Atoms, Compounds, And Elements Pretest

A pure substance that is composed of two or more elements that are chemically combined is called a compound. In a compound, the elements are bonded together in a fixed ratio and cannot be separated by physical means. Isotopes are forms of an element with different numbers of neutrons, atoms are the basic units of matter, and elements are substances made up of only one type of atom. Therefore, compound is the correct answer as it best describes a pure substance composed of chemically combined elements.

Neils Bohr made a significant contribution to the atomic theory by theorizing a model of atomic structure. His model, known as the Bohr model, proposed that electrons orbit the nucleus in specific energy levels or shells. This model explained the stability of atoms and the emission and absorption of energy by electrons. Bohr's model laid the foundation for understanding the behavior of electrons in atoms and contributed to the development of quantum mechanics.

Electrons are found outside the nucleus of an atom. They orbit around the nucleus in specific energy levels or shells. These shells are arranged in increasing energy levels, with the innermost shell being closest to the nucleus. The electrons are negatively charged particles and are attracted to the positively charged protons in the nucleus. However, they are also repelled by each other, which causes them to occupy different energy levels and maintain a certain distance from the nucleus. This arrangement of electrons outside the nucleus is what gives atoms their chemical properties and allows for the formation of chemical bonds.

An element is a pure substance that cannot be separated into simpler substances. It consists of only one type of atom, which cannot be broken down into smaller particles by chemical means. Isotopes are different forms of the same element that have the same number of protons but different numbers of neutrons. Atoms are the basic units of matter, but they can combine to form compounds. Therefore, the correct answer is element.

The nucleus of an atom contains positively charged particles called protons. These protons have a positive charge, which is balanced by an equal number of negatively charged electrons in the electron cloud surrounding the nucleus. Therefore, the correct answer is positive, as the nucleus carries a positive charge.

Isotopes are atoms of the same element that have the same number of protons but different numbers of neutrons. This means that the atomic mass of isotopes varies due to the differing number of neutrons. Therefore, the correct answer is that the number of neutrons is different between isotopes of the same element.

John Dalton was the first scientist to use the term "atom." He proposed the atomic theory in the early 19th century, which stated that all matter is composed of small, indivisible particles called atoms. Dalton's theory revolutionized the understanding of chemistry and laid the foundation for modern atomic theory. His work paved the way for further scientific discoveries and advancements in the field of atomic research.

The parts of an atom include the neutron, electron, nucleus, proton, and electron shell. The neutron is a subatomic particle that has no charge, while the electron is a negatively charged particle that orbits the nucleus. The nucleus is the central part of an atom that contains protons and neutrons. Protons are positively charged particles found in the nucleus. The electron shell is the region surrounding the nucleus where electrons are found. Therefore, all of these options are correct.

The mass of an atom is made up of neutrons and protons. Neutrons have a neutral charge and are located in the nucleus of an atom along with protons, which have a positive charge. Electrons, on the other hand, have a negative charge and are found in electron shells surrounding the nucleus. Therefore, while electrons and electron shells are important components of an atom, they do not contribute to its mass.

Atoms are able to form compounds when they have unfilled outer electron shells. This is because atoms are constantly seeking stability, and having unfilled outer electron shells makes them more likely to interact with other atoms in order to achieve a full outer shell. By forming compounds, atoms can share, gain, or lose electrons to achieve a stable electron configuration, ultimately increasing their stability and reducing their reactivity.