4. Atoms and the Atomic theory

Democritus

400 B.C. - he thought matter could not be divided indefinitely

Aristotle

350 B.C - he modified an earlier theory that matter was made of four “elements”- earth, fire, water, air

2000

how many years did Aristotle’s theory persist?

John Dalton

1800 - proposed a modern atomic model based on experimentation not on pure reason

John Dalton

His ideas account for the law of conservation of mass and the law of constant composition

Dalton’s Atomic Theory

All matter is made of atoms.

Atoms of an element are identical.

Each element has different atoms.

Atoms of different elements combine in constant ratios to form compounds.

Atoms are rearranged in reactions

Law of conservation of mass

atoms are neither created nor destroyed

Law of constant composition

elements combine in fixed ratios

Dalton’s “Billiard ball” model

Model- Atoms are solid and indivisible

Thompson “Plum pudding” model

Model - Negative electrons in a positive framework

Rutherford model

Model - Atoms are mostly empty space. Negative electrons orbit a positive nucleus

Cathode rays

Materials, when rubbed, can develop a charge difference. This electricity is called __ when passed through an evacuated tube. These rays have a small mass and are negative

Ernest Rutherford

According to him, most particles passed through. So, atoms are mostly empty. Some positive α-particles deflected or bounced back! Thus, a “nucleus” is positive & holds most of an atom’s mass

Bohr’s model

Model-Electrons orbit the nucleus in “shells”. Electrons can be bumped up to a higher shell if hit by an electron or a photon of light. It’s when electrons fall back down that they release a photon

continuous spectra

a spectrum that shows an unbroken range of colors or wavelengths with no gaps

line spectra

a spectrum that shows only specific, discrete wavelengths (lines) of light released when electrons drop from one energy shell to another.

line spectra

one of the 2 types of spectra, jumps down from “shell” to “shell” account for the line spectra seen in gas discharge tubes

ion

An atom or small molecule with an overall positive or negative charge due to an imbalance of protons and electrons.

loss

Positive charge = _ of electrons

gain

Negative charge = _ of electron

Isotopes

Atoms with the same number of electrons and protons, but different numbers of neutrons

Quantum numbers

Set of 4 numbers used to describe the electrons in terms of:

• Distance from the nucleus

• Shape of the orbitals

• Orientation in space

• Direction of electron spin

Principal quantum number (n)

Refers to the main energy levels. Can only have integral values of n = 1, 2, 3, 4 etc.

Principal quantum number (n)

Related to the average distance of the electron from the nucleus

Azimuthal or secondary quantum number (l)

Defines the shape of the orbital.

Azimuthal or secondary quantum number (l)

Also refers to the energy sublevels

Magnetic quantum number (m1)

Describes the orientation of the orbitals in space.

Spin quantum number (ms)

differentiates how two electrons behave under a magnetic field. Can only have two possible values +½ and -½.