mass of 64.93 amu

• Magnesium has three isotopes• 78.99% magnesium 24 with a mass of 23.9850 amu• 10.00% magnesium 25 with a mass of 24.9858 amu• The rest magnesium 26 with a mass of 25.9826 amu• What is the atomic mass of magnesium?

Calculating Averages

• If not told otherwise, the mass of the isotope is the mass number in amu

• The average atomic masses are not whole numbers because they are an average mass value

• Remember, the atomic masses the decimal numbers on the periodic table

Average Atomic Masses