Democritus’s model

• matter consists of invisible particles called atoms and a void (empty space),
• atoms are indestructible and unchangeable,
• they are homogenous, meaning they have no internal structure,
• atoms was solid, and stated all atoms differ in size, shape, mass, position and arrangement, with a void exists between them.

Dalton’s Atomic Theory

• Everything is composed of atoms, which are the indivisible building blocks of matter and cannot be destroyed.
• All atoms of an element are identical.
• The atoms of different elements vary in size and mass.
• Compounds are produced through different whole-number combinations of atoms.
• A chemical reaction results in the rearrangement of atoms in the reactant and product compounds.

J.J. Thomson's Atomic Theory

J.J. Thomson's experiments with cathode ray tubes showed that all atoms contain tiny negatively charged subatomic particles or electrons. Thomson proposed the plum pudding model of the atom, which had negatively-charged electrons embedded within a positively-charged "soup."

Rutherford Atomic Theory

According to the Rutherford atomic model: The positively charged particles and most of the mass of an atom was concentrated in an extremely small volume. He called this region of the atom as a nucleus. Rutherford model proposed that the negatively charged electrons surround the nucleus of an atom.

Niels Bohr Atomic Theory

In 1913, Niels Bohr proposed a theory for the hydrogen atom based on quantum theory that energy is transferred only in certain well defined quantities. Electrons should move around the nucleus but only in prescribed orbits. When jumping from one orbit to another with lower energy, a light quantum is emitted.

Periodicity Definition.

In the context of chemistry and the periodic table, periodicity refers to trends or recurring variations in element properties with increasing atomic number. Periodicity is caused by regular and predictable variations in element atomic structure.

Ionization Energy

This is the energy needed to completely remove an electron from an atom or ion. Ionization energy increases moving left to right across the table and decreases moving down a group.

Electronegativity

A measure of how readily an atom forms a chemical bond. Electronegativity increases moving left to right across a period and decrease moving down a group.

Atomic Radius

This is half the distance between the middle of two atoms just touching each other. Atomic radius decreases moving left to right across a period and increases moving down a group. Ionic radius is the distance for ions of the atoms and follows the same trend. Although it might seem like increasing the number of protons and electrons in an atom would always increase its size, the atom size doesn't increase until a new electron shell is added. Atom and ion sizes shrink moving across a period because the increasing positive charge of the nucleus pulls in the electron shell.

Electron Affinity

This is a measure of readily an atom accepts an electron. Electron affinity increases moving across a period and decreases moving down a group. Nonmetals usually have higher electron affinities than metals. The noble gases are an exception to the trend since these elements have filled electron valence shells and electron affinity values approaching zero. However, the behavior of the noble gases is periodic. In other words, even though an element group might break a trend, the elements within the group display periodic properties.