Ch 4 Atoms
(including some PC info)

Dalton developed a theory to explain why the elements in a compound always join in the same way.
Dalton proposed the theory that all matter is made up of individual particles called atoms, which can not be divided.

• All elements are composed of atoms.
• All atoms of the same element have the same mass, and atoms of different elements have different masses.
• Compounds contain atoms of more than one element.
• In a particular compound, atoms of different elements always combine in the same way.

• He used an cathode rays to learn that negatively charged particles came from atoms.
• Thomson’s experiments provided the first evidence that atoms are made of even smaller particles.
• Thomson’s “plum pudding” model suggested that the negative charges in an atom were evenly scattered throughout an atom filled with a positively charged mass of matter.

• Rutherford’s model concluded that all of an atom’s positive charge is concentrated in its nucleus.
• He used the gold foil experiment to determine the behavior of the alpha particles when they struck different locations in the gold foil. He originally thought that the path of the alpha particles would only be slightly bent. He was wrong. Some of the particles bounced straight back.

• Protons are positively charged subatomic particle that are found in the nucleus.
• Electrons are negatively charged subatomic particles that are found in the space outside the nucleus.
• Neutrons are neutral subatomic particles that are found in the nucleus.
• Protons, electrons, and neutrons can be distinguished by mass, charge, and location in an atom.

• The atomic number of an element is the same as the number of protons in an atom of that element.
Atoms of different elements have different numbers of protons.
• The mass number of an atom is the sum of the protons and neutrons in the nucleus of that atom.
To find number of neutrons:
Mass of neutrons = Mass number – Atomic number

H-1
0 n
H-2
1 n
H-3
2 n
Same atomic #, however = 1.

• Bohr’s model focused on the electrons in an atom.
• Bohr’s model stated that electrons move with constant speed in fixed orbits around the nucleus, like planets around a sun.
• When an atom gains or loses energy, the energy of an electron can change.
• Possible energies that electrons in an atom have are called energy levels .
• An electron in an atom can move from one energy level to another when the atom gains or loses energy.

An electron cloud is a visual model of the most likely locations for electrons in an atom.

3
4
1
2
Energy Level # of orbitals Max # of e’s
1 (En. Lev.
Squared)
4
9
16
18
32
2 (2 x # of orbitals)
8

• An electron configuration is the arrangement of electrons in the orbitals of an atom.
• The most stable electron configuration is the one in which the electrons are in orbitals with the lowest possible energies.

I. Organization of the Periodic Table
A. periodic law – properties of elements tend to change in a regular pattern when elements are arranged in order of increasing atomic number.

B. Elements are arranged in periods and groups
1. period – a horizontal row of the elements in the periodic table a. another name for periods are valences

2. Every atom has valence electrons a. Electrons are arranged in energy levels. i. Each energy level can hold only a specific number of electrons ii. Electrons start at the energy level closest to the nucleus and fill up one energy level after another.
b. valence electron – an electron in the outermost energy level of an atom

B. Elements are arranged in periods and groups
1. period – a horizontal row of the elements in the periodic table a. Another name for periods are valences b. Each period has a number, which refers to how many energy levels are in that element.

2. group (family) – a vertical column of the elements in the periodic table a. Groups of elements in the periodic table, have similar properties b. Valence electrons largely effect chemical and physical properties.
c. Elements in the same group have the same number of valence electrons.

II. Ions!!!
A. Ion – an atom that has lost or gained one or more electrons and therefore has a net electric charge.

E. Atoms have no overall charge
1. An un-reacted, elemental atom has the same number of protons as electrons
2. Each proton has 1 positive charge and each electron has 1 negative charge.
3. Positive charges and negative charges cancel each other out.

II. Ions!!!
A. Ion – an atom that has lost or gained one or more electrons and therefore has a net electric charge.
B. Ionization – the process of adding or losing one or more electrons from an atom
C. Atoms want all of their energy levels to be full. Adding or losing electrons helps this to happen.

C. Ions have charge!
1. cation – an ion that has given up one or more electrons
18 neutrons = 0 charge a. Cations have more protons than electrons.
- 1 charge b. Cations have an overall positive charge.
2. anion – an ion that has gained one or more electrons electrons.
12 neutrons = 0 charge b. Anions have an overall negative charge.
+ 1 charge

III. Differences in atomic structure
A. Atomic number
1. atomic number – the number of protons in an atom
2. Each element has its own atomic number.

B. Atomic mass or weight
1. atomic mass – the mass of an atom of a given element
2. The average atomic mass is never a whole number.
C. Isotopes!!
1. isotopes – any atoms having the same number of protons but different numbers of neutrons
C

2. average atomic mass – the mass of the average atom of a given element a. Atomic mass unit (amu) – a unit equivalent to 1/12 the mass of a carbon-
12 atom (~1 proton or neutron) b. It reflects number of protons and neutrons in naturally occurring elements .
12 protons
+12 neutrons
24 amu
12 protons
+13 neutrons +14 neutrons
25 amu
12 protons
26 amu
25
Mg
26
Mg
78.65% x 24 amu
10.14% x 25 amu
+ 11.13% x 26 amu
24.305 amu
24
Mg

• A mole of atoms contains Avogadro’s number of atoms: 6.02 x 10 23 atoms
• Used to convert mass <-> moles <-> number of atoms or particles.
• From the periodic chart, the atomic mass tells the number of grams in a mole of an element.
• For O, 15.999 g/mol
• For P, 30.974 g/mol

• Find the mass of oxygen in 2.7 mol.
2.7 mol 15.999g mol
2.7 x 15.999 =
4.3197 =
4.3 g O (2 sig figs because of 2.7)

• Find the number of moles in 230.0 g of O.
230.0g mol
15.999g
230.0 =
15.999
1.4376
=
1.438 mol O (4 sig figs because of 230.0)