A free "teach-in on housing" will be held from 9 a.m. to noon today at St. Raphael's Catholic Church on Falls Road in west Rockville. It will be sponsored by Montgomery County's Community Ministries. People who wish to take an afternoon bus ride of housing sites sponsored by the country's Housing Opportunities Commission should bring lunch, the organizers said.

The Racal Building at 5 Research Place, Rockville, has been purchased by a private investment group for $2.5 million from Research Limited Partnership. Malcolm Marshall of B. F. Saul Co. handled the transaction.

The American Geophysical Union recently acquired an office building at 2000 Florida Ave. NW. for $2 million. The property was sold by developer Jeffrey N. Cohen and others. The building was formerly occupied by the National Rural Electric Cooperative Association.

U.S. Foods Corp. recently paid $5.1 million to Mr. and Mrs. William Robinowitz for an office building at 605 G St. NW. Most of the space is rented by the D.C. Government.

Richard Hollander, formerly with Gulf Oil Corp.'s real estate division, has joined the Washington office of Cushman & Wakefield as branch manager. The nationwide real estate organization also announced plans to expand its operations in the Washington market.

Peter B. Nash, formerly with Walker & Dunlop, has joined Leggat McCall & Werner Co. here as vice president for real estate finance.

The Poretsky & Starr building firm has opened the second section of its Woodleigh community of 151 houses near the Chantilly Golf and Country Club near Centreville, Va.

Pulte Home Corp. announced that sold 25 town houses in the first eight weeks of sales at its Snowden's Mill development in the Silver Spring area. The houses are priced in the $80,000s. The firm said it has also sold 30 detached houses, priced up to $120,000, in the development, which is near Old Columbia Pike and Randolph Road.

The Braedon Companies will be the leasing agent for a building that Prudential Insurance Co. of America will start soon at the northwest corner of 15th and L streets NW, where the Pick-Lee Hotel was razed recentlly. Designed by Skidmore, Owings & Merrill, the 12-story building will have 140,000 square feet of space. It is expected to be finished late in 1981.

The Student Loan Marketing Association has leased 100,000 square feet of office space in Georgetown Plaze at 1050 Thomas Jefferson St. N.w. Coldwell Banker handled the transaction.