The Atom: History Recap

The indivisible atom: Democritus (400 BC) proposed that matter consisted of particles that could not be cut into smaller pieces.

In 1904, JJ Thompson proposed that the atom consisted of two parts…a large positively charged mass with electron ‘raisins’ mixed in with it

The ‘plum pudding model’

Rutherford model

In 1911, Rutherford used the results from his gold-foil experiment to propose that the positive charge was concentrated in the nucleus with the electrons surrounding it.

Bohr model:

In 1913, Bohr proposed that the electrons traveled along fixed orbits around the nucleus. Another student of Rutherford’s suggested that the nucleus contained protons and neutrons.

Electron cloud model:

In the current model of atomic structure, electrons do not travel along fixed paths like planets around the sun. Instead, electrons can be found most frequently in certain areas around the nucleus.

So, how are the electrons arranged around the nucleus?

The electrons in an atom have different energies depending upon how close to the nucleus they are. The energy levels are spaced apart like steps on a flight of stairs.

nucleus

2 e-

8 e-18 e-

32 e-

EnergyLevel 1

Level 2

Level 3Level 4

Let’s build some atoms!

AKA: Bohr rings aren’t boring!

1 p0 n

e

Hydrogen

H1

1

Atomic number: the number of protons in the nucleusMass number: the number of protons + the number of neutrons in the nucleus

0 n

In neutral atoms, #p = #e

2 p2 n

e

e

Helium

He2

4

3 p4 n

e

ee

Lithium

Li3

7

Chemical reactivity depends upon the number of electrons in the outermost energy level.

Electron dot structures show only the outermost electrons so that reactivity can be easily predicted.

The outermost electrons are also called the valence electrons.

Rules to Drawing Electron Dot Structures

1. Electrons 1-4 are added singly.2. Exception: If there only two electrons,

they’re shown as a pair.3. Electrons 5-8 are paired with the single

electrons that are already there.

Xe

Sr

N

Ex: Sr has two valence electrons

N has five valence electrons

Xe has eight valence electrons

1 p0 n

e

Hydrogen-1

H has one electron in its outermost energy level

H

Electron dot structure

•The element symbol represents the nucleus.

•The dot represents the electron.

2 p2 n

e

Helium-4

e He has two electrons in its outermost energy level

He

Electron dot structure

Lithium-7

3 p4 n

e

ee

Li has one electron in its outermost energy level

Li

Electron dot structure

•The element symbol represents the nucleus and the inner electrons.

Beryllium-9

4 p5 n

e

ee

Be has two electrons in its outermost energy level

Be

Electron dot structure

e

Boron-11

5 p6 n

e

ee

B has three electrons in its outermost energy level

B

Electron dot structure

e

e

Bonding: A Preview

The electron dot structures of H and O are:

H O

If elements combine together so that both elements have a noble gas configuration, they will form a stable compound.

H OH H