Atomic Structure & Periodic Table – One Page Study Guide ⸻ Atomic Scientists (In Order) Democritus – matter made of indivisible particles (atomos) Dalton – modern atomic theory; atoms combine in whole-number ratios J.J

1. Democritus (~400 BC)

matter is made of indivisible particles called 'atomos'.

2. John Dalton (1803)

atoms are solid spheres

3. J.J. Thomson (1897)

Discovered the electron and 'Plum Pudding' model.

4. Ernest Rutherford (1911)

Discovered the nucleus

5. Niels Bohr (1913)

energy levels.

6. Werner Heisenberg (1927)

'Uncertainty Principle

7. James Chadwick (1932)

Discovered the neutron

Metals

Elements that are shiny, conductive, and tend to lose electrons.

Nonmetals

Elements that are dull, brittle, and tend to gain electrons.

Metalloids

Elements with mixed properties of both metals and nonmetals.

Ion

A charged atom resulting from the loss or gain of electrons.

Cation

A positive ion formed by losing electrons.

Anion

A negative ion formed by gaining electrons.

Isotopes

Atoms of the same element with the same number of protons but different numbers of neutrons and mass.

Alkali Metals

Highly reactive elements located in Group 1.

Alkaline Earth Metals

Reactive metallic elements located in Group 2.

Transition Metals

Elements located in Groups 3-12.

Boron/Carbon/Nitrogen/Oxygen Groups

Groups 13-16, often named after the first element in each column.

Halogens

Very reactive nonmetals located in Group 17.

Noble Gases

Stable, non-reactive gases located in Group 18.

Total Electrons

In a neutral atom, the number of electrons is equal to the atomic number.

Valence Electrons

Electrons in the outermost shell that determine an atom's chemical reactivity.