Chapter 3, Section 3 Counting Atoms

2 All atoms contain the same particles… Yet all atoms are not the same. What makes them different??

Think about those 3 basic components of the atom:

Protons Electrons Neutrons

Protons – identify each element. Change this number and you’ve changed elements. Electrons – in a neutral atom equal the number of protons. Change this number and you’ve changed the charge of the atom. (This is another chapter!0 Neutrons – our focus today!

6 Protons identify an element. All atoms of an element must have the same number of protons, but the number of neutrons can be different.

7 The atomic number (Z) of an element is the number of protons of each atom of that element. Z = number of protons

Find your periodic table… An element’s atomic number is usually above its symbol and the elements are placed in order of increasing atomic number. (Thanks, Mr. Mendelyeev) The atomic number identifies an element. So if the number of protons (Z) changes, the identity of the element changes. 8

The number of neutrons and the number of electrons can vary and the atom will still be the same element. We said when electrons change, the charge of the atom changes… But what happens when the number of neutrons changes? Let’s see about that!

10 Remember that a neutral atom has the same number of … (You tell me!) The number of electrons & neutrons can vary and the atom will still be the same element. But if the number of protons changes, then the atom becomes an atom of a different element.

Let’s look at an element on the periodic table and sort out this information… Pick an element from your periodic table and draw it’s entry in your notes… Include: Atomic number Symbol and name Average atomic mass. ** We’ll talk about average atomic mass in a bit…

12 Atomic number

Isotopes Isotopes are atoms of the same element that have different masses. The isotopes of a particular element all have the same number of protons and electrons but different numbers of neutrons. 13

Take hydrogen, for example All hydrogen atoms have 1 proton, but they can have different numbers of neutrons – or no neutrons at all! Protium is the most common and most stable ( % of the H atoms found on Earth) 1 proton + 0 neutrons = 1 Deuterium also known as heavy hydrogen (0.0115% found on Earth) 1 proton + 1 neutron = 2 Tritium is radioactive and not very common. 1 proton + 2 neutrons = 3 14

Most elements are a mixture of isotopes Most elements consist of mixtures of isotopes. Tin has 10! (That’s the most of any element) The atoms in any sample of an element will most likely be a mixture of several isotopes in various proportions. 15

Mass number The mass number (A) of an element is the total number of protons and neutrons that make up the nucleus of an isotope. We have to know the name or atomic number of the element and the mass of the isotope to identify the isotope. (These will be given.) 16

17 Mass Number (A) A = Z + n 0

18 Two isotopes of carbon A nuclide or isotope symbol for carbon.

Two isotopes of chlorine

Atomic number (Z) number of protons in nucleus Mass number (A) number of protons + number of neutrons (or (Z) + number of neutrons) (or (Z) + number of neutrons) Isotopes are atoms of the same element (X) with different numbers of neutrons in their nuclei. XAZ Mass Number Atomic Number Element Symbol Nuclide symbol Atomic number, Mass number and Isotopes C 12 6 C 14 6

FYI: The Isotopes of Hydrogen

To find neutrons: Mass number – atomic number = number of neutrons 235 (protons + neutrons) -92 protons = 143 neutrons 22

There are 2 ways we can write isotopes – actually 3… 1.Hyphenated notation 2.Nuclide (or nuclear symbol)

Designating isotopes and finding neutrons **add in your notes Hyphen notation = element (hyphen) mass number For example:uranium-235 Nuclear symbol: mass no. (A) atomic no. (Z) 24

Let’s practice this: Write the nuclear symbol for oxygen-16 1)What 3 things do you need? 2)What is the order you write them? 25

Let’s try the ones in your notes Tin-121 Write the Nuclide symbol, Atomic No. and Mass Number. List the number of protons, neutrons & electrons found in Xenon

Sn Xenon-113 p+ = 54 e- = 54 n 0 = 59 27

Let’s work on some of these: Subatomic particles Sample problem A, pg. 75 Then do the practice questions, pg. 76:

Answers: 1.35 p, 35 e-, 45 neutrons C 3. phosphorus-30 29

Let’s check it! 1.How many p+, e- and n 0 are in the following: Beryllium (Be) Atomic number 4 Mass number 9 2. Element X has two isotopes, X-23 and X-30. Element X has 8 protons. Write the symbol for each isotope using superscripts and subscripts to represent the mass number and atomic number.