C HAPTER 4 – A TOMIC S TRUCTURE Overview of the Atom and Atomic Structure

T HE T HREE S UB -A TOMIC P ARTICLES OF THE A TOM Proton+1In the nucleus Neutron0 (no charge)In the nucleus ElectronIn the electron cloud Sub-Atomic Particle Name ChargeLocation in the atom

T HE NUCLEUS The nucleus is the area of the atom that contains almost all of the mass of the atom. The nucleus is composed entirely of protons and neutrons. The rest of the atom is composed of an electron cloud. The electron cloud is the space outside the nucleus where the electrons rapidly move. Most of an atom is composed of empty space.

A TOMIC SYMBOL On a periodic table, you may notice that every element is given a shortened symbol called an atomic symbol. The first letter is always capitalized. If there are other letters present, they are always written in lower case. Atomic Symbol

A TOMIC NUMBER There is a number that is written above each element symbol on the periodic table which is called the atomic number. The atomic number is the number of protons in that particular element. So, if an element has 6 protons, then it has to be carbon. Atomic Number

M ASS AND THE ATOM That very small nucleus of the atom still contains almost all of the mass of an atom. Protons and neutrons have masses that are nearly equal. To keep from dealing with very tiny numbers we assign each proton or neutron a mass of 1 atomic mass unit (amu) per particle. It takes almost 2,000 electrons to equal the mass of a proton. Since electrons are so small, they are considered unimportant and are assigned a value of 0 amu.

T HE ATOMIC MASS UNIT The atomic mass unit (amu) is a measurement system used to measure the mass of atoms. Since atoms are extremely tiny, it is easier to use a simple system that measures mass in whole numbers instead of the true mass numbers for protons and neutrons.

A TOMIC M ASS AND THE M ASS N UMBER A mass number is the number of protons and neutrons in an element. For example, one type of atom of carbon has six protons and six neutrons. The mass number for this atom of carbon would be 12 (6p + 6n). The periodic table shows an average atomic mass for each element (usually at the bottom) which quite often is not a whole number. This is an average mass of all of the types of this atom that are found. This is the average atomic mass for Carbon

I SOTOPES Atomic Mass numbers become important when you look at the isotopes of elements. Isotopes are different forms of the same element that differ in neutron numbers. Another way of saying this is that isotopes of an element have the same number of protons but a different number of neutrons.

T HE I SOTOPES OF C ARBON For example, there are 3 types of carbon atoms found in nature. Carbon-12 (C-12), Carbon-13 (C-13) and Carbon-14 (C-14). All of these isotopes have 6 protons as that is what makes the atom carbon. What is different is the mass of the isotopes due to different numbers of neutrons.

C ALCULATING WITH ATOMIC MASS Atomic mass = number of protons + number of neutrons If an atom of oxygen has 8 protons and 8 neutrons, it has a mass of 16 amu. But what if you are given the atomic mass and asked to find the number of neutrons of an atom? Use this formula – # of neutrons = mass number minus # of protons. So, using this, how many protons does an atom of chlorine-37 have?

W RITING I SOTOPES To distinguish isotopes from other atoms when written a dash is always written between the element name and the mass number. For example, hydrogen-1 or H-1 are proper ways to write isotopes. Another way to write isotopes is to show the mass number and atomic number to the left of the atomic symbol. Try to figure out these: 1. He-4 How many neutrons does it have? (Hint: find He and see how many protons it has) 2. C-14 How many protons? Neutrons? 3. Cl-37 How many protons? Neutrons? 4. What isotope has 30 protons and 35 neutrons?