Scientists have been studying atoms since the 1800s using models to understand their structure and behavior. Early philosophers proposed the idea of atoms, while scientists like Dalton, Thomson, Rutherford, and Bohr developed atomic models through experiments. They discovered that atoms are mostly empty space with a small, dense nucleus at the center containing protons and neutrons, and electrons orbiting the nucleus. The number of protons determines the element, while isotopes of an element have different numbers of neutrons.

1. Structure of AtomsThe History and Models of Atoms

2. Studying AtomsScientists have been studying atoms since the 1800s. If atoms are too small to be seen, how do we study them?Scientists use models to study atoms.Sometimes scientists can tell what things look like by studying how they act.Example: Wind

3. ModelsIn science, a model is an idea, a picture, or an object that is built to explain how something else looks or works.The model might not look exactly like the object it is built to describe, but it helps people understand the way the object acts.

4. History of the AtomDemocritus, a Greek philosopher, first used the term atom (atomos) around 400 BC.John Dalton showed that materials were made of atoms in 1803.In the late 1800s we discovered that the atom has many smaller parts.In the late 1800s, English physicist J. J. Thomson discovered the electron.DaltonThomson

5. More History of the AtomIn 1901 Ernest Rutherford found discovered the nucleus, or center of the atom.In 1917 Niels Bohr, working as Ernest Rutherford’s student, discovered how electrons orbit the nucleus.His model of the atom is the current base for the modern atomic model.

6. Atomic Mass UnitSubatomic Particles are so small that a special unit was devised to measure them.This is the atomic mass unit (amu).Atoms are almost entirely empty space.If the nucleus were the size of a basketball, the first electrons would be somewhere near Springfield.

7. Model of an AtomThe central part of the atom is called the nucleus.The nucleus contains positively charged protons that weigh 1 atomic mass unit.The nucleus also contains neutrons that have no charge and weigh 1 atomic mass unit.Electrons are extremely small units of an atom that have a negative charge and move around the atom in orbits.Protons and electrons stay together because they attract each other.+O-

8. Model of an AtomElectronProtonNeutron

9. NucleusThe nucleus is where the protons and neutrons are located within the atom.

10. Atomic NumberThe atomic number of an atom is the number of protons it has.If an atom has 6 protons, it is carbon.It does not matter what else it has, if it has 6 protons then it is carbon.

11. Atomic Number

12. Practice Number of Protons

13. Electron CloudThe electron cloud is the region around the nucleus occupied by electrons.Atoms are much, much denser near the nucleus.

14. ElectronsElectrons are also arranged in shells.The first shell can hold a maximum of 2 electrons. All other shells can hold up to 8 electrons.

15. Valence ElectronsThe outermost shell of an atom is called the valence shell.

16. Protons and ElectronsIn an ATOM the number of electrons should always equal the number of protons.

17. IsotopesAn Isotope is when an atom has a different number of protons and neutrons.Most elements have many different isotopes. This makes the nucleus unstable or radioactive in many cases.

18. Writing IsotopesTo describe an atom, write the name of the atom and then write the total number of protons and neutrons that are found in it’s nucleus.The total number of protons and neutrons in the nucleus is called the Mass Number.

19. Isotopes and Mass Number

20. Practice Isotopes

21. Abbreviating Mass NumberTo abbreviate mass number, write the symbol of the atom and then to the upper left write the mass number and under that, write the atomic number.