Chapters 11 and 12

Physical Science

Atom – the smallest particle into which an element can be divided and still be the same substance.

Atoms can join with other atoms to make new substances.

By the 1700s, scientists knew elements could combine in specific combinations to form compounds (Hydrogen + Oxygen = Water). John Dalton found that elements combine due to their individual atoms.

All substances are made up of atoms. Atoms are small particles that cannot be created, divided or destroyed.

Atoms of the same element are exactly alike, while atoms of different elements are different.

For the most part, Dalton was correct. Later there would be some revisions.

100 years later, British scientist J.J. Thomson discovered there were particles inside of atoms (so atoms could be divided)

He discovered that there were positively-charged particles in the atoms. He called those particles protons. He found they were attracted to negative charges.

Thomson also discovered there were negative charges that were attracted to positive charges. He called those electrons.

Since an atom by itself doesn’t have a charge, then Thomson realized that each atom must have the same number of protons and neutron. He created his atomic model based on his experiments (the plum-pudding model)

Ernest Rutherford, a student of Thomson, realized that, when he sent atomic charges (postive and negative) to metal, some of the particles went through the metal (which he expected but then some charges bounced back (which he didn’t expect). This made him realize that the atomic structure of the atom is a little different than Thomson believed.

Niels Bohr, a Danish scientist who worked with Rutherford, he suggested that electrons travel around the nucleus in definite levels, and that these electrons could jump from level to level.

We now know that electrons travel in levels (shells) in a region called the electron cloud.

Scientists now know how atoms are structured:

Nucleus – the center of the atom, it is very small and very dense with a lot of particles. These particles are the protons (+ charge) and the neutrons (no charge at all, but are “heavier” than a proton).

The atomic number is the number of protons in the nucleus of an atom. A proton and neutron each have a mass of 1 amu (atomic mass unit)

So, what is the purpose of neutrons if they have no charge?

Neutrons act as a “buffer” between the protons. If the protons were around each other, they would constantly repel (since they are all positive charges). The larger neutrons help keep the protons from repelling.

There is a “cloud” outside of the nucleus where the electrons (- charge) are found (electron cloud). These electrons move around the nucleus in circular layers called shells, and there are different numbers of electrons on each shell.

The mass of an electron is so small, it is considered to be 0.

If there are the same number of protons and electrons in an atom, then the atom has no electric charge (it is neutral). However, sometimes an atom has more protons (becomes + charged), or electrons (becomes – charged).

When an atom has a charge, it is called an ion.

There are over 110 identified elements, so how do each of their atoms differ?

It depends on the number of protons and electrons each atom of each element has.

Hydrogen (H) = 1 proton, 1 electron (no need for a neutron – why?)

Carbon (C) = 6 protons, 6 neutrons and 6 electrons

Oxygen (O) = 8 protons, 9 neutrons, 8 electrons

Gold (Au) = 79 protons, 118 neutrons, 79 electrons

Notice that you don’t have to have the same number of neutrons as protons in the nucleus.

You already know that the mass of an atom is all found in the nucleus.

However, atoms are identified by their atomic number. The atomic number is the number of PROTONS you find in a nucleus.

Example: Hydrogen has an atomic number 1

Sometimes the same element will have atoms that have more particles (neutrons) in it’s nucleus. For example, you know that Hydrogen has only 1 proton, so it has an atomic number of 1. However, if you add neutrons to it’s nucleus, it will still have an atomic number of 1, but the atomic mass has changed. This “heavier” atom is called an isotope.

Isotopes are atoms that have the same number of protons but a different number of neutrons.

Isotopes share the same chemical and physical properties of the atoms in the same element, but some isotopes are unstable. This means that their neutrons can change their composition and become radioactive (ex: Carbon 14 dating).

Since atoms are identified by their protons (atomic number), isotopes are special. Their big difference is the number of neutrons, which means the mass of an isotope is larger than the mass of an atom of the same element. Therefore, isotopes are identified by their mass number

To identify a specific isotope of an element:

1) Write the name of the element. 2) Take the mass number, subtract

the number of protons (the atomic number).

3) The answer is the number of neutrons in the atom.

Carbon –12 12 Mass Number (protons +

neutrons) - 6 Number of Protons (atomic

number) 6 Number of neutronsThe mass number of Carbon-12 is 6.

Atomic mass is the weighted average of the masses of all naturally occurring isotopes of an element.

Example: Isotopes of Copper (Cu) - the Statue of Liberty is made of isotopes of copper (69% is Cu-63, 31% is Cu-65). If you average the masses of all isotopes of copper, the atomic mass of copper is 63.6 amu.

In an atom, you have both attraction and repelling forces:

Gravity – because the particles in an atom are so small, gravitational attraction is very small.

Electromagnetic Force – particles with the same charge will repel, opposite charges will attract. Electrons circle the nucleus because they “feel” the attraction to the protons inside the nucleus.

Strong Force – Because the protons in the nucleus repel each other (electromagnetic force), there is a Strong Force within the nucleus that is stronger than the repelling, so the nucleus stays together.

Weak Force – In radioactive isotopes, a neutron can change into a proton or an electron. This makes the isotope unstable.

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