Atomic number, Z:

The identity of an element is controlled by the number of protons in the nucleus. The number of protons equals the atomic number. Every element has its own unique atomic number. The atomic number represented by the symbol, Z. The periodic table is arranged in sequence of increasing atomic number or proton count. Hydrogen has one proton and has atomic number one, Z=1. The atomic number increases by one unit for every additional proton. helium has two protons and is therefore atomic number Z= 2.

Example:

What is the atomic number for nitrogen, N?

Answer: Nitrogen is in the seventh position in the periodic table. This means nitrogen atoms have 7 protons in the nucleus, and they have an atomic number of 7.

Mass number, A:

The count of neutrons and protons equals the mass number for an atom. This has its origin in the fact that the "massive" particles in the atom are protons and neutrons. The symbol for mass number is A. The entries in the periodic table do not show the mass numbers for atoms. The periodic table is useless when we want to figure mass numbers. The mass number for an atom is commonly indicated with a number after the element name. For example lead-210 indicates a type of lead atom with a mass number A = 210. For people who like formulas the mass number and atomic number are related as shown below. number of neutrons = A - Z number of neutrons = mass number - atomic number

Example:

How many neutrons and protons are in an atom of sodium-23?

Answer: Sodium, Na, has atomic number 11. This means sodium atoms have 11 protons in the nucleus. The atom has a mass number equal to the number of protons and neutrons, so there must be 12 neutrons in the nucleus.

number of neutrons = A - Z number of neutrons = 23 - 11 = 12

Atomic number and electron count:

Atoms are neutral particles. The positive charge on the positively charged protons in the nucleus is canceled out by an equal number of negatively charged electrons moving in space outside the nucleus. Hydrogen has one proton in the its nucleus and one electron outside.   The illustration shows a neutral beryllium-8 atom, Be-8, with four electrons and four protons.

Example:

How many electrons and protons are in a neutral sodium atom?

Answer: Sodium, Na, has atomic number 11. This means sodium atoms have 11 protons in the nucleus. The neutral atom has to have equal numbers of protons and electrons, so there must be 11 electrons moving outside the nucleus.

An atom contains 24 neutrons and 25 protons, what is the mass number of the atom? Answer: Mass number = A = number protons + number of neutrons = 24 + 25 = 49