Greek Model “To understand the very large, we must understand the very small.” Democritus • Greek philosopher • Idea of ‘democracy’ • Idea of ‘atomos’ • Atomos = ‘indivisible’ • ‘Atom’ is derived • No experiments to support idea Democritus’s model of atom No protons, electrons, or neutrons Solid and INDESTRUCTABLE
John Dalton (1766-1844) was known as the “Father of the modern atom.” He was the first to actually test, previously the Greeks would just theorize. In his work he developed Dalton's Atomic Theory. John Dalton
Structure of the Atom The Changing Atomic Model • In the 1800s, John Dalton, an English scientist, was able to offer proof that atoms exist. Because Dalton’s atomic theory was proven through many experiments His THEORY became widely accepted.
Dalton’s Atomic Theory (experiment based) All elements are composed of tiny indivisible particles calledatoms Atoms of the same element are identical. Atoms Can’t be Created nor Destroyed! John Dalton (1766 – 1844) 4) Atoms of different elements combine in simple whole-number ratios to form chemical compounds 5) Allatoms of the same element have the same mass, atoms of diff. elements have diff. masses
J.J. used the idea behind charges: -- Like charges repel -- Unlike charges attract With this idea in mind, J.J. used electric current to study the atom. JJ. Thomson
In 1897, J.J. Thomson used a cathode ray tube to deduce the presence of a negatively charged particle: the electron Since atoms are generally neutral, there must also be a positively charged particle. ** Thomsons experiments were the 1st proof that atoms are made of smaller particles.
Discovery of the Electron Looking at what JJ saw, what do you think he determined was the charge on the particles in the beam?
Thomson’s Atomic Model J. J. Thomson Thomson believed that the electrons were like plums embedded in a positively charged “pudding,” thus it was called the “plum pudding” model.
Ernest Rutherford (1871-1937) • Learned physics in J.J. Thomson’ lab. • Noticed that ‘alpha’ particles were sometime deflected by something in the air. • Gold-foil experiment
Ernest Rutherford’sGold Foil Experiment - 1911 • Alpha particles are positively charged helium nuclei - They were fired at a thin sheet of gold foil • Particles that hit on the detecting screen (film) are recorded
Rutherford’s Findings • Most of the particles passed right through • A few particles were deflected • VERY FEW were greatly deflected “Like howitzer shells bouncing off of tissue paper!” Conclusions: The nucleus is small The nucleus is dense The nucleus is positively charged
The Rutherford Atomic Model • Based on his experimental evidence: • The atom is mostly empty space • All thepositive charge, and almost all the mass is concentrated in a small area in the center. He called this a“nucleus”
Thenucleusis composed ofprotons and neutrons The electrons distributed around the nucleus, and occupy most of the volume His model was called a “nuclear model” The Rutherford Atomic Model
Niels Bohr • In the Bohr Model (1913) the neutrons and protons occupy a dense central region called the nucleus, and the electrons orbit the nucleus much like planets orbiting the Sun. • They are not confined to a planar orbit like the planets are.
Bohr Model Planetary model After Rutherford’s discovery, Bohr proposed that electrons travel in definite orbitsat constant speeds around the nucleus like planets around the sun. Orbits exist in “Energy Levels”.
Two atoms are walking down the street. One atom says to the other, “Hey! I think I lost an electron!” The other says, “Are you sure??” “Yes, I’m positive!” A neutron walks into a restaurant and orders a couple of drinks. As she is about to leave, she asks the waiter how much she owes. The waiter replies, “For you, No Charge!!!” Chemistry Humor
Structure of the Atom The Electron Cloud Model • By 1926, scientists had developed the electron cloud model of the atom that is in use today. • An electron cloud is the area around the nucleus of an atom where its electrons are most likely found.
Structure of the Atom The Electron Cloud Model • The electron cloud is 100,000 times larger than the diameter of the nucleus. • In contrast, each electron in the cloud is much smaller than a single proton. • Because an electron's mass is small and the electron is moving so quickly around the nucleus, it is impossible to describe its exact location in an atom. • EX: Propeller of an Airplane’s wings !
Structure of the Atom Atomic Components • An element is matter that is composed of one type of atom, which is the smallest piece of matter that still retains the property of the element. • Atoms are composed of particles called protons, neutrons, and electrons. Click image to view movie
Structure of the Atom Atomic Components • Protons and neutrons are found in a small positively charged center of the atom called the nucleusthat is surrounded by a cloud containing electrons. • Protons are particles with an electrical charge of 1+.
Structure of the Atom Atomic Components • Electrons are particles with an electrical charge of 1–. • Neutrons are neutral particles that do not have an electrical charge.
Masses of Atoms 17.2 Atomic Mass • The nucleus contains most of the mass of the atom because protons and neutrons are far more massive than electrons. • The mass of a proton is about the same as that of a neutron—approximately
Masses of Atoms 17.2 Atomic Mass • The mass of each is approximately 1,836 times greater than the mass of the electron.
Masses of Atoms 17.2 Atomic Mass • The unit of measurement used for atomic particles is the ______________________. atomic mass unit (amu) • The mass of a proton or a neutron is almost equal to 1 amu. • The atomic mass unit is defined as one-twelfth the mass of a carbon atom containing six protons and six neutrons.
Masses of Atoms 17.2 Protons Identify the Element • The number of protons tells you what type of atom you have and vice versa. For example, every carbon atom has six protons. Also, all atoms with six protons are carbon atoms. • The number of protons in an atom is equal to a number called the atomic number.
Masses of Atoms 17.2 Mass Number • The mass number of an atom is the sum of the number of protons and the number of neutrons in the nucleus of an atom.
Masses of Atoms 17.2 Mass Number • If you know the mass number and the atomic number of an atom, you can calculate the number of neutrons. number of neutrons = mass number – atomic number
Masses of Atoms 17.2 Isotopes • Not all the atoms of an element have the same number of neutrons. • Atoms of the same element that have different numbers of neutrons are called isotopes. • Remember – Protons Never Change
Masses of Atoms 17.2 Identifying Isotopes • Models of two isotopes of boron are shown. Because the numbers of neutrons in the isotopes are different, the mass numbers are also different. • You use the name of the element followed by the mass number of the isotope to identify each isotope: boron-10 and boron-11.
Tomorrow M&M Lab
Masses of Atoms 17.2 Identifying Isotopes • The average atomic mass of an element is the weighted-average mass of the mixture of its isotopes. • For example, four out of five atoms of boron are boron-11, and one out of five is boron-10. • To find the weighted-average or the average atomic mass of boron, you would solve the following equation:
Plug the following in on a calculator • Order: • 4 / 5 X 11 = ? • 1 / 5 X 10 = ? • Add the 2 numbers = ?
Section Check 17.2 Question 1 How is the atomic number of an element determined? Answer The atomic number of an element is equal to the number of protons in an atom of that element.
Section Check 17.2 Question 2 The element helium has a mass number of 4 and atomic number of 2. How many neutrons are in the nucleus of a helium atom?
Section Check 17.2 Answer Recall that the atomic number is equal to the number of protons in the nucleus. Since the mass number is 4 and the atomic number is 2, there must be 2 neutrons in the nucleus of a helium atom.
Section Check 17.2 Question 3 How much of the mass of an atom is contained in an electron and what is the charge of an electron? Answer The electron’s mass is so small that it is considered negligible when finding the mass of an atom. Electrons are negative.
Masses of Atoms Atomic Mass 17.2
The Periodic Table 17.3 Organizing the Elements • Periodic means "repeated in a pattern." • In the late 1800s, Dmitri Mendeleev, a Russian chemist, searched for a way to organize the elements. • When he arranged all the elements known at that time in order of increasing atomic masses, he discovered a pattern.
The Periodic Table 17.3 Organizing the Elements • Because the pattern repeated, it was considered to be periodic. Today, this arrangement is called a periodic table of elements. • In the periodic table, the elements are arranged by increasing atomic number and by changes in physical and chemical properties.
The Periodic Table 17.3 Mendeleev's Predictions • Mendeleev had to leave blank spaces in his periodic table to keep the elements properly lined up according to their chemical properties. • He looked at the properties and atomic masses of the elements surrounding these blank spaces.
The Periodic Table 17.3 Mendeleev's Predictions • From this information, he was able to predict the properties and the mass numbers of new elements that had not yet been discovered.
The Periodic Table 17.3 Mendeleev's Predictions • This table shows Mendeleev's predicted properties for germanium, which he called ekasilicon. His predictions proved to be accurate.
The Periodic Table 17.3 Improving the Periodic Table • On Mendeleev's table, the atomic mass gradually increased from left to right. If you look at the modern periodic table, you will see several examples, such as cobalt and nickel, where the mass decreases from left to right.
The Periodic Table 17.3 Improving the Periodic Table • In 1913, the work of Henry G.J. Moseley, a young English scientist, led to the arrangement of elements based on their increasing atomic numbers instead of an arrangement based on atomic masses. • The current periodic table uses Moseley's arrangement of the elements.
The Periodic Table 17.3 The Atom and the Periodic Table • The vertical columns in the periodic table are called groups, or families, and are numbered 1 through 18. • Elements in each group have similar properties.
The Periodic Table 17.3 Electron Cloud Structure • In a neutral atom, the number of electrons is equal to the number of protons. • Therefore, a carbon atom, with an atomic number of six, has six protons and six electrons.
Bohr Model Practice • In atomic physics, the Bohr model, devised by Niels Bohr, depicts the atom as a small, positively charged nucleus surrounded by electrons that travel in circular orbits around the nucleus—similar in structure to the solar system, but with electrostatic forces providing attraction, rather than gravity