Notes 4-1 Intro to Atoms

What’s an atom? Smallest particle of an element

Atomic Theories There have been many theories of proposed models of the atom. Some were completely wrong! And some were partially correct or close to correct. We will look at some of the most known theories and models.

Dalton’s Atomic Theory Dalton thought that atoms were like smooth, hard balls that could not be broken into smaller pieces. He was credited with coming up with the earliest model of the atom Most of this theory is still accepted today #2

Thomson’s Model Thomson suggested that atoms had negatively charged electrons embedded in a positive sphere. This is the first model that included smaller particles of the atom His model is similar to blueberries in a muffin

Rutherford’s Model Developed the first model that included a nucleus Similar to a peach with a pit, the pit = the heavy mass of protons in the nucleus, the fruit = the empty space made up of electrons

Rutherford’s Gold Foil Experiment Turn to page 126-127 in your text book Let’s read about Rutherford’s model/ experiment Who wants to read first? Everyone else, follow along….

Rutherford’s Gold Foil Experiment Rutherford was surprised that a few particles were deflected strongly. This led him to propose an atomic model with a positively charged nucleus. #5

Bohr’s Model Said electrons could only have a certain amount of energy, so they must move in certain orbits Similar to planets orbiting the sun (sun = nucleus), or the rings/layers of an onion

Cloud Model Says Bohr’s model is wrong, electrons do not orbit the nucleus like planets do the sun Instead, electrons can be found anywhere in a cloud-like region around the nucleus, rapidly orbiting in every direction

Later Atomic Models Through the first part of the twentieth century, atomic models continued to change.

Modern Atomic Model Accepted since the 1930s At the center of the atom is a massive nucleus, containing positively charged protons and neutrally charged neutrons (no charge) Surrounding the nucleus is a cloud-like region of moving negatively charged electrons # protons in atom = # electrons in atom # neutrons can change

Particles in an Atom An atom is composed of positively charged protons, neutral neutrons, and negatively charged electrons. Protons and neutrons are about equal in mass and are measured in atomic mass units (amu). An electron has about 1/2,000 the mass of a proton or neutron. #18

Atomic Number Every atom of a given element has the same number of protons in its nucleus This is known as the element’s atomic number For example, Helium’s atomic number is 2, so every atom of helium has 2 protons in its nucleus. If the element has more than 2 protons, it is NOT helium! Carbon’s atomic number is 6. How many protons does an atom of carbon have?

Atomic Number The atomic number is usually found above the letter on the periodic table Look at helium below The atomic number tells the number of protons in the nucleus. #7

Isotopes: Atoms of elements have same number of protons, but different numbers of neutrons Atoms of all isotopes of carbon contain six protons and six electrons, but they differ in their number of neutrons. Carbon-12 is the most common isotope.

Mass number 6 How do you know? Because if the mass number is 12, An isotope is identified by its mass number Mass number = #protons + #neutrons Example: Carbon-12, the mass number is 12. We know carbon has 6 protons by looking at its atomic number on the periodic table. How many neutrons does carbon-12 have? 6 How do you know? Because if the mass number is 12, 12 - 6protons = 6neutrons

Problem: The atomic number of nitrogen is 7. How many protons, neutrons, and electrons make up Nitrogen-15? Protons = 7 Neutrons = 8 Electrons = 7