ATOMS The discovery and structure of atoms.

1. Elements are composed of extremely small particles called atoms. All atoms of a given element are identical. The atoms of one element are different from the atoms of all other elements. 2. Compounds are composed of atoms of more than one element. The relative number of atoms of each element in a given compound is always the same. 3. Chemical reactions only involve the rearrangement of atoms. Atoms are not created or destroyed in chemical reactions. 2.1

Dalton The theory that atoms are the fundamental building blocks of matter reemerged in the early 19th century, championed by John Dalton The theory that atoms are the fundamental building blocks of matter reemerged in the early 19th century, championed by John Dalton

Each element is composed of extremely small particles called atoms. All atoms of a given element are identical to one another in mass and other properties, but the atoms of one element are different from the atoms of all other elements.

Discharge Tubes Discharged tubes produced rays that seemed to come from the cathode. By studying the properties of the rays, they were found to consist of negatively charged particles of low mass. These particles are known as electrons. Discharged tubes produced rays that seemed to come from the cathode. By studying the properties of the rays, they were found to consist of negatively charged particles of low mass. These particles are known as electrons.

Crookes modified the gas discharge tube to include a small pin wheel. When the cathode rays struck the pin wheel, it rotated, suggesting that the particles had mass. Thomson subjected the cathode rays to electric and magnetic fields, causing the rays to bend, therefore inferring the particles were of small mass and negatively charged. Crookes modified the gas discharge tube to include a small pin wheel. When the cathode rays struck the pin wheel, it rotated, suggesting that the particles had mass. Thomson subjected the cathode rays to electric and magnetic fields, causing the rays to bend, therefore inferring the particles were of small mass and negatively charged.

The Electron Streams of negatively charged particles were found to emanate from cathode tubes. Streams of negatively charged particles were found to emanate from cathode tubes. J. J. Thompson is credited with their discovery (1897). J. J. Thompson is credited with their discovery (1897).

Radioactivity: The spontaneous emission of radiation by an atom. The spontaneous emission of radiation by an atom. First observed by Henri Becquerel. First observed by Henri Becquerel. Also studied by Marie and Pierre Curie. Also studied by Marie and Pierre Curie.

Roentgen discovered X-rays coming from cathode ray tubes. Bacquerel found that similar rays came from uranium samples, but reasoned they could not be the X-rays he had been looking for because they seemed to be self-generated (*remember the film was exposed in the dark drawer even though there was no exposure to ultra violet light). Bacquerel’s rays turned out to be radioactivity. Roentgen discovered X-rays coming from cathode ray tubes. Bacquerel found that similar rays came from uranium samples, but reasoned they could not be the X-rays he had been looking for because they seemed to be self-generated (*remember the film was exposed in the dark drawer even though there was no exposure to ultra violet light). Bacquerel’s rays turned out to be radioactivity.

Radioactivity Three types of radiation were discovered by Ernest Rutherford: Three types of radiation were discovered by Ernest Rutherford:  particles  particles  particles  particles  rays  rays

Discovery of the Nucleus Ernest Rutherford shot  particles at a thin sheet of gold foil and observed the pattern of scatter of the particles.

Rutherford found that most of the alpha particles went straight through the gold foil. A few were deflected at varying angles and some were deflected backward towards the source. Rutherford found that most of the alpha particles went straight through the gold foil. A few were deflected at varying angles and some were deflected backward towards the source. Rutherford suggested that the bulk of the mass and all of the positive charge was located in a tiny, dense central core called the nucleus. The negative charges (electrons) were in circular motion around the nucleus. The majority of the atom was thought to be empty space. Rutherford suggested that the bulk of the mass and all of the positive charge was located in a tiny, dense central core called the nucleus. The negative charges (electrons) were in circular motion around the nucleus. The majority of the atom was thought to be empty space.

Rutherford’s calculations of the mass of gold did not agree with the mass of its protons. The electromagnetic theory suggests that orbiting electrons should emit electromagnetic waves and lose energy. As a result, the electrons should spiral into the nucleus. Rutherford’s model of the atom did not explain why the electrons did not spiral inward towards the nucleus. Rutherford’s calculations of the mass of gold did not agree with the mass of its protons. The electromagnetic theory suggests that orbiting electrons should emit electromagnetic waves and lose energy. As a result, the electrons should spiral into the nucleus. Rutherford’s model of the atom did not explain why the electrons did not spiral inward towards the nucleus.

The Nuclear Atom Rutherford postulated a very small, dense nucleus with the electrons around the outside of the atom. Rutherford postulated a very small, dense nucleus with the electrons around the outside of the atom. Most of the volume of the atom is empty space. Most of the volume of the atom is empty space.

Protons were discovered by Rutherford in Protons were discovered by Rutherford in Neutrons were discovered by James Chadwick in 1932 Neutrons were discovered by James Chadwick in 1932

Subatomic Particles Protons and electrons are the only particles that have a charge. Protons and electrons are the only particles that have a charge. Protons and neutrons have essentially the same mass. Protons and neutrons have essentially the same mass. The mass of an electron is so small we ignore it. The mass of an electron is so small we ignore it.

Symbols of Elements Elements are symbolized by one or two letters.

Atomic Number All atoms of the same element have the same number of protons: The atomic number (Z)

Atomic Mass The mass of an atom in atomic mass units (amu) is the total number of protons and neutrons in the atom.

Isotopes: Atoms of the same element with different masses. Atoms of the same element with different masses. Isotopes have different numbers of neutrons. Isotopes have different numbers of neutrons C 12 6 C 13 6 C 14 6 C

Atomic number (Z) = number of protons in nucleus Mass number (A) = number of protons + number of neutrons = atomic number (Z) + number of neutrons Isotopes are atoms of the same element (X) with different numbers of neutrons in the nucleus X A Z H 1 1 H (D) 2 1 H (T) 3 1 U U Mass Number Atomic Number Element Symbol 2.3

How many protons, neutrons, and electrons are in C 14 6 ? How many protons, neutrons, and electrons are in C 11 6 ? 6 protons, 8 (14 - 6) neutrons, 6 electrons 6 protons, 5 (11 - 6) neutrons, 6 electrons Do You Understand Isotopes? 2.3

Now Lets Talk About Charges!

An ion is an atom, or group of atoms, that has a net positive or negative charge. cation – ion with a positive charge If a neutral atom loses one or more electrons it becomes a cation. anion – ion with a negative charge If a neutral atom gains one or more electrons it becomes an anion. Na 11 protons 11 electrons Na + 11 protons 10 electrons Cl 17 protons 17 electrons Cl - 17 protons 18 electrons 2.5

Lets Do Some Examples! 7 Li 35 Cl Li + 35 Cl As 3- Ag 1+

13 protons, 10 (13 – 3) electrons 34 protons, 36 (34 + 2) electrons Do You Understand Ions? 2.5 How many protons and electrons are in Al ? 3+3+ How many protons and electrons are in Se ?

Charges on Atoms The number of electrons is the same as the number of protons. When there is a charge present that indicates either more electrons (a negative charge) or less electrons (a positive charge).

REFERENCES: Wolf, Clancy, Jasper, Lindenberg, Lynn, Mustoe, Smythe, (1999). SciencePower 9 (Atlantic Edition). McGraw-Hill Ryerson Limited. Wolf, Clancy, Jasper, Lindenberg, Lynn, Mustoe, Smythe, (1999). SciencePower 9 (Atlantic Edition). McGraw-Hill Ryerson Limited. modified modified