The Structure of the Atom

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The Structure of the Atom

Objectives Compare and contrast the atomic models of Democritus and Dalton Define an atom. Distinguish between the sub atomic particles in terms of relative charge and mass. Describe the structure of the nuclear atom, including the locations of the subatomic particles.

Objectives Explain the role of atomic number in determining the identity of an atom. Define an isotope and explain why atomic masses are not whole numbers. Calculate the number of electrons, protons, and neutrons in an atom given its mass number and atomic number.

Dalton’s Atomic Theory
Each element consists of individual particles called atoms Atoms can neither be created nor destroyed All atoms of a given element are identical Atoms combined chemically in definite whole-number ratios to form compounds Atoms of different elements have different masses

History The electron was discovered in 1897 by Thomson. He imagined the atom as a “raisin pudding” with electrons stuck in a cake of positive charge In 1912, Rutherford discovered the nucleus (proton). He concluded the atom was mostly empty space, with a large dense body at the center, and electrons which orbited the nucleus like planets orbit the Sun

History Rutherford’s gold foil experiment was used to show that the nucleus was small, dense, and positively charged Thomson used the cathode ray to discover electrons The neutron was then discovered in 1932 by Chadwick.

Subatomic Particles and the Nuclear Atom

Development of Atomic Structure
• About 1900 Sir J. J. Thomson and Earnest Rutherford established the modern model of the atom. • The atom is composed of three subatomic particles. 1. Electrically positive (+1) protons. 2. Electrically neutral (0) neutrons. 3. Electrically negative (-1) electrons. • Atoms have a net charge of zero. • The number of protons equals the number of electrons.

Parts of the atom

Parts of an Atom Each element in the Periodic Table has a different number of protons in its nucleus Change the number of protons  change elements The element also has the same number of electrons Change the number of electrons  ionize the element Some elements also have neutrons They are in the nuclei of atoms p e n

1H The Hydrogen Atom One electron orbiting a nucleus
1 proton = Z = atomic number 0 neutrons = N Total mass = A = Z+N =1 Singly ionized Hydrogen is missing one electron = 1H+ Add a neutron and you have Deuterium = 2H = D p e 1H

4He The Helium Atom Two electrons orbiting a nucleus
2 protons = Z = atomic number 2 neutrons = N Total mass = A = Z+N =4 Singly ionized Helium is missing one electron = 4He+ e p n n p e 4He

3H (Tritium) 3He Isotopes and Elements
If Helium loses one of its protons (and one of its electrons), it becomes a different element p n n 3H (Tritium) p n e If Helium loses one of its neutrons, it becomes an isotope 3He

Knowing the structure of atoms tells us about their chemical properties light-emitting properties light-absorbing properties From this information we can learn about galaxies, stars, planets, asteroids, based on the light they emit or reflect. An example of absorption spectra from many different types of stars.

How Atoms Differ Section 4.3

Atomic Structure An atom is composed of a dense core called a nucleus and surrounding this nucleus one or more negatively charged electrons. The nucleus is composed of positively charged protons and neutral neutrons.

Atomic Structure An atom is mostly empty space because the electron moves around the nucleus at such a great distance. A chemical element is determined based on how many protons the nucleus contains (Hydrogen has 1, Carbon has 6, Oxygen has 8 protons).

An atom has the same number of electrons as protons, an atom has no net charge.
If an atom loses or gains one or more electrons it has become ionized. With less electrons, the atom has a positive charge (Cation), more electrons it has a negative charge (Anion). Ions

Parts of the atom Particle Charge Mass (g) Mass (amu) Proton +1
1.672 x 10-24 1 Neutron 1.675 x 10-24 Electron -1 9.110x10-28 1/1839

Atomic Number & Atomic Mass
• The number of protons in the nucleus of an atom is called the atomic number (Z).

Atomic Number & Atomic Mass
Mass Number • The atomic mass of an atom is estimated from the total number of protons and neutrons in the nucleus. • The sum of the number of protons and neutrons in an atom is called the atomic mass number (A). Element Symbol Mass Number Atomic Number

Isotopes Isotope - Atoms with the same atomic number but different mass numbers. • Isotopes have the same number of protons but differ in number of neutrons.

Isotopes Isotope Abundance • Naturally occurring sample of most elements will contain a mixture of isotopes. • Percent abundance is used to quantify the amount of each isotope in an elemental sample. Example: Oxygen Isotope % Abundance # in 100,000 16O 99.759 99,759 17O 0.037 37 18O 0.204 204

Atomic Mass Atomic mass unit is a relative unit
1/12 the mass of a 12C atom 1 atomic mass unit (amu) = 1.66 X kg

Atomic Mass and Isotopes
The atomic mass which appears on the periodic table is the weighted average of the masses of all the isotopes. weighted fractional isotopic average = isotopic X mass abundance S

Atomic Mass and Isotopes
Boron has two isotopes 10B abundance 19.9% mass u 11B abundance 80.1% mass u Atomic mass = (0.199 x 10.01) + (0.801 x 11.01) Atomic mass = u

Atomic Weight (Mass) – The average mass of a representative sample of atoms.
Example 1: Calculate the atomic weight of oxygen given the following data. Isotope % Abundance Atomic Weight 16O 99.762 17O 0.038 18O 0.200

Atomic Weight (Mass) – The average mass of a representative sample of atoms.
Example 2: Calculate the atomic weight of chlorine given the following data. Isotope % Abundance Atomic Weight 35Cl 75.77 36Cl 0.0 37Cl 24.23

Atomic number and mass number
+/-q Z X X=symbol Z=atomic number (# of protons) A=mass number (# of protons + # neutrons) q= charge :+=electrons<protons, :-=electrons>protons

Atomic Structure Atomic number (Z) is the number of protons in the nucleus of an atom. It is also known as the proton number. No. of protons always equals the no. of electrons in any neutral atom of an element. Mass number (A) is the sum of the number of protons and the number of neutrons in the nucleus of an atom.

Atomic Structure - Example
So how can you work out the number of neutrons in an atom? Example No. of neutrons = Mass number – atomic number No. of neutron = Mass No. – Atomic No. = 23 – 11 = 12

Atomic Mass from Isotopic Composition

Atomic Mass from Isotopic Composition
20.00 (0.9092) + 21.00 (0.0026) 22.00 (0.0882) 20.18 amu A.M. Ne = 20.18g/mol

X Symbols Mass number Atomic number
Contain the symbol of the element, the mass number and the atomic number Mass number X Atomic number

F Symbols 19 9 Find the number of protons number of neutrons
number of electrons Atomic number Mass Number 19 F 9

Br Symbols 80 35 Find the number of protons number of neutrons
number of electrons Atomic number Mass Number 80 Br 35

Symbols if an element has an atomic number of 34 and a mass number of 78 what is the number of protons number of neutrons number of electrons Complete symbol

Symbols if an element has 91 protons and 140 neutrons what is the
Atomic number Mass number number of electrons Complete symbol

Symbols if an element has 78 electrons and 117 neutrons what is the
Atomic number Mass number number of protons Complete symbol