Chapter 11

The Atom

The Atom

• The smallest particle into which an element can be broken down.

• size: 3x10-8 cm = .00000003 cm

3 hundred millionths cm

• Structure - Subatomic particles located in the nucleus and the electron cloud

Nucleus

• small, dense, positively charged center of the atom

Contains:+ Protons – positively charged particles

• Mass: 1amu (atomic mass unit) = 1.7x10-24 g

0 Neutrons – no charge• Mass: 1 amu

• Most of the mass of an atom is found in the nucleus.

• The volume is small which is why it is dense.

Electron Cloud

• mostly empty space.

Contains:– Electrons – negatively charged particles

• Mass: 0 amu

• Can only predict where the electrons can be found around the nucleus.

• Most atoms have an equal number of protons and electrons.

Forces in the Atom

• Electromagnetic force – holds the electrons around the nucleus.

Negative electrons are attracted to the positive protons, opposites charges attract.

• Strong force – holds the nucleus together. Stronger than the electromagnetic force that pushes the protons apart, like charges repel.

• Weak Force – plays a key role in radioactive atoms where neutrons turn into protons and electrons

Atomic Number

• Number of protons in the nucleus

• Determines the elements

• Found in each elements square on the periodic table.

Isotopes

• Atoms of the same element with a different number of neutrons.

• You can tell them apart by their mass numbers

• Mass number = number of protons plus neutrons

Naming Isotopes

• Element name – mass number

Hydrogen – 1

(1 proton + no neutrons =1)

Hydrogen – 2

(1 proton + 1 neutron = 2)

Number of Neutrons in an Isotope

# neutrons = mass # – atomic #

p + n – p = n

Unstable Isotopes

• Some isotopes are unstable making them radioactive (break apart over time)

Carbon – 14 used in carbon dating

Uranium – 238 used in nuclear chain reaction

Mass

• average of all the known naturally occurring isotopes of an element.

WeightAtomic

Mass

Copper’s atomic mass = 63.6 amu

Known Isotopes: copper – 63 (69%)

copper – 65 (31%)

Equation:mass# X percentage + mass# X percentage +.…= atomic mass

63 amu (.69) + 65 amu (.31) = 63.6 amu

WeightCalculating Atomic

Try These

Chlorine =

Chlorine – 35 (76%)

Chlorine – 37 (24%)

Thallium =

Thallium – 203 (30%)

Thallium – 205 (70%)

Gallium =

Gallium – 69 (60%)

Gallium – 71 (40%)

Uranium =

Uranium – 238 (99.284%)

Uranium – 235 (0.711%)

Uranium – 234 (0.005%)

Titanium =

Titanium – 46 (8%)

Titanium – 47 (7.3%)

Titanium – 48 (73.8%)

Titanium – 49 (5.5%)

Titanium – 50 (5.4%)

35.5 amu

204.4 amu

69.8 amu

237.978 amu

47.9 amu

Ions• Atoms with a charge because they gained or lost an

electron.• Have an unequal number of protons and electrons

charge = protons – electrons

Positive Ion Sodium atom has 11 protons Sodium Ion has 10 electrons +11 – 10 = +1

Negative Ion Chlorine atom has17 protons Chlorine Ion has 18 electrons +17 – 18 = – 1