Atomic Theory• Atom

– smallest part of an element with that element’s properties

• 2 parts– Nucleus

• Protons (positive charge, weigh 1 amu*)• Neutrons (neutral charge, weigh 1 amu*)

– Orbitals (inside electron cloud)• Electrons (negative charge, negligible mass)

• (* amu = atomic mass unit)

A note about electrons• They don’t all have the same amount of

energy.• The farther away from the nucleus they are,

the more energy they have.• We don’t know exactly where they are, we

just have an idea. They move fast, like the blades of a fan. We know they’re there somewhere.

• Outermost ones are called valence electrons. They are responsible for how elements react with each other and the physical and chemical properties.

Electrons• Electron cloud is split into energy

levels that hold different amounts of electrons.– 1st one holds 2 (maximum)– 2nd one holds 8– 3rd one holds 18– 4th one holds 32

• Electrons fill the lowest one first• The energy levels are split into

orbitals, called s, p, d and f.

So it gets complicated•Really the 8 electrons in the second energy level are 2 in the s orbital and 6 in the p orbital•And the 18 in the third

energy level are 2 in s, 6 in p and 10 in d orbital…

s, p, d, f orbitals• You can see this pattern on the

table!

Do we have to know this?

• But you’ll hear more about that in chemistry. For now realize that each energy level is made up of sublevels that hold specific amounts of electrons.

• The sublevels are called orbitals and are named s, p, d, f

• You can see these patterns in the periodic table.

The Periodic Table• Was organized according to the chemical &

physical properties of the known elements

• Protons, neutrons and electrons had not yet been discovered

• Dmitri Mendeleev noticed that when the elements were put into order by their atomic mass, some of their properties repeated periodically (at regular intervals)

• This is because the pattern of valence electrons repeats as orbitals are filled

What it tells you

• Atomic number = the number of

protons• Atomic mass =

average mass for all the forms of that element

• Name of the element• Symbol (first letter always capital)

Ions• Atoms aren’t always neutral (no

charge) • Lose electrons

– More protons, positive ion• Gain electrons

– More electrons, negative ion• Has 1 charge for each electron lost or

gained• Written above the symbol, e.g. Na+, O2-

Isotopes• Some forms of the atom naturally have

more or less neutrons• Because it has the same number of

protons it’s still the same atom, but called an isotope.

• That’s why atomic mass is not a whole number, it averages all the

isotopes• Isotopes are written differently, too.

Isotopes

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• Write the mass number, A (# protons + neutrons) and atomic number, Z (# protons) to left of symbol

• Name tells you the mass number, e.g. sodium 22 has 22 total protons and neutrons.

• Most carbon is carbon 12 (6p+, 6n0), but carbon-14 is used to carbon date

things. How do you write it? C

How you use it• #p+ = to Atomic Number (by

definition)• #e- = # protons (in a neutral atom)• #n0 = atomic mass, rounded off –

atomic number

Bohr Model• Shows protons

and neutrons in nucleus

• Shows electrons in the cloud in layers called

energy levels

Lewis Diagram• Also called dot

diagrams• Only shows

valence electrons (outer most)

• Valence are only ones involved in chemical reactions

Patterns on the table• Rows are called periods• As you go across a period:

– Atomic number (#p+) increases by 1– So does #e- for neutral atoms– So does # valence electrons– Same energy level

• Columns are called families or groups– Similar properties (density, melt pt, etc.)– React chemically the same way– Because have same valence electrons– Energy level stays the same