# Chapter 3: Atoms and the Periodic Table

## 32 terms

### atomic theory

- all matter is composed of atoms (an atom is the smallest particle of an element that takes place in chemical reactions)
- all atoms of a given element are alike
- compounds are combinations of atoms of more than one element, the relative number of each type of atom always stays the same (only whole atoms can combine)
- chemical reactions change only the way the atoms are combined in compounds; the atoms themselves remain unchanged (cannot be created nor destroyed)

### subatomic particles

- proton (+, in nucleus)
- neutron (no charge, in nucleus)
- electrons (-, rotates around nucleus)

- the attraction between electrons and protons keep atoms neutral

### atomic number

- Z
- what makes one atom different from another atom
- number of protons and electrons

### atomic mass/atomic weight/mass number

- A
- sum of protons + neutrons

### isotopes

- atoms with identical atomic numbers but different mass numbers (due to different numbers of neutrons)
- a specific isotope is represented by showing its mass number (A) as a superscript and its atomic number (Z) as a subscript in front of the atomic symbol

A
ELEMENT
Z

### atomic weight on periodic table

the atomic weights listed on the period table are the weighted average of all the naturally occurring isotopes for that element

ex) Copper (Cu)
Cu-63 69.17% x 62.93 amu = .6917 x 62.93 = 43.53
Cu-65 30.83% x 64.93 amu = .3083 x 64.93 = 20.02
40.53 + 20.02 = 63.55 amu (weighted average for Cu)

### main group elements

- AKA representative elements
- numbered 1-8 (left to right) or 1A-8A
- number of group = number of valance electrons

### transition metal elements

- group in the middle (numbers 3-12) or 1B-8B
- all have 2 valance electrons

### groups

- AKA families
- 18 vertical columns
- elements in a given group have similar chemical properties

### periods

- 7 horizontal rows

### intertransition metals

- 14 groups at the bottom of the periodic table
- contain Lanthanides and Actinides

### group 1 characteristics

- alkali metals (except H - nonmetal)
- typically shiny, low-melting metals
- react with water (some very violently)
- not found in their pure, uncombined state in nature (always in a compound)

### group 2 characteristics

- alkaline earth metals
- reactive metals (less than group 1 though)
- react with water to produce very basic or alkaline solutions
- not found in their pure, uncombined state in nature (always in a compound)

### group 17 (7A) characteristics

- halogens
- mostly nonmetals
- exist in a range of physical states from solid to gaseous
- colorful and reactive
- not found in their pure, uncombined state in nature (always in a compound)

### group 18 (8A) characteristics

- noble gasses (inert gasses)
- nonmetals existing as colorless gasses
- very unreactive and found pure in nature in their uncombined state
- electron configuration is stable

- one-half the distance between two nucleii in adjacent atoms
- decreases going from left to right
- increases going from top to bottom

### quantum mechanical model

- electrons cannot move freely
- electrons are restricted to moving about only in certain regions depending on the amount of energy the electron has
- different electrons have different amounts of energy
- electron energies are quantized (restricted to certain values)

### quantum mechanical model of atomic structure

- the electrons in an atom are grouped around the nucleus into shells
- the farther a shell is from the nucleus, the larger it is, the more electrons it can hold, and the higher the energies of those electrons (as you move further away from the nucleus, the attraction to the nucleus decreases)

### electron shells

- # of shells = period #

### electron subshells

- subshell # = # of shells
- s, p, d, f
- subshells are increasing in energy s<p<d<f

### orbitals

- within each subshell, electrons are further grouped into orbitals
- are regions of space within an atom where the specific electrons are more likely to be found
- # of orbitals within a subshell increases as the odd numbers
- s has 1 orbital
- p has 3 orbitals
- d has 5 orbitals
- f has 7 orbitals
- each orbital can hold 2 electrons
- different orbitals can have different shapes

### orbital energy levels order

1s 2s 2p 3s 3p 4s 3d 4p 5s 4d 5p

electrons fill from lowest to highest energy

### orbital diagram

- uses arrows
- show reactivity of the atom
- must fill each slot in a subshell before pairing electrons up

ex) Li Z=3 ↑↓ ↑
1s 2s

ex) Ne Z=10 ↑↓ ↑↓ ↑↓↑↓↑↓
1s 2s 2p

### electron configuration rules

- electrons occupy the lowest energy orbitals available beginning with 1s
- each orbital can hold only two electrons, which must be of opposite spin
- two or more orbitals with the same energy are each half filled by one electron before any one orbital is completely filled by addition of the second electron

ex) Li Z=3 ↑↓ ↑
1s 2s 1s²2s¹

ex) Ne Z=10 ↑↓ ↑↓ ↑↓↑↓↑↓
1s 2s 2p 1s²2s²2p⁶

### noble gas configuration

- all noble gases have filled orbitals, so can substitute noble gases before element

ex) Cl Z=17 1s²2s²2p⁶3s²3p⁵
Ne is noble gas before Cl with 1s²2s²2p⁶
so can write Cl as [Ne]3s²3p⁵

### s block

- H and He
- first two columns
- 1s-7s

### p block

- rightmost columns
- groups 3A-8A
- 2p-6p

### d block

- transition metals
- 3d-6d

### f block

- intertransition metals
- 4f-5f

### valence shell

- outermost, highest energy shell (NOT subshell) of an atom
- 1st shell holds 2 electrons, then other shells hold up to 8 electrons

### valence electrons

- an electron in an outermost shell of an atom
- are loosely held
- most important in determining an element's properties
- max number of valence electrons = 8
- in main groups, group # tells # of valence electrons

### electron dot symbol

- an atomic symbol with dots placed around it to indicate the number of valence electrons
- each side gets a dot first, then they get paired
°°
ex) °° Ne °°
°°

ex) ° Ag °