The Atom
a quick note before we start… - we have never actually seen inside an atom (even with the world’s best microscopes) - therefore, we must use models to explain it
Indirect Evidence- Evidence you can get without actually seeing or touching an object
Early Models a.) 2,500 years ago- Ancient Greeks believed all matter was made up of four elements: fire water earth air
- Democritus (Greek philosopher) - first used the term “atom” (atomos- “indivisible”) believed that if you kept dividing matter in half, you’d eventually get down to an object that couldn’t be divided anymore
his theory was not widely accepted, and the idea of atoms was largely forgotten until…
Dalton’s Atomic Theory (1803) b.) Early 1800’s- John Dalton did a number of experiments that led to the acceptance of the idea of atoms believed (like Democritus) that the atom was indivisible - came up with Dalton’s Atomic Theory (1803)
1. All elements are composed of atoms. Atoms are indivisible and indestructible 2. Atoms of the same element are exactly alike 3. Atoms of different elements are different 4. Compounds are formed by the joining of atoms of two or more different elements
note… 1.) Dalton never tried to describe what an atom looked like 2.) NOBODY had any idea what an atom looked like for almost another 100 years, until…
c.) 1897- J.J. Thomson - disproved Dalton’s idea that the atom was indivisible - discovered that an atom had positive and negative electrical charges first person to attempt to create a model of the atom
- thought that the atom had negatively- charged particles in a positively-charged “pudding” (he couldn’t find any positively-charged particles) - - - - - - - - sometimes called the “plum pudding model”
negatively-charged particles - - - - - - - - - positively-charged “pudding”
d.) 1908- Ernest Rutherford - found the positively-charged particles that Thomson could not - guessed that the atom had a small, dense, positively-charged center that he called the nucleus - also named the negatively-charged particles electrons, and guessed that they were scattered around the nucleus
- his model: atom nucleus electrons
e.) - 1913- Niels Bohr - improved on Rutherford’s model - placed the electrons in specific orbits or “shells” around the nucleus - his model:
shells nucleus electrons
f.) Wave Model - currently used model - improved on Bohr’s model - states that the electrons remain at a certain distance from the nucleus, but follow no specific path
- electrons are moving so fast that they appear as a cloud around the nucleus electron cloud model- nucleus
Atomic Masses - 99.9% of the mass of an atom is in the nucleus - because the masses of atoms are so small, scientists had to invent a unit of measure just for atoms: the Atomic Mass Unit (AMU) - 1 gram = = 600, 000 ,000 ,000 ,000 ,000 ,000 ,000 amu’s (6x1023)
Subatomic Particles - we currently know of the existence of over 200 subatomic particles - the three most important are protons, neutrons, and electrons
1.) Protons - positively charged (+) - found in the nucleus - are made up of particles called quarks - are identical to all other protons, no matter what element they are in - have a mass of 1 amu
2.) Neutrons - have no electric charge - found in the nucleus - are also made up of quarks - are identical to all other neutrons, no matter what element they are in - have a mass of 1 amu
3.) Electrons - negatively charged (-) - orbit the nucleus - have a mass of 1/1836 amu - in a “neutral” atom, the number of protons and the number of electrons are the same
- electrons move around the nucleus billions of times each second in what is called an electron cloud the distance of an electron from the nucleus depends on how much energy the electron has: more energy → farther away less energy → closer
more energy less energy
- the electrons are then arranged in energy levels or shells - each shell surrounding the nucleus can only hold a limited number of electrons: 1st shell- 2 electrons 2nd shell- 8 electrons 3rd shell- 8 electrons and so on…
- the chemical properties of an element depend on how many electrons are in the various shells - according to modern atomic theory, electrons can move from one shell to another
Things that are determined by an atom’s protons, neutrons, and electrons… a.) atomic number the number of protons in the nucleus of an atom every element has a unique atomic number
the number of protons and neutrons in the nucleus (note: you can change the number of neutrons and electrons in an atom and still have the same element, but if you change the number of protons you change the element) b.) mass number the number of protons and neutrons in the nucleus example: carbon number of protons- 6 usual number of neutrons- 6 mass number= 12
c.) isotope two (or more) atoms of the same element which have the same number of protons (obviously!), but different numbers of neutrons example: carbon (again)
Sample A: number of protons- 6 number of neutrons- 6 this type of carbon atom is called C12 or carbon-12
Sample B: number of protons- 6 number of neutrons- 8 this type of carbon atom is called C14 or carbon-14
C12 and C14 are both carbon atoms, but are isotopes because they have different numbers of neutrons
d.) atomic mass the average mass number of all known isotopes of a particular element is also influenced by the relative percentages of each isotope example: carbon (yet again)
950 carbon-12 atoms (mass number- 12) in a sample of 1,000 carbon atoms you might find the following- 950 carbon-12 atoms (mass number- 12) 30 carbon-13 atoms (mass number- 13) 20 carbon-14 atoms (mass number- 14) to average them, find the total number of protons and neutrons (the total mass), and divide it by the number of atoms:
950 atoms x 12 = 11,400 protons + neutrons 30 atoms x 13 = 390 p + n 20 atoms x 14 = 280 p + n total- 12,070 p + n 12,070 (p +n) ÷ 1,000 atoms = 12.07 (the atomic mass of carbon)
Last term… Ion- an atom where you have changed the number of electrons (more on this later) In Summary… If you take an atom and change the number of protons, you get a whole new element neutrons, you get an isotope electrons, you get an ion