Atomic Structure. For thousands of years, people had many ideas about matter Ancient Greeks believed that everything was made up of the four elements

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1 An atom is the smallest particle of an element that retains its identity in a chemical reaction. Although early philosophers and scientists could not observe individual atoms, they were still able to propose ideas about the structure of atoms.

2 For thousands of years, people had many ideas about matter Ancient Greeks believed that everything was made up of the four elements Water, Fire, Earth, Air

3 Around 300 BC, a Greek philosopher, Democritus stated that everything is made up of tiny, invisible particles He said the particles were indivisible He called them atomos which means unable to divide

4 Then in the early 1800 s, an English school teacher, John Dalton developed the first atomic theory His theory was successful because it was supported with scientific evidence By using experimental methods, Dalton transformed Democritus ideas on atoms into a scientific theory. His theory has 4 main points.

5 1. All elements are composed of tiny indivisible particles called atoms. 2. Atoms of the same element are identical. The atoms of any one element are different from those of any other element. Atoms of element A Atoms of element A Atoms of element B

6 3. Atoms of different elements can physically mix together or can chemically combine in simple whole-number ratios to form compounds. Mixture of atoms of elements A and B

7 4. Chemical reactions occur when atoms are separated from each other, joined, or rearranged in different combinations. Atoms of one element are never changed into atoms of another element as a result of a chemical reaction. Compound made by chemically combining atoms of elements A and B

8 Much of Dalton s atomic theory is accepted today. One important change, however, is that atoms are now known to be divisible. Also, atoms of the same element are not identical.

9 By the late 1800 s, an English physicist, JJ Thomson contributed to the development of the model He performed an experiment with a cathode ray tube His experiment led to the discovery of the electron An electron has a negative charge

10 If matter has negative particles, it must also have positive particles Atoms have protons Later the neutron was discovered Neutrons are subatomic particles with no charge but with a mass nearly equal to that of a proton.

11 When subatomic particles were discovered, scientists wondered how the particles were put together in an atom. Most scientists thought that the electrons were evenly distributed throughout an atom filled uniformly with positively charged material.

12 Since the mass of an atom is very small chemists created a new unit to describe the mass of atoms amu atomic mass unit

13 The table below summarizes the properties of subatomic particles. Properties of Subatomic Particles Particle Symbol Relative charge Mass (amu) Electron e 1 0 Proton p Neutron n 0 0 1

14 Elements are different because they contain different numbers of protons. The atomic number identifies an element. Atomic number = protons The total number of protons and neutrons in an atom is called the mass number. Mass number = protons + neutrons Remember :Electrons are so much less massive than protons and neutrons, so their mass is not included in atomic mass

15 Remember that atoms are electrically neutral. In an atom, protons = electrons Protons, neutrons, and electrons can be calculated from atomic number and mass number. How many protons, electrons, and neutrons are in each atom? boron, fluorine, sodium

16 Isotopes are atoms of the same element with different masses Neutrons were actually discovered due to isotopes In an isotope the number of protons and electrons are the same only the neutrons differ

17 For example, hydrogen has 3 isotopes: Isotope Proton Neutron Electron Hydrogen - 1 Hydrogen 2 Hydrogen - 3 Note that the correct way to write an isotope is to write the name, followed by the mass number.

18 Another way to specify an isotope is to use the nuclear symbol. The nuclear symbol includes the chemical symbol, atomic number, and atomic mass. Write the nuclear symbol for boron, fluorine, and sodium

19 The mass listed in the periodic table is the average atomic mass It is a weighted average of the atomic masses of naturally occurring isotopes

20 Although Thomson and other scientists answered some questions about the atom, many still remained In the early 1900 s, Ernest Rutherford performed the Gold Foil Experiment He observed that the alpha particles went through the gold foil

21 The Rutherford atomic model is known as the nuclear atom. His model inferred that the atom was almost entirely empty space Also concluded that the nucleus was extremely tiny and contained the protons and the neutrons The electrons are distributed around the nucleus and occupy almost all the volume of the atom.

22 According to this model, the nucleus is tiny and densely packed compared with the atom as a whole. If an atom were the size of a football stadium, the nucleus would be about the size of a marble.