Atomic Theory

Atomic Theory

• Chemistry I• Chapter 3

Dalton’s Atomic Theory Dalton developed a theory to explain why the

elements in a compound always join in the same way.

Dalton proposed the theory that all matter is made up of individual particles called atoms, which can not be divided.

Dalton’s Atomic Theory• All elements are composed of atoms.• All atoms of the same element have the

same mass, and atoms of different elements have different masses.

• Compounds contain atoms of more than one element.

• In a particular compound, atoms of different elements always combine in the same way.

Thomson’s Model of the AtomHe used an electric current to learn more about

atoms.

Thomson’s experiments provided the first evidence that atoms are made of even smaller particles.

Thomson’s “plum pudding” model suggested that the negative charges in an atom were evenly scattered throughout an atom filled with a positively charged mass of matter.

Rutherford’s Atomic Theory• Rutherford’s model concluded that all of

an atom’s positive charge is concentrated in its nucleus.

• He used the gold foil experiment to determine the behavior of the alpha particles when they struck different locations in the gold foil. He originally thought that the path of the alpha particles would only be slightly bent. He was wrong. Some of the particles bounced straight back.

Rutherford’s Gold Foil Experiment

Subatomic Particles• Protons are positively charged subatomic

particle that are found in the nucleus. +1 charge

• Electrons are negatively charged subatomic particles that are found in the space outside the nucleus. -1 charge

• Neutrons are neutral subatomic particles that are found in the nucleus. 0 charge or neutral

• Protons, electrons, and neutrons can be distinguished by mass, charge, and location in an atom.

Atomic Number and Mass Number• The atomic number of an element is the same as the

number of protons in an atom of that element.Atoms of different elements have different numbers of

protons.Hydrogen has 1 proton so its atomic number is 1.Silver has 47 protons so its atomic number is 47.

• The mass number of an atom is the sum of the protons and neutrons in the nucleus of that atom.

To find number of neutrons:Mass of neutrons = Mass number – Atomic number

Atoms are neutral

• When they have an equal number of protons and electrons.

• If Hydrogen has 1 proton, it has 1 electron.+1 + -1 = 0 It is neutral.

• If Silver has 47 protons, it has 47 electrons.+47 + -47 = 0 It is neutral.

Isotopes Isotopes are atoms of the same

element that have different numbers of neutrons and different mass numbers.

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Bohr’s Model of the Atom• Bohr’s model focused on the electrons in an

atom.• Bohr’s model stated that electrons move with

constant speed in fixed orbits around the nucleus, like planets around a sun.

• When an atom gains or loses energy, the energy of an electron can change.

• Possible energies that electrons in an atom have are called energy levels.

• An electron in an atom can move from one energy level to another when the atom gains or loses energy.

Bohr Model and Subatomic Particles

Atomic number is the number of protons.Mass number is the number of protons and neutrons.

Electron Cloud Model

An electron cloud is a visual model of the most likely locations for electrons in an atom.

The Modern Periodic Tableis organized so that the elements are arranged

in ascending order of atomic number.The symbols are a combination of ancient and

modern symbols, from several languages (such as Latin, English, and German). All symbols consist of one or two letters, with the first capitalized and the second lower case.

Nitrogen is N, Calcium is Ca, Silver is Ag

Energy LevelsThe location of electrons is not known at any given time, except that they move within specific energy levels and have a certain amount of energy. Electrons in the first energy level have lower total energy than electrons farther away from the nucleus.

Energy Level # of orbitals Max # of electrons

1 1 (Energy Level Squared)

2 (2 x # of orbitals)

2 4 8

3 9 18

4 16 32

Electron Configuration• An electron configuration is the

arrangement of electrons in the orbitals of an atom.

• The most stable electron configuration is the one in which the electrons are in orbitals with the lowest possible energies.