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Plum Pudding Atoms
In 1897, scientist J.J. Thomson discovered the electron. To explain the balance between negative and positive charges in an atom, he proposed that an atom was like "plum pudding." In this model, negatively-charged electrons floated freely in a "blob" of positive charge—like raisins in a plum pudding. A few years later, a scientist named Ernest Rutherford, a student of Thomson's, was conducting an experiment in which he bounced alpha particles off a sheet of gold foil. The particles were deflected off the gold atoms at large angles. This experiment showed that the positive charge had to be concentrated in a small mass—not distributed evenly with negative charges as the "plum pudding" model had suggested. Rutherford suggested that the positive charge was collected in a massive nucleus and the lighter electrons orbited the nucleus as planets orbit the Sun.
What they are: the building blocks of matter
Number: at least 112 known types; 92 occur in nature
What they're made of: protons, neutrons, and electrons
Mass: protons and neutrons have the same mass; electrons have about 2,000 times less mass than protons and neutrons
This is a box from the Periodic Table that represents oxygen. The atomic number "8" is equal to the:
weight of an oxygen atom
number of protons in an oxygen atom
year the element oxygen was discovered
Correct!
The Periodic Table lists all the known elements. The atomic number "8" is equal to the number of protons in an oxygen atom. The "O" is the symbol for the element oxygen. The "16.0" equals the average mass of the atoms that make up that element.
About how many kinds of atoms are there?
100
1,000
10,000
Correct!
There are about 110 to 112 known atoms, and new ones are being discovered every day. Each type has a different number of protons, neutrons, and electrons, which gives the atom unique properties.
The chemical formula for hot chocolate is H2O.
Fiction
H2O is the chemical formula for water. Water is a compound, a substance made of two or more elements. It is made of two hydrogen atoms and one oxygen atom.
Molecules are made of atoms.
Fact
Molecules are formed when atoms group together with other atoms. Molecules can be made of the same element or different elements to form a compound.