What is the difference between an atom and an element? What are atoms composed of? What is the significance of atomic number and mass number.

An atom is the smallest unit of matter that retains the properties of an element. It consists of a nucleus, which contains protons and neutrons, and electrons that orbit the nucleus.

An element, on the other hand, is a pure substance composed of only one type of atom. For example, an oxygen atom is an individual atom of the element oxygen.

Now, let's understand what atoms are composed of. At the center of an atom is its nucleus, which contains positively charged protons and uncharged neutrons. Surrounding the nucleus, negatively charged electrons occupy energy levels or electron shells.

The atomic number of an atom is the number of protons in its nucleus. It uniquely identifies each element on the periodic table. For example, hydrogen has an atomic number of 1, indicating it has one proton in its nucleus. The atomic number determines the chemical properties of an element.

The mass number of an atom is the sum of protons and neutrons in its nucleus. It is denoted by the symbol 'A'. For example, carbon-12 has a mass number of 12, indicating it has 6 protons and 6 neutrons in its nucleus. The mass number helps differentiate between isotopes - atoms of the same element with varying numbers of neutrons.

To summarize, the main difference between an atom and an element is that an atom is a tiny particle that makes up matter, while an element is a pure substance composed of only one type of atom. Atoms are composed of protons, neutrons, and electrons. The atomic number represents the number of protons and determines the element's identity, while the mass number represents the sum of protons and neutrons in an atom's nucleus.