Lesson 9 October 13th, 2010

By the late 1700’s chemists wondered why some elements existed in different states and why they reacted in different manners. In the middle of the 1800’s the list of elements had grown to 63 and chemists still did not have a way of categorizing them.

started to categorize elements by the way they reacted; this placed all the metals on one side, and the non-metals on the other. He was so confident in his theory that he even left gaps in the periodic table where he thought an element would be found later.

The periodic table is a chart that places all of the elements in rows and columns. In the modern periodic table, elements are listed from left to right and top to bottom according to a property called atomic number.

Atomic number - the number of protons in an atom of an element.

Each element has a set number of protons and every atom from that element will have that many protons.

The pattern for increasing protons moves from left to right and then down to the next row just like reading a book.

Atomic mass - the average mass of an element’s atoms.

Atomic mass is given in atomic mass units (amu).

H has a mass of 1.01 amu. This means that iron atoms are about 55.85 times heavier than hydrogen atoms.

Atomic masses are always expressed as decimal fractions. One reason that they do not have whole number values is that, except for fluorine, atoms of the same element have different numbers of neutrons.

◦ Example: A hydrogen atom has one proton and one electron but no neutron. A small percentage of hydrogen atoms have 1 p, 1e, and 1n. 1 p, 1e, and 2n.

Atomic mass generally increases in order of atomic number.

Exception: iodine (I) has a lower atomic mass than tellurium (Te).

Ion charge - the electric charge that an atom takes on when it loses or gains electrons.

An atom or group of atoms that has lost or gained electrons is called an ion.

Electrons have a negative charge, and so an atom that loses electrons becomes a positive ion. An atom that gains electrons becomes a negative ion.

Metal atoms can lose electrons in certain situations. (positive ions)

Non-metals can gain electrons in certain situations. (negative ions)

Elements with atoms that can form similar ions are grouped together in the periodic table. Metals generally lose electrons and become positive ions.

Many non-metals can gain electrons and so become negative ions.

Some elements do not form ions. Helium, for example, does not normally form ions. For these elements, no ion charges are shown in their squares in the periodic table.

Determining the number of protons Look at the atomic number given on the

periodic table (atomic number)◦ Example: H = 1, He = 2, Li = 3

Determining the number of neutrons Subtract the atomic number (# of protons)

from the Atomic mass (# of protons and neutrons) ◦Atomic mass – atomic number (A – Z = N)◦ Example: Iron

55.85 – 26 = 30 neutrons

Determining the charge of Ions Subtract the number of electrons from the

number of protons◦Protons (P) – Electrons(E) = Ion Charge◦ Example: Iron

26 – 24 = 2+ charge = Fe2+

26 – 23 = 3+ charge = Fe3+

◦ Example: Fluorine 9 – 10 = 1 – Charge = F-

1. Use the periodic table to find the atomic number of each of the following elements.

(a) C (b) O (c) Na (d) Si (e) S (f) Cl (g) Fe 2. How many protons are in an atom of each of the following

elements? (a) lithium (b) nitrogen (c) fluorine (d) aluminum (e) copper (f)

gold 3. Name the element with the following number of protons. (a) 1 (b) 2 (c) 10 (d) 19(e) 20 (f) 31 (g) 47 4. Name the element with the following atomic mass.(a) 12.01

amu (b) 16.00 amu (c) 39.10 amu (d) 83.80 amu 5. What is the electric charge on an ion of each of the

following elements? (a) Li (b) Be (c) N (d) S (e) Al (f) I 6. Although the element hydrogen is a non-metal, it is located

on the left side of the periodic table. Explain how placing hydrogen in this position relates to its ion charge.

7. Describe the patterns in atomic masses and ion charges in the periodic table.