Atoms

What is the composition of matter?

The Greek philosopher Democritus (460-370 BC) first proposed that if a pure substance was divided into smaller and smaller pieces, the result would be a small particle that could not be divided any further. He called this particle an "atom" which meant "uncuttable". He used his concept of the atom to explain many observations such as how a metal like lead is able to distort when bent or hammered. Plato and Aristotle argued against his theory and the idea of atoms was never accepted until 1803 when John Dalton, an English schoolteacher, revived the idea of atoms.

In 1899, Marie Curie proposed that certain substances such as uranium, polonium and radium emit three kinds of radiation and disintegrate when they emit this radiation. This radiation was called a-particles, b-particles and g-rays. Both a and b particles are smaller than an atom. Therefore the atom had to be composed of subatomic particles. These particles, for the sake of simplicity, are protons, neutrons and electrons.

What are these subatomic particles?

The center or nucleus of an atom contains protons and neutrons that account for nearly all the mass of an atom...both protons and neutrons have masses equal to one atomic mass unit (amu or Dalton). This unit is actually not the mass of a proton or neutron in grams but is a mass ratio based on the element carbon. An amu is defined as 1/12 of the mass of a carbon atom.

Proton...Protons have a positive charge. The number of protons in a nucleus determines the identity of the atom (element) and this number is called the atomic number. The actual mass of a proton has been determined to be 1.673 x 10^-24 gram. Scientists use the metric system of measurement where the standard unit for mass is the gram.

Neutron...Neutrons have no charge. For our purposes, a neutron can be considered to be a proton and an electron held together by another subatomic particle called a gluon. The number of neutrons in a nucleus added to the number of protons determines the total mass of an atom (atomic mass). The number of neutrons in a nucleus may vary even though the number of protons is set for a given element. This leads to some atoms of the same element having a different total mass and these atoms are called isotopes. The actual mass of a neutron has been determined to be 1.675 x 10^-24 gram.

Electron...Electrons have a negative charge and virtually no mass. The mass of a single electron has been determined to be 9.11 x 10^-28 gram. Electrons were determined by Millikan to have a charge of 1.6 x 10^-19 coulomb.

In 1910, Rutherford shot a-particles at a gold foil. Most of the a-particles passed through the foil but a small fraction deflected back at various angles.

Rutherford then proposed the following structure of an atom.

(a) Most of the mass of atom is in a dense positively charged nucleus.

(b) Most of the volume of an atom is in an empty space in which negatively charged electrons move around the nucleus. The electrons are attracted to the nucleus by electrostatic attractions and the velocity of the electrons prevents the electrons from collapsing in on the nucleus.

Modern atomic theory contends that electrons surround the nucleus in orbitals and balance out the positive charges of the protons. The orbitals are probability areas in which the electron is most likely to be found.

The orbitals are filled with electrons based on the energy level of the orbital. Each orbital can only hold two electrons because the negative charges of the electrons repel each other.

The space occupied by the nucleus is very small with respect to the size of these orbitals. The volume of the nucleus is 1/100,000 of the atom's total volume. If the nucleus of hydrogen were the size of a baseball, the s orbital would extend out about 2.5 miles.

Spectroscopic Notation of Orbitals

There is a shorthand way to express the electron configuration for atoms and ions that is called the spectroscopic notation. This method just lists the orbital name and the total electrons in that orbital with a superscript. The fluorine atom would be...

1s²2s²2p⁵

What are ions?

The atom of an element has equal protons and electrons. Protons are never gained or lost in chemical reactions but the number of electrons in the outer energy level can be changed. When this happens, a charge imbalance will occur and this charged atom is called an ion.

What type of ion would be formed if an electron was lost from an atom?

What type of ion would be formed if an electron was added to one of the outer orbitals?

The elements that are metals tend to lose electrons readily and the elements that are the non-metals tend to gain electrons readily. The inert gases are so stable that they do not gain or lose electrons except under extreme conditions.

Problems...

1. What are the two substances found in the nucleus of an atom? How do they differ from each other?

2. Show how the orbitals are filled for carbon, which has a total of 6 electrons, using both an orbital box diagram and spectroscopic notation.

3. Show how the orbitals are filled for oxygen, which has a total of 8 electrons, using both an orbital box diagram and spectroscopic notation.

4. What is the atomic particle that determines the identity of an element?

5. What is a cation?

6. What is an anion?

7. An ion is created by the gain or loss of _?_ by an atom.

8. What are isotopes?

9. When a fluorine atom gains an electron it forms the fluoride ion. Show how the orbitals are filled for the fluoride ion, which has a total of 10 electrons, using both an orbital box diagram and spectroscopic notation.

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