About the periodic table (atoms, elements, orbitals, bonds, etc.)
by John Abbe
[incomplete]
=========================
Much of the universe is made up of atoms, which are very small
particles (about 1 ten billionth of a meter across -- also see
here). It used to be thought that atoms were the smallest things
possible. In fact, the dictionary meaning of the word atom is
that it is "irreducible", meaning you can't break it into
anything smaller. In the early 1900s it was learned that atoms
are themselves made up of smaller particles -- protons, neutrons
and electrons. But the name stuck.
We categorize atoms based on their atomic number -- how many
protons they have. The resulting 100+ categories are the
elements. Each element is represented by a cell in the periodic
table (helpful to look at while reading this).
Atoms of a given element share various properties, such as how
they react with other atoms, their color, whether they're gas,
liquid or solid, and how these all vary with temperature and
pressure. Elements in the same column, or near each other
in the periodic table often have similar properties.
Atoms are made of:
protons -- positive charge, heavy, stay in atom's nucleus
neutrons -- no charge, heavy, stay in atom's nucleus
electrons -- negative charge, light, in orbitals around nucleus
Atomic number is the number of protons in the atom nucleus.
Atomic weight is the total number of protons and neutrons.
The charge on protons and electrons is equal* but opposite.
Protons strongly attract electrons (see electromagnetic force).
If possible, an atom will have as many electrons in orbitals
around it as it has protons in its nucleus, to balance the
positive and negative charges.
So, Oxygen, symbol O, atomic number 8,
has 8 protons in its nucleus, and (ideally)
8 electrons in orbitals around it.
Orbitals and co-valent bonds
----------------------------
Electrons in an atom will fall into the available orbital that
requires the least energy (releasing any extra energy as photons--
light, but that's another story).
Orbitals within a set all take the same amount of energy.
There are four kinds of sets of orbitals:
S-orbitals come in sets of 2
P-orbitals come in sets of 6
F-orbitals come in sets of 10
D-orbitals come in sets of 14
...so the next we would expect to take...18? We may see...
Atoms of elements from the first row (Hydrogen and Helium) have
one set of S-orbitals, for a total of 2 orbitals.
Helium, the last element in the row, has two protons in its
nucleus, and so will normally have two electrons in orbitals
around it. Hydrogen has only one proton, and one electron.
When an orbital set is full (as in the case of Helium), the electrons
in that set are tightly bound to the atom, and do not interact
easily with other atoms. Electrons in orbital sets that are not
full are less tightly bound, and more able to interact with other
particles. So Hydrogen is more reactive than Helium.
Orbitals "like" to be full [what force is this?], and atoms can
share electrons. A bond (of the co-valent kind) is formed between
two atoms when one or more electrons fill orbitals on *both* atoms.
So if two hydrogen atoms come near each other, they can join
together. Then we have two electrons (e) each filling two
orbitals (S1s or S2s) -- one around each Hydrogen nucleus (H):
S1 e . S1 S1 e S1
H . H They get together --> H H
S2 . e S2 S2 e S2
(This is just to show relationships, *not* how they really look)
The sharing of two electrons creates a covalent bond between two
Hydrogen atoms, making a Hydrogen molecule, H2. Now the
S-orbital set for both Hydrogen atoms is full, so H2 tends
to be less reactive than plain H.
Atoms in the second row (Lithium, Beryllium, Boron, Carbon,
Nitrogen, Oxgen, Fluorine and Neon) have enough electrons to fill
the first set of S-orbitals; additional electrons go into a second
set of S-orbitals, and then if there are enough electron
a set of P orbitals as well.
Lithium's electrons fill up to one of its second set of S-orbitals,
Beryllium both. Boron fills all those plus one of the new set of
P-orbitals:
S1 e S1 e | S1 e S1 e |N| S1 e S1 e |F| S1 e S1 e
S2 e S2 e | S2 e S2 e |i| S2 e S2 e |l| S2 e S2 e
P1 e | P1 e |t| P1 e |u| P1 e
Boron P2 | Carbon P2 e |r| Oxygen P2 e |o| Neon P2 e
AN=5 P3 | AN=6 P3 |o| AN=8 P3 e |r| AN=10 P3 e
P4 | P4 |g| P4 e |i| P4 e
P5 | P5 |e| P5 |n| P5 e
P6 | P6 |n| P6 |e| P6 e
AN = Atomic Number
Oxygen (AN=8).
8 electrons
2 of 2 S-orbitals (full)
2 of 2 S-orbitals (full)
4 of 6 P-orbitals (2 orbitals empty)
We can put together an Oxygen atom and two Hydrogen atoms with one
covalent bond between the Carbon atom and each Hydrogen atom:
S1 e S1 e . e S1 proton \ / S1 e S1 e
S2 e S2 e . S2 | | S2 e S2 e Hydrogen
P1 e . | | P1 e atoms
Oxygen P2 e . e S1 proton | --> | Oxygen P2 e
atom P3 e . S2 | | atom P3 e S1 proton
P4 e . | | P4 e S2
P5 . Hydrogen | | P5 e S1 proton
P6 . atoms / \ P6 e S2
Each Hydrogen atom is getting its empty S-orbital filled by one of
the "top" two electrons from the Oxygen atom. Each of the two
empty P-orbitals on the Oxygen atom is being filled by an
electron from one of the two electrons.
So each covalent bond is in some sense made up of the two electrons
that are being shared between the oxygen atom and each hydrogen atom.
What orbitals are there in an atom of an element in a given row?
Total = total # of orbitals in all sets in last element in row
Row Old + New Orbitals Total
--- --- --- -------- -----
n1 S = 2
n2 n1 + SP = S SP = 2 +2+6 = 10
n3 n2 + SP = S SP SP = 10 +2+6 = 18
n4 n3 + SPF = S SP SP SPF = 18 +2+6+10 = 36
n5 n4 + SPF = S SP SP SPF SPF = 36 +2+6+10 = 54
n6 n5 + SPFD = S SP SP SPF SPF SPFD = 54 +2+6+10+14 = 86
n7 n6 + SPFD = S SP SP SPF SPF SPFD SPFD = 86 +2+6+10+14 = 118
n8 n7 + ? = ? = ?
AN = atomic number
Sy = Symbol
Row = row of perodic table
AN Sy Name Row Notes
--- -- ----------------- --- ----------------------------------------
1 H Hydrogen n1 S-orbitals: 1 of 2 full
2 He Helium S-orbitals: full (n1 full)
3 Li Lithium n2 S-orbitals: 1 of 2 full
4 Be Beryllium S-orbitals: all 2 full
5 B Boron P-orbitals: 1 of 6 full
6 C Carbon P-orbitals: 2 of 6 full
7 N Nitrogen P-orbitals: 3 of 6 full
8 O Oxygen P-orbitals: 4 of 6 full
9 F Fluorine P-orbitals: 5 of 6 full
10 Ne Neon S- and P-orbitals full (n2 full)
11 Na Sodium n3 S-orbital 1 of 2 full
12 Mg Magnesium ...and so on. (see periodic table)
13 Al Aluminum
etc.
Other notes
===========
Isotopes
--------
Atoms of an element can be further categorized into isotopes,
based on the atoms' atomic weight (the number of neutrons plus the
number of protons).
Ions
----
An atom with a different number of electrons than protons is an
ion. Ions have a charge, positive if thay're "missing" electrons,
negative if they have "extra" electrons.
Noble gases
-----------
When all the sets of a row are full, the electrons are even
more tightly bound. All such elements that tend to be like this
naturally (the rightmost column of the atomic chart) have so far
been gases at earth surface temperatures, and very non-reactive.
A horribly simplistic,
and wrong anyway,
physics overview
---------------------
A selection of theories/forces. The desire of many physicists
has been to identify and describe them as a single, unified force.
As you move down the list, each successive force is strongest at
a smaller scale.
Gravity -- all mass attracts other mass
Electromagnetism -- a theory of fields and poles with varying
positive and negative charge, which attract
one another.
magnets, electricity, protons, electrons, etc.
Weak force -- [get!!?] ('unified' with electromagnetism)
Strong force -- small scale, attracts protons & neutrons together
===
Quantum theory -- describes time & space differently
describes quarks, which make up protons,
neutrons, and some other particles
===
String theory -- describes time & space *very* differently,
with compact (? :-) 10-dimensional spaces at
very tiny scales
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